BERKELEY 

LIBRARY 

UNIVERSITY 


OF 


PLATE  I.— The  Captain  of  the  Canyon,  Monument  Canyon,  Arizona. 


GEOLOGY 

PHYSICAL  AND  HISTORICAL 


BY 


HERDMAN   FITZGERALD    CLELAND,    Pn.D, 

PROFESSOR   OF  GEOLOGY  IN   WILLIAMS   COLLEGE 
WILLIAMSTOWN,   MASSACHUSETTS 


,    ' 
*.-»*,-, 


AMERICAN    BOOK    COMPANY 

NEW  YORK  CINCINNATI  CHICAGO 


COPYRIGHT,  1916 
BY  H.  F.  CLELAND 


ALL   RIGHTS    RESERVED 


CLELAND'S  GEOLOGY 
W.  P.  I 


,  -  -  : 

-.• 


EARTH 

SCIENCES 
LIBRARY 


TO 

MY  MOST  HELPFUL  CRITIC 
AND  INDISPENSABLE  AID 

MY  WIFE 


341808 


PREFACE 

IN  the  preparation  of  this  volume  an  attempt  has  been  made  to 
present  an  outline  of  the  essentials  of  modern  geology.  By  avoid- 
ing all  details  not  necessary  to  an  understanding  of  the  fundamental 
principles  of  the  science,  it  is  hoped  that  this  work  will  prove  inter- 
esting to  the  student,  although  not  less  accurate  because  interesting. 
In  the  section  on  physical  geology  the  human  relation  has  been 
emphasized  whenever  possible,  while  in  the  historical  section  the 
history  of  life  from  the  evolutionist's  point  of  view  has  been  taken 
up  in  broad  outline. 

Much  that  may  prove  excellent  in  this  work  is  due  to  the  help 
of  a  number  of  eminent  geologists,  whose  suggestions  and  criticisms 
have  added  many  interesting  points  and  have  assisted  in  the  elimi- 
nation of  errors. 

The  writer  wishes  especially  to  express  his  debt  to  Dr.  W.  D. 
Matthew,  of  the  American  Museum  of  Natural  History,  City  of 
New  York;  to  Professor  Joseph  Barrell,  of  Yale  University;  and  to 
Professor  N.  M.  Fenneman,  of  the  University  of  Cincinnati,  upon 
whom  he  has  freely  called  for  suggestions  and  criticisms  and  from 
whom  much  valuable  assistance  has  been  received. 

One  or  more  chapters  have  also  been  read  and  helpfully  criticized 
by  the  following  geologists  and  educators,  and  their  generous  aid 
is  acknowledged  with  keen  appreciation :  Messrs.  H.  E.  Gregory,  of 
Yale  University;  C.  K.  Schwartz,  of  Johns  Hopkins  University; 
J.  B.  Woodworth,  of  Harvard  University ;  J.  S.  Grasty,  of  the  Uni- 
versity of  Virginia ;  Sumner  W.  Cushing,  of  the  Salem  (Massachu- 
setts) Normal  School;  J.  W.  Gidley  and  C.  W.  Gilmore,  of  the 
United  States  National  Museum ;  T.  W.  Stanton  and  F.  H.  Knowl- 
ton,  of  the  United  States  Geological  Survey ;  Charles  Schuchert  and 
G.  G.  MacCurdy,  of  Yale  University;  T.  D.  A.  Cockerell,  of  Boulder, 
Colorado;  L.  Hussakof,  of  the  American  Museum  of  Natural  His- 
tory ;  E.  C.  Case,  of  the  University  of  Michigan ;  O.  P.  Hay,  of  the 
Carnegie  Institution  of  Washington;  Sidney  Powers,  Cambridge, 
Massachusetts ;  and  C.  L.  Dake,  of  the  Missouri  School  of  Mines. 

5 


6  PREFACE 

For  suggestions  as  to  the  most  characteristic  species  of  the  vari- 
ous periods,  credit  is  due  to  Professor  G.  D.  Harris,  for  the  Tertiary ; 
Messrs.  T.  W.  Stanton  and  F.  H.  Knowlton,  for  the  Mesozoic;  and 
Drs.  R.  Ruedemann  and  E.  M.  Kindle,  and  Mr.  L.  Burling,  for  the 
Paleozoic. 

The  numerous  block  diagrams  which  illustrate  the  text  were  made 
in  wash  rather  than  in  line  because  the  former  are  not  only  more 
attractive  in  appearance,  but  because  being  more  realistic,  they  are 
more  readily  understood  by  the  student. 

The  writer  is  greatly  indebted  to  Professors  H.  F.  Osborn,  W.  B. 
Scott,  F.  A.  Lucas,  and  S.  W.  Williston,  for  permission  to  use  photo- 
graphs of  restorations  of  extinct  animals,  made  by  them  or  under 
their  direction,  and  to  Prof.  William  Bullock  Clark  and  Dr.  John  M. 
Clarke  for  the  loan  of  a  number  of  original  drawings  of  the  Geologi- 
cal Surveys  of  which  they  are  directors. 


CONTENTS 

INTRODUCTION 

PAGE 

Astronomic  or  Cosmic  Geology.  Structural  Geology.  Dynamical  Geology. 
Industrial  Geology.  Historical  Geology.  Length  of  Geological  Time. 
Present  Status  of  Geology.  Fundamental  Terms.  Rocks.  Sedimentary 
Rocks.  Igneous  Rocks.  Metamorphic  Rocks.  Divisional  Planes  .  .  21 

PART  I.    PHYSICAL  GEOLOGY 

CHAPTER  I 

WEATHERING 

MECHANICAL  AGENCIES      .        .       .       .       .        .       .       .       ,       .       ..      V      .       27 

Frost.  Talus.  Rock  Glaciers.  Creep  of  Soils.  Changes  in  Daily 
Temperature.  Mechanical  Action  of  Animals  and  Plants.  Rain. 
Wind.  Lightning. 

CHEMICAL  AGENCIES ,.       .35 

Solution.  Oxidation.  Hydration.  Carbonation.  Organisms.  Compari- 
son of  Effects  of  Chemical  and  Mechanical  Weathering. 

RESULTS  OF  WEATHERING 38 

Spheroidal  Weathering.  Differential  Weathering.  Widening  of  Valleys. 
Rock  Mantle  and  Soil.  Kinds  of  Soil.  Removal  of  Soil. 

CHAPTER  II 

WORK  OF  THE  WIND 

WIND  AND  SAND         .       .       .       .        .        .       ...       .        .        .        .       .        .       44 

Wind  without  Sand.  Wind  with  Sand.  Sand  Dunes.  Shape  and  Origin  of 
Dunes.  Migration  of  Sand  Dunes.  Beneficial  Effect  of  Dunes.  Ma- 
terial of  Dunes.  Height  of  Dunes.  Eolian  Sandstone.  Dust.  Loess. 

CHAPTER  HI 

THE  WORK  OF  GROUND  WATER 
WORK  OF  GROUND  WATER         .        .        .  -    .       .       .       .  .       .. ;•      .        .        56 

Quantity  of  Ground  Water.  The  Water  Table.  Wells.  Movement  of 
Ground  Water.  Depth  of  Ground  Water.  Artesian  Wells.  Chemical 
Work  of  Ground  Water.  Solution.  Replacement  and  Deposition. 
Belts  of  Weathering  and  Cementation.  Desert  Limestone.  Mechanical 
Work  of  Ground  Water. 

SPRINGS .       .       .       .       .       .       62 

Origin  of  Springs.  Constant  and  Intermittent  Springs.  Mineral  Matter 
in  Spring  Water.  Mineral  Springs.  Temperature  of  Springs.  Ther- 
mal Springs.  Geysers. 

7 


8  CONTENTS 

PAGE 

STRIKING  EFFECTS  OF  GROUND  WATER 69 

Swallow  Holes.  Caverns.  Natural  Bridges.  Cave  Deposits.  Karst. 
Landslides. 

CONCRETIONS 75 

Composition  of  Concretions.  Time  of  Formation.  Oolitic  Limestone. 
Geodes. 

CHAPTER  IV 

THE  WORK  OF  STREAMS 

FACTORS  IN  STREAM  EROSION 81 

Material  Carried  by  Streams.  How  the  Sediment  is  Moved.  Factors  Deter- 
mining the  Velocity  of  Streams.  Water  Wear.  Solution.  Vertical 
Erosion  (Corrasion).  Weathering  and  Vertical  Erosion.  Base  Level 
of  Erosion.  Effect  of  Load.  Factors  Affecting  the  Rate  of  Erosion. 
Scour  and  Fill.  Lateral  Erosion. 

FEATURES  DUE  TO  STREAM  EROSION 89 

Falls  and  Rapids.  Exceptions  —  Falls  not  the  Result  of  Erosion.  Pot- 
holes. Canyons.  Instances  of  Rapid  Erosion.  Effect  of  Deforesta- 
tion on  Rivers.  Growth  of  Valleys.  Valleys  Formed  in  Ways  Other 
than  by  Stream  Erosion.  The  Direction  of  Valleys.  Basins  and 
Divides.  Elevations  Due  to  Unequal  Hardness.  Outliers.  Rock 
Terraces.  Stream  Piracy. 

THE  EROSION  CYCLE 109 

Youth.  Maturity.  Old  Age.  Effect  of  Elevation  and  Depression  on 
Streams. 

PENEPLANATION .        .        .114 

The  Peneplain  of  Southern  New  England.  The  Appalachian  Peneplain. 
The  Laurentian  Peneplain.  Rate  of  the  Denudation  of  Continents. 
How  the  Load  of  Streams  is  Measured. 

DEPOSITION IIQ 

Causes  of  Deposition.  Flood  Plains.  Meanders.  Oxbow  Lakes.  Natural 
Levees.  Alluvial  Cones  and  Fans.  Piedmont  or  Alluvial  Plains. 
Alluvial  Terraces.  Discontinuity  of  Terraces.  Characteristics  of 
River  Deposits. 

DELTAS I30 

Growth  of  Deltas.     Structure  of  Deltas. 

DEPOSITION  IN  LAKES  BY  STREAMS  AND  BY  OTHER  AGENTS 133 

Mechanical  Deposits.  Chemical  Deposits.  Organic  Deposits  —  Diatoms. 
Marl.  Peat.  Playas.  Salt  Lakes.  Alkaline  Lakes.  Origin  of 
Rock  Salt.  Extinct  Lakes. 

CHAPTER  V 

THE  WORK  OF  GLACIERS 

GENERAL  CONSIDERATIONS I4I 

Distribution  and  Size  of  Glaciers.  Position  of  the  Snow  Line.  Formation 
of  Ice  in  Snow  Fields. 


CONTENTS  9 

PAGE 

MOUNTAIN  GLACIERS 143 

Formation.    Cirques.     Origin  of  Cirques.     Development  of  Cirques.     Fate 

of  Cirques.     Ablation. 

• 

SURFACE  OF  MOUNTAIN  GLACIERS     . •     .  '     .        .        .     147 

Irregularities  Due  to  Tension.     Irregularities  Due  to  Streams  and  Ice  Tables. 

MOVEMENT  OF  GLACIERS    .        .        . .      150 

Rate  of  Movement.  Differential  Movement  of  Glaciers.  Factors  In- 
fluencing the  Rate  of  Movement.  Lower  Limit  of  Glaciers. 

TRANSPORTATION  OF  MOLWTAIN  GLACIERS I54 

Surface  Moraines.     Subglacial  Material.     Englacial  Material. 

EROSION  BY  MOUNTAIN  GLACIERS 157 

Plucking  and  Abrasion.  Effect  on  the  Material  Carried.  Factors  In- 
fluencing the  Rate  of  Erosion. 

DEPOSITS  OF  MOUNTAIN  GLACIERS 159 

Terminal  Moraines.  Submarginal  Moraines.  Ground  Moraine.  The 
Work  of  Glacial  Streams. 

LANDSCAPE  MODIFIED  BY  GLACIAL  ACTION 163 

Characteristics  of  Glaciated  Valleys.  Mature  Glaciated  Valleys.  De- 
struction of  Features  of  Glaciated  Valleys.  Fiords. 

PIEDMONT  GLACIERS 167 

CONTINENTAL  ICE  SHEETS         *     _. 168 

Greenland.     The  Antarctic  Continent. 

ANCIENT  GLACIATION  .  171 

DEPOSITION  ..'..-.' 171 

Bowlders.  Unstratified  Drift.  Moraines.  Terminal  Moraines.  Moraines 

of  the  Last  Great   Ice  Sheet   in  North  America.     Ground  Moraine. 

Drumlins.      Stratified    Drift.     Outwash    Plains.      Terraces.      Deltas. 

Eskers.     Kames.     Relation  between  Stratified  and  Unstratified  Drift. 

EROSION  BY  CONTINENTAL  GLACIERS 182 

Effect  on  the  Underlying  Rock.  Modification  in  the  Shape  of  the  Hills. 
Effect  of  Glaciation  on  Drainage.  Lakes  and  Ponds.  Rivers. 

ICEBERGS     .  •  .       . 188 

Formation  of  Icebergs.     Size  and  Work  of  Icebergs. 

GLACIAL  MOVEMENT  .        . .        .        .189 

Viscosity  Theory.  Expansion  and  Contraction.  Regelation.  Melting  and 
Pressure.  Growth  of  Granules. 

CHAPTER  VI 
THE  OCEAN  AND  ITS  WORK 

GENERAL  CHARACTER  OF  THE  OCEAN 194 

Topography  of  the  Ocean  Floor.  Irregularities  of  the  Ocean  Floor.  Com- 
position of  Ocean  Water.  Temperature  of  the  Ocean.  Distribution 
of  Marine  Life.  Age  of  the  Ocean. 


I0  CONTENTS 

PAGE 

MOVEMENT  OF  THE  WATER  -. 198 

Wave  Motion.  The  Breaking  of  Waves.  Force  of  Storm  Waves.  Height 
of  Storm  Waves.  Tides.  Tidal  Currents.  Tidal  Bores.  Earthquake 
Waves.  Ocean  Currents. 

MARINE  EROSION 202 

Factors  in  Marine  Erosion.     Shore  Ice.     Ice  in  Lakes. 

RESULTS  OF  MARINE  EROSION 205 

Effect  of  Erosion  on  Different  Materials.  Influence  of  Joints  and  Other 
Planes  on  Erosion.  Coves  and  Headlands.  Sea  Caves  and  Blow- 
holes. Arches.  Stacks.  Marine  Terraces.  Striking  Examples  of 
Marine  Erosion.  Sea-captured  Streams.  Raised  Beaches.  Ancient 
Plains  of  Marine  Denudation.  The  New  England  Marine  Plain. 

TRANSPORTATION .    '    .       .       .       .217 

Littoral  or  Shore  Currents.     Tidal  Currents. 

FEATURES  RESULTING  FROM  TRANSPORTATION 218 

Beaches.  Bayhead  Beaches.  Bars  and  Spits.  Sand  Reefs  or  Barrier 
Beaches.  Tied  Islands.  Examples  of  the  Constructive  Work  of  the 
Sea. 

SHORES •     .     224 

Smooth  Shores.  Cuestas.  Rough  Shores.  Examples  of  Irregular  Coasts. 
Proofs  of  Elevation  and  Depression.  The  Stability  of  the  Atlantic 
Coast  of  North  America.  Cycle  of  Shore  Erosion. 

DEPOSITION  IN  SEAS  AND  LAKES 233 

Source  and  Extent  of  Land-derived  Sediments.  Stratification.  Cross  or 
False  Bedding. 

LITTORAL  DEPOSITS 235 

Extent.  Character  of  Littoral  Deposits.  Distinguishing  Characteristics 
of  Littoral  Deposits. 

SHOAL-WATER  DEPOSITS      .        .        ...        .       . 237 

Extent  and  Character  of  Deposits.  Limestone.  Lens-shaped  Sediments. 
Dovetailing  of  Sediments.  Basal  Conglomerates.  Subsidence  Neces- 
sary for  Great  Accumulations. 

DEEP-SEA  DEPOSITS    .        .       ..       . 241 

Blue  Mud.     Globigerina  Ooze.     Radiolarian  Ooze.     Red  Clay. 

CORAL  REEFS  AND  ISLANDS       ......       .       i       .       .        .        .        .        .243 

Coral-reef  Problem.  Subsidence  Theory  of  Darwin.  Submarine  Bank 
Theory  of  Murray  and  Others.  Change  in  Sea  Level  Due  to  Glaciation, 
or  the  Glacial-control  Theory. 

CONSOLIDATION  OF  SEDIMENTS ...  248 

Cementation.     Effect  of  Pressure.     Effect  of  Heat. 

CLASSIFICATION  OF  SEDIMENTARY  ROCKS  .        .       .....        .        .        .        .249 

Limestones.  Sandstones.  Shales.  Deposits  in  Lakes  and  Deserts. 
Influence  of  Sedimentary  Rocks  upon  Topography. 


CONTENTS  H 

CHAPTER  VII 
THE  STRUCTURE  OF  THE  EARTH 

PAGE 

STRUCTURAL  FEATURES  OF  ROCKS     .        .        .        .        .    •  •  ,       >•      .       .       .        .     252 
Dip  and  Strike.     Effect  of  Dip  and  Strike  upon  Outcrop. 

FOLDS         .        .        .        .   •    .        .        .        . .      254 

Effect  of  Folding  on  Competent  and  Incompetent  Strata.  How  the 
Structure  of  a  Region  is  Determined.  Origin  of  Folds.  Warping. 
Zones  of  Flow  and  Fracture. 

JOINTS .258 

Origin  of  Joints.     Effect  of  Joints  on  Topography. 

FAULTS ../..'.        .        .        .      261 

Normal  or  Gravity  Faults.  Examples  of  Normal  Faults.  Reverse  or 
Thrust  Faults.  Examples  of  Thrust  Faults.  Vertical  and  Hori- 
zontal Faults.  Influence  of  Faults  on  Topography.  Minor  Features 
of  a  Fault  Fracture.  Detection  of  Faults.  Origin  of  Faults. 
Rapidity  of  Fault  Movements. 

CONFORMITY  AND  UNCONFORMITY      .        .        .        .        ........       .270 

Importance  of  Unconformities.     Overlap. 

CONSTITUTION  OF  THE  EARTH'S  INTERIOR 272 

Zone  of  Variable  Temperature.     The  Interior  Heat  of  the  Earth. 

THEORIES  OF  THE  PHYSICAL  STATE  OF  THE  EARTH'S  INTERIOR 273 

Internal  Fluidity  Theory.  Solid  Interior.  Gaseous  Center.  Radioactivity 
and  a  Solid  Center.  Subcrust  Theory.  Summary. 

CHAPTER  VIII 

EARTHQUAKES 

EARTHQUAKES  AND  ATTENDING  FEATURES 275 

The  San  Francisco  Earthquake.  Distribution  of  Earthquakes.  Summary 
of  the  Causes  of  Earthquakes.  Displacements.  Depth  of  the  Plane 
or  Point  of  Origin.  Earthquake  Waves.  Amplitude  of  Vibration. 
Vorticose  and  Twisting  Movements.  Duration.  Frequency.  Areas 
Affected  by  Certain  Earthquakes.  Instruments  for  Determining  and 
Measuring  Earthquakes. 

EFFECTS  OF  EARTHQUAKES .  .        .        ..       .287 

Faults  and  Fissures.  Changes  in  Level.  Landslides.  Earthquake  Topog- 
raphy. Sounds.  Loss  of  Life.  Effect  on  Underground  Water.  Gases. 
Construction  of  Buildings  in  Earthquake  Regions.  Effect  of  Earth- 
quakes on  the  Sea.  Evidence  that  a  Region  has  been  Free  from  Severe 
Earthquakes. 

CHAPTER  IX 

VOLCANOES  AND  IGNEOUS  INTRUSIONS 

VOLCANOES ,        .....     •       .     294 

How  Volcanoes  Begin.     New  Volcanoes.     Classification  of  Volcanoes. 


12  CONTENTS 

PAGE 

MATERIALS  ERUPTED 295 

Gases.  Fragmental  Materials.  Lava.  Lava  Streams.  Effect  of  Compo- 
sition on  Fluidity.  Temperature.  Surface  of  Lava  Flows.  Velocity 
of  Lava  Flows.  Nature  of  Lavas. 

TYPES  OF  VOLCANOES '       ..   -    .       /       .        .        .     302 

The  Explosive  or  Fesuvian  Type:    Vesuvius.     Krakatao.      Katmai.     Mt. 

Pelee.     Bandai-san. 
The   Quiet   or   Hawaiian    Type:     Crater    of   Kilauea.     Eruptions.     Lava 

Streams.     Origin  of  Calderas.     Steep  Lava  Cones :   Volcanoes  of  the 

Chimborazo  Type. 
Fissure  Eruptions:  Recent  Icelandic  Lava  Sheets. 

CHARACTERISTICS  OF  VOLCANIC  CONES     .        .        .        .        .        .        ...        .     311 

Profiles  of  Volcanoes.  Shape  of  Craters.  Erosion  of  Volcanic  Cones. 
Necks  and  Plugs.  Age  of  Volcanoes  in  the  United  States. 

DISTRIBUTION  AND  NUMBER  OF  VOLCANOES 3I8 

Number  of  Volcanoes.  Distribution.  Cause  of  Distribution.  Ancient 
Volcanoes. 

IMPORTANCE  OF  VOLCANISM  TO  MAN 321 

Beneficial  Effects.     Harmful  Effects.     Volcanoes  and  Climate. 

SUBORDINATE  VOLCANIC  PHENOMENA 322 

Mud  Volcanoes.     Solfataras. 
INTRUSIVE  OR  PLUTONIC  ROCKS .        .324 

Injected  Masses:   Dikes.     Sills.     Laccoliths. 

Subjacent  Masses:  Stocks.     Batholiths.     Some  Effects  of  Intrusions. 
IGNEOUS  ROCKS 32Q 

Subdivisions  Depending  upon  Chemical  Composition.  Subdivisions  De- 
pending upon  Texture. 

CLASSIFICATION  OF  IGNEOUS  ROCKS  .  ,70 

•  •  •  OO^ 

Coarse-grained   Igneous    Rocks:      Granite.      Syenite.      Diorite.      Gabbro. 

Peridotite. 

Compact  or  Fine-grained  Igneous  Rocks:   Felsites.     Basalts. 
Glassy  Rocks:  Obsidian  or  Volcanic  Glass.     Pitchstone. 

FRAGMENTAL  VOLCANIC  ROCKS .        ...        .  332 

Tuff.     Volcanic  Breccia.     Columnar  Structure  of  Lava. 
AGE  OF  IGNEOUS  ROCKS    .       .       ...       .       ."       .       ...  334 

THEORIES  OF  VOLCANISM  .        ."./,.'        ...        .  -  334 

Theory  Based  upon  the  Assumption  that  the  Interior  is  Molten. 
Theories  Based  upon  the  Assumption  that  the  Earth  is  Solid:  Heat  by  Fric- 
tion.    Formation  of  Lava  Reservoirs  by  Relief  of  Pressure.     Liquid- 
thread  Theory. 
Abyssal  Injection  Hypothesis. 
RESUME'  OF  PRESENT  KNOWLEDGE  OF  VOLCANISM  ....  337 

Origin  of  Volcanic  Gases.  Cause  of  the  Ascension  of  Lava.  Cause  of 
Periodicity.  Influences  of  the  Atmosphere,  etc. 


CONTENTS  I3 

CHAPTER   X 
METAMORPHISM 

PAGB 

KINDS  OF  METAMORPHISM          .        .        .        .      „  -"•.•••'    .       .       .        .        .        .341 

Contact  Metamorphism.     Regional  Metamorphism. 
CLASSIFICATION  OF  METAMORPHIC  ROCKS         .        .        .       .'       .       ,       .     '-.  '     .     344 

Quartzite.     Marble.     Slate.     Schist.     Gneiss. 
SUMMARY  OF  CAUSES  OF  METAMORPHISM ,       ...     347 

Heat.     Moisture.     Pressure. 
ARRANGEMENT  OF  MINERALS     .        .  .        .        .        .       .       .       .        f       t     348 

Crystallization.  Granulation.  Relation  of  Cleavage  to  Pressure.  From 
Igneous,  through  Sedimentary,  to  Metamorphic  Rocks.  Weathering  of 
Metamorphic  Rocks.  Economic  Importance. 

CHAPTER  XI 

MOUNTAINS   AND   PLATEAUS 

CLASSIFICATION  OF  MOUNTAINS 352 

Mountains  of  Accumulation.  Residual  Mountains.  Fault  or  Block  Moun- 
tains. Laccolith  Mountains.  Domed  Mountains.  Complexly  Folded 
Mountains. 

ORIGIN  AND  DEVELOPMENT  OF  FOLDED  MOUNTAINS .     358 

Geosynclines.  Lateral  Pressure.  Experiments  in  Mountain  Building. 
Rate  of  Folding.  To  What  the  Topographic  Features  of  Folded  Moun- 
tains are  Due.  Cycle  of  Erosion  of  Mountains. 

THEORIES  OF  MOUNTAIN  BUILDING 364 

Cause  of  Lateral  Pressure.  The  Elevation  of  Plateaus  and  Mountains. 
The  Theory  of  Isostasy.  The  Distribution  of  Mountains.  Perma- 
nence of  Continents  and  Ocean  Basins.  Age  of  Mountains. 

CHAPTER  XII 

ORE  DEPOSITS 
CHARACTERISTICS  OF  DEPOSITS  .        ....        .        .        .       .        .       .       .  370 

Ores  in  Ready-made  Cavities.  Fissure  Deposits.  Form  and  Extent  of 
Veins.  Source  of  Vein  Material.  Cause  of  Precipitation.  Replace- 
ment Deposits.  Weathering  and  Concentration  of  Ores.  Magmatic 
Segregation.  Placer  Gold  Deposits.  Sedimentary  Iron  Deposits. 


PART   II.    HISTORICAL  GEOLOGY 

CHAPTER  XIII 
HISTORICAL   GEOLOGY 

FOSSILS       .        .        .        .        .        .        .  .        .        .        ...        ....     377 

The  Original  Substance  may  be  Preserved.  Replacement.  Casts  and 
Molds.  Footprints,  Trails,  etc.  Entombment  of  Plants  and  Animals. 
Imperfection  of  the  Record. 


I4  CONTENTS 

PAGE 

GEOLOGICAL  CHRONOLOGY 381 

Order  of  Superposition.  Chronology  Determined  by  Fossils.  Use  of 
Fossils  in  Determining  Physical  Conditions.  Difficulties  in  Corre- 
lating Strata. 

DIVISIONS  OF  GEOLOGICAL  TIME   .    . .-     .       .     383 


CHAPTER  XIV 
THE  EARTH  BEFORE  THE   CAMBRIAN 

THEORIES  OF  THE  EARTH'S  ORIGIN 385 

Nebular  Hypothesis.  Planetesimal  Hypothesis.  Nebular  and  Planetesimal 
Theories  Contrasted. 

PRE-CAMBRIAN  ERAS 388 

THE  ARCHEOZOIC  ERA •  389 

Distribution  of  the  Archaeozoic  Rocks.  Characteristics  of  Archaeozoic  Rocks. 
Thickness.  Causes  of  Metamorphism  and  Deformation.  Conditions 
during  the  Archaeozoic  Era.  Duration.  Bearing  upon  the  Theories 
of  the  Earth's  Origin. 

THE  PROTEROZOIC  ERA 393 

Archaeozoic  and  Proterozoic  Contrasted.  The  Proterozoic  in  Different 
Regions.  Iron  and  Copper  Deposits.  Life  of  the  Proterozoic  Era. 
Duration.  Climate.  Life  before  Fossils. 

CHAPTER  XV 

THE  CAMBRIAN  PERIOD 

THE  PALEOZOIC  ERA J-    .       .     401 

THE  CAMBRIAN  PERIOD .-.      .        .     402 

Divisions  of  the  Cambrian.  Location  of  Cambrian  Rocks.  Physical 
Geography  of  Ancient  Periods.  Basal  Unconformity.  Physical 
Geography  of  the  Cambrian.  Character  of  the  Cambrian  Rocks.  Pres- 
ent Condition  of  the  Sediments.  Volcanism.  Close  of  the  Cambrian. 
Other  Continents. 

LIFE  OF  THE  CAMBRIAN .  408 

PLANTS  ......       .       ......       .       .     409 

ANIMALS      .       ...    • .       .       »       .       .410 

Crustacea:  Trilobites.     Other  Crustaceans. 

Mollusc  a:  Gastropods. 

Molluscoidea:   Brachiopods. 

Echinodermata :   Cystoids. 

Worms. 

Coelenterata:  Corals.     Graptolites.     Jellyfish.     Sponges. 

Protozoa. 

SUMMARY 4l6 

Evolution  during  the  Cambrian.     Climate  and  Duration. 


CONTENTS  15 


CHAPTER  XVI 
THE  ORDOVICIAN  PERIOD 

PAGE 

ORDOVICIAN  PHYSICAL  GEOGRAPHY .        .       .        .-       .     418 

Close  of  the  Ordovician.     Cincinnati  Anticline.     Volcanism.     Ordovician 

of  Other  Continents. 
PETROLEUM  AND  NATURAL  GAS         .        .        .        .        .        .        .       ...        .     424 

Conditions  Favoring  the  Accumulation  of  Oil  and  Gas.     Origin  of  Oil  and 

Gas.     Life  of  Oil  Wells  and  Fields. 
LIFE  OF  THE  ORDOVICIAN  .        .        ••'•;* 427 

Protozoa. 

Ccelenterata:  Sponges.     Graptolites.     Stromatopora.     Corals. 

Echinodermata :  Cystoids.  Crinoids.  Blastoids,  Starfish,  Brittle  Stars,  and 
Sea  Urchins. 

Molluscoidea:   Brachiopods.     Bryozoa. 

Mollusca:   Pelecypods.     Gastropods.     Cephalopods. 

Crustacea:  Trilobites.     Other  Arthropods. 

Fishes. 
PLANTS        '..      V      "...    '.       V      '.     ."..       •        •        .        .        .      ......     436 

Seaweeds. 
SUMMARY 436 

Progress  and  Character  of  Ordovician  Life.  Climate  and  Duration  of  the 
Ordovician. 

CHAPTER  XVII 
THE  SILURIAN  PERIOD 

SILURIAN  PHYS  CAL  GEOGRAPHY 439 

Geography  of  the  Silurian.  Character  and  Thickness  of  the  Sediments. 
Clinton  Iron  Ore.  Deserts.  Origin  of  Rock  Salt.  Igneous  Rocks. 
Other  Continents. 

LIFE  OF  THE  SILURIAN 444 

Ccelenterata:  Corals.     Other  Coelenterates. 
Echinodermata:   Crinoids.     Cystoids. 
Molluscoidea:   Brachiopods.     Bryozoa. 
Mollusca:  Gastropods.     Pelecypods.     Cephalopods. 
Arthropoda:  Trilobites.     Eurypterids.     Scorpions. 
Fishes. 

SUMMARY 450 

Life  on  the  Land.     Migration.     Climate  and  Duration.     Close  of  the  Silurian. 

CHAPTER  XVIII 
THE  DEVONIAN  PERIOD 

DEVONIAN  PHYSICAL  GEOGRAPHY 452 

Subdivisions  of  the  Devonian.     Geography.     The  Devonian  in  New  York. 
Continent  of  Appalachia.     Igneous  Rocks.     Devonian  Oil  and  Gas. 
Devonian  of  Other  Continents. 
LIFE  OF  THE  DEVONIAN     .        .     \  ...-   :.,..'..        .        ...»••       •        .     456 

Ccelenterata:  Corals. 

CLELAND    GEOL. 2 


CONTENTS 


Echinodermata:  Crinoids.     Blastoids. 

Molluscoidea  and  Mollusca:  Brachiopods.  Bryozoans.  Pelecypods.  Gas- 
tropods. Cephalopods. 

Arthropoda:  Trilobites.     Barnacles.     Eurypterids.     Insects. 

Fishes:  Ostracoderms.     Sharks.      Lungfish.     Ganoids.     Teleosts  or  Bony 
Fish.     Comparison  of  Devonian  and  Modern  Fish.     Why  the  Verte- 
brate Type  was  "  Fit." 
PLANTS         .        .        .        .        .        .        .        .        .        «        •"      .        .        .        .        .467 

SUMMARY     .        .        .       •.       • ..-.-..       .        .     467 

Migration  and  Evolution.     Climate  and  Duration. 

CHAPTER  XIX 

THE   CARBONIFEROUS  PERIODS 
MISSISSIPPIAN  OR  LOWER  CARBONIFEROUS        .        .        ......       .        .        .469 

Close  of  the  Mississippian.     Other  Continents. 

PENNSYLVANIAN  OR  UPPER  CARBONIFEROUS 472 

COAL  FIELDS  OF  NORTH  AMERICA 473 

Productive  Coal  Fields :  Eastern  Canadian  and  New  England  Fields. 
Appalachian  Field.  Michigan  Coal  Field.  The  Indiana-Illinois 
Field.  The  lowa-Missouri-Texas  Field. 

SUMMARY  OF  THE  PENNSYLVANIAN     .        .        . 475 

Iron  and  Oil.     Duration.     Other  Continents. 

PERMIAN 476 

Permian  Glaciation.  Permian  Deserts.  Igneous  Activity.  Appalachian 
Deformation.  Age  of  the  Deformation.  Other  Continents. 

INVERTEBRATES  OF  THE  CARBONIFEROUS .        .      480 

Protozoans.     Coelenterates     and    Echinoderms.     Molluscoids.     Mollusks. 

Arthropods.     Insects. 

VERTEBRATES  OF  THE  CARBONIFEROUS      .        .       ..  .  ,    »  .        .        .     485 

Fishes.  Amphibians.  Origin  of  Amphibians.  Rise  of  Amphibians. 
Reptiles.  Rise  of  Reptiles. 

CARBONIFEROUS  PLANTS 491 

Ancestral  Ferns  and  Seed  Ferns.  Club  Mosses  (Lycopods).  Sphen- 
ophylls.  Horsetails  (Calamites).  Cordaites  and  Other  Gymnosperms. 
Conditions  under  which  the  Coal  Plants  Grew. 

COAL  .  .  .  .  •.  ;  .  ;  ;  .  .  :  -.  .  .  .  .  499 
Mode  of  Occurrence.  Origin  of  Coal.  Necessary  Conditions  for  Coal 
Formation.  How  Vegetable  Tissue  Accumulated.  How  it  was  Kept 
from  Decay.  How  it  was  Changed  to  Coal  and  What  Varieties 
Resulted.  Conditions  Favoring  Coal  Formation  in  the  Pennsylvanian. 
Extent  and  Structure  of  Coal  Beds.  Climate  during  the  Deposition 
of  Coal. 

PROBLEMS  OF  THE  PERMIAN      .        . 504 

SUMMARY  OF  THE  PALEOZOIC  ERA    .        .        .        .     .  .       .  •     .       .  -     .       .  •      .     505 

The  Building  of  the  Continents.  Evolution  and  Extinction  of  Life.  Cli- 
mate. 


CONTENTS  17 


CHAPTER  XX 
THE  MESOZOIC  ERA:    THE  AGE  OF  REPTILES 

PAGE 

SUBDIVISIONS  OF  THE  MESOZOIC        .        .        .    .    .-       .       .       .       .       .       .        .     508 

PHYSICAL  GEOGRAPHY  DURING  THE  MESOZOIC  .        .       ..       ..       .       ,.       .       .        .508 

TRIASSIC .....        .     508 

Atlantic  and  Gulf  Coasts.  Western  Interior.  Pacific  Coast.  Triassic 
in  Other  Continents. 

JURASSIC 512 

Atlantic  and  Gulf  Coasts.  Western  Interior.  Mountain  Forming  in  the 
West.  Jurassic  of  Other  Continents. 

LOWER  CRETACEOUS  (COMANCHEAN) .     514 

Atlantic  and  Gulf  Coasts.     Western  Interior.     Pacific  Coast.     Lower  Creta- 
ceous of  Other  Continents. 
UPPER  CRETACEOUS  (CRETACEOUS)    .        .        .        .        '.        .        .,,  .......        .        .     SI6 

Atlantic  and  Gulf  Coasts.  Pacific  Coast.  Western  Interior.  Upper 
Cretaceous  of  Other  Continents.  The  Cretaceous  Peneplain.  Moun- 
tain-making Movements  at  the  Close  of  the  Mesozoic.  Duration  of 
the  Mesozoic. 

LIFE  OF  THE  MESOZOIC 521 

Comparison  of  the  Life  of  the  Paleozoic  and  the  Mesozoic.     Plan  of  Study. 

INVERTEBRATES   .        ... 523 

Chalk.  Sponges.  Corals.  Crinoids.  Sea  Urchins  (Echinoids).  Starfish. 
Brachiopods.  Pelecypods.  Gastropods.  Cephalopods.  Ammonites. 
Naked  Cephalopods  (Belemnites).  Crustaceans.  Insects. 

FISHES  AND  AMPHIBIANS     . 533 

REPTILES 536 

Reptiles  with  Mammalian  Characters. 

DINOSAURS 539 

Carnivorous  Dinosaurs.  Unarmored  Quadrupedal  Dinosaurs  (Sauropoda). 
Unarmored  Bipedal  Herbivorous  Dinosaurs  (Unarmored  Predentata). 
Armored  Dinosaurs  (Armored  Predentata).  Summary  of  Dinosaurs. 
Migration  and  Extinction  of  Dinosaurs.  Size  as  a  Factor  in  Ex- 
tinction. 

CROCODILES          .        ....". 552 

MARINE  REPTILES 552 

Ichthyosaurus.     Plesiosaurus.      Mosasaurus-(Sea  Lizards).     Turtles. 

FLYING  REPTILES  (PTEROSAURS)        . 558 

TOOTHED  BIRDS .560 

Archaeopteryx.     Hesperornis.     Ichthyornis. 

MAMMALS     .        .        ...      '.      '. .'      .     563 

PLANTS '.'      '."      .      565 

Horsetails.     Cycads.     Ferns.     Gymnosperms.     Angiosperms. 

CLIMATE      .        .        .        .       •.       '.   .    • •    .  .      569 

Triassic.     Jurassic.     Cretaceous. 

COAL   .        .       -.        .        .       ..       •„      • .        .571 

Triassic.     Cretaceous. 


!8  CONTENTS 

CHAPTER  XXI 
CENOZOIC  ERA:  THE  AGE  OF  MAMMALS.    TERTIARY  PERIOD 

COMPARISON  OF  THE  LIFE  AT  THE  CLOSE  OF  THE  MESOZOIC  AND  THE  BEGINNING  OF 

THE  CENOZOIC     .  57* 

Subdivisions  of  the  Cenozoic  Era. 

PHYSICAL  GEOGRAPHY  OF  THE  TERTIARY 574 

EOCENE '    ....     574 

Atlantic  and  Gulf  Coasts.  Pacific  Coast.  Western  Interior.  Eocene  of 
Other  Continents. 

OLIGOCENE 577 

Atlantic  and  Gulf  Coasts.  Western  Interior.  Pacific  Coast.  Oligocene  of 
Other  Continents. 

MIOCENE 579 

Atlantic  and  Gulf  Coasts.  Economic  Products  of  the  Miocene.  Western 
Interior.  Pacific  Coast.  Mountain  Building.  Basis  for  Separation 
into  Periods.  Igneous  Activity.  Miocene  of  Other  Continents. 

PLIOCENE 585 

Atlantic  and  Gulf  Coasts.  Western  Interior.  Pacific  Coast.  Pliocene 
Elevation.  High  Plains  and  Bad  Lands.  Pliocene  of  Other 
Continents. 

LIFE  OF  THE  TERTIARY 590 

Rise  of  Mammals.  Archaic  Mammals  of  Ancient  Ancestry.  Amblypoda. 
Ancestors  of  the  Carnivores.  Marine  Mammals.  Zeuglodon.  An- 
cestors of  Existing  Whales.  Ancestors  of  the  Hoofed  Mammals 
(Ungulates).  Divergence  of  the  Even  and  Odd-toed  Hoofed  Mammals 
(Ungulates). 

FACTORS  IN  THE  EVOLUTION  OF  MAMMALS 600 

Mammalian  Teeth.     Feet.     Limits  to  Evolution. 
ODD-TOED  MAMMALS  (PERISSIDACTYLS) 603 

Titanotheres.    Rhinoceroses.    Tapirs.    Horses.    Summary  of  the  Evolution 
of  the  Horse.     Probable  Cause  of  the  Evolution  of  the  Horse.     Cause 
of  the  Extinction  of  the  Horse  in  North  America.     Elephants.     Sum- 
mary of  the  Evolution  of  the  Elephant. 
EVEN-TOED  MAMMALS  (ARTIODACTYLS)       .       .       ...»       *       .       .       .615 

Camels.     Deer.     Cattle,  Sheep,  and  Goats.     Swine  and  Related  Animals. 

A  Climbing  Ungulate. 

INSECTIVORES       .        .        .        .  . 620 

RODENTS  (GNAWING  ANIMALS)  .       .       .       .       .       .       . 620 

EDENTATES 621 

TRUE  CARNIVORES       .        . .  .        .622 

PRIMATES  (MONKEYS,  APES,  LEMURS) 622 

BlR°s    .       .       .       .      |      .      '.      '.       . 623 

REPTILES  AND  AMPHIBIANS        ..' 624 

FlSHES 625 


CONTENTS  I9 

PAGE 

INVERTEBRATES    .       ...       •.,.,«      ..• 626 

Insects.     Horseflies,  Tsetse  Flies,  and  Ants. 

VEGETATION 630 

Grasses.  Daemonhelix.  Geological  History  of  Sequoias.  Diatoms.  Ex- 
ceptional Preservation  of  Plants.  Plant  Localities  in  North  America. 

CLIMATE 634 

Difficulty  in  Determining  Tertiary  Climates.  Eocene.  Oligocene.  Mio- 
cene. Pliocene. 

EFFECTS  OF  ISOLATION  AND  MIGRATION 636 

Eocene  Invasion.  Oligocene  Invasion.  Miocene  African  Invasion.  Plio- 
cene South  American  Invasion  and  Intermigration  between  the  Old  and 
New  Worlds.  Duration  of  the  Tertiary. 

CHAPTER  XXII 

QUATERNARY 

CHANGES  AT  THE  CLOSE  OF  THE  TERTIARY 643 

Elevation.     Glaciation.     Changes  in  Life. 

DISTRIBUTION  OF  THE  ICE  SHEETS 645 

Other  Continents.     North  America. 

DEVELOPMENT  OF  THE  ICE  SHEETS .     647 

Thickness  of  Ice  Sheets  at  Center. 

GLACIAL  AND  INTERGLACIAL  STAGES 648 

Characteristics  of  Former  Drift  Sheets. 

HISTORY  OF  THE  GREAT  LAKES 651 

Preglacial  Drainage.  Origin  of  the  Basins.  Great  Lakes  Stages.  The 
Champlain  Subsidence. 

OTHER  PLEISTOCENE  LAKES 656 

Lake  Agassiz.     Lake  Bascom.     Great  Basin  Lakes. 

LOESS   .       %      . 657 

DURATION     .-.?•.-; 658 

CAUSES  OF  GLACIATION     '•'.-''--.•.       .       .  .        . 660 

Elevation.     Astronomical.     Atmospheric  Hypothesis. 

EFFECTS  OF  GLACIATION 662 

LIFE  OF  THE  PLEISTOCENE 663 

Interglacial  Deposits.  North  and  South  Migrations  during  Glacial  and 
Interglacial  Times.  Deposits  beyond  the  Ice  Sheets  or  Protected  from 
Them.  Deposits  on  the  Last  Drift.  Vegetation.  Mammoths  and 
Mastodons.  Edentates.  Pleistocene  Carnivores.  Horses,  Camels, 
etc.  Birds. 

PREHISTORIC  MAN 674 

Eolithic.      Paleolithic  Man.     Neolithic  Man.      Man  in  North  America. 

Birthplace  of  Man.     Effect  of  the  Advent  of  Man. 
FUTURE  HABITABILITY  OF  THE  EARTH 683 

APPENDIX 
COMMON  MINERALS    .  685 


INTRODUCTION 

GEOLOGY  is  the  science  that  treats  of  the  earth  and  its  inhabitants 
as  revealed  in  the  rocks,  and  therefore  deals  with  its  constitution  and 
structure,  with  the  operation  of  the  forces  which  led  to  its  present 
condition,  and  with  the  occurrence  and  evolution  of  its  life.  In  the 
search  for  this  knowledge  it  calls  to  its  aid  astronomy,  chemistry, 
physics,  and  biology.  Geology  is,  in  fact,  a  composite  science,  making 
use  of  the  physical  sciences  in  unrolling  the  complicated  history  and 
structure  of  this  planet. 

Because  of  the  breadth  of  its  scope,  geology  has  been  divided  into 
a  number  of  branches  which  are,  however,  in  such  a  large  measure 
interdependent,  that  a  general  knowledge  of  all  is  often  essential  to 
a  thorough  understanding  of  any  one. 

Astronomic  or  Cosmic  Geology.  —  Since  the  earth  is  one  of  the 
planets  of  the  solar  system,  all  theories  of  its  origin  must  at  the  same 
time  consider  the  origin  of  the  other  planets,  and  vice  versa.  Con- 
sequently, astronomy  and  geology  are  dependent  upon  one  another 
in  all  attempts  at  determining  the  genesis  of  the  earth. 

Structural  Geology  is  a  study  of  the  materials  of  which  the  earth 
is  built  and  their  arrangement,  and  is  especially  concerned  with  the 
interpretation  of  the  structures  produced  in  the  rocks  by  earth 
movements.  The  branch  of  geology  which  investigates  minerals  is 
mineralogy,  and  that  which  deals  with  rocks  is  petrology  (Greek, 
petros,  rock,  and  logos,  discourse).  Both  mineralogy  and  petrology 
are  closely  allied  to  chemistry  and  optical  physics. 

Dynamical  Geology  is  a  study  of  the  agencies  that  have  produced 
geological  changes,  together  with  their  laws  and  modes  of  operation. 
Among  the  most  important  forces  considered  are  water  in  motion, 
wind,  glaciers,  igneous  activity,  and  earth  movements  resulting  from 
strains.  This  branch  of  the  subject  is  closely  related  to  physio- 
graphic geology,  since  the  latter  deals  with  the  evolution  of  the 
topography  of  the  earth's  surface  and  with  the  forces  which  have 
produced  it.  A  study  of  physiographic  geology  is  necessary  to  an 
interpretation  of  land  surfaces. 

21 


22 


INTRODUCTION 


Industrial  Geology  includes  mining  and  economic  geology  and  is  the 
commercial  application  of  geological  principles. 

All  of  the  above  subjects  are  included  in  Part  I  of  this  volume,  under 
the  head  of  Physical  Geology. 

Historical  Geology  includes  paleontology  (Greek,  palaios,  ancient, 
ontos,  living  being,  and  logos,  discourse),  or  the  study  of  the  life  of  the 
past  as  shown  by  its  fossil  remains ;  or,  in  other  words,  fossil  botany 
and  zoology.  It  also  embraces  p ale '0 geography  (the  geography  of  pre- 
historic lands),  which  is  concerned  with  the  boundaries  of  the  lands 
and  seas  of  the  epochs  and  periods  of  the  past,  and  with  the  evolution 
of  the  continents.  It  also  includes  stratigraphy,  or  the  arrangement 
and  succession  of  the  strata,  as  indicated  mainly  by  fossils.  His- 
torical geology  calls  to  its  aid  all  other  branches  of  geology,  in  order 
that  the  topography  of  the  ancient  land  surfaces  and  the  boundaries 
of  the  lands  and  seas  may  be  known,  and  that  the  climates  to  which  the 
earth  was  subjected  may  be  determined.  Such  an  exhaustive  study 
is  necessary,  since  the  causes  of  the  rapid  extinction  of  certain  forms 
of  life,  and  of  the  sudden  appearance  and  evolution  of  others,  cannot 
be  known  with  certainty  until  the  environment  under  which  they 
lived  is  learned. 

In  general,  it  may  be  said  that  Historical  Geology  deals  with  the 
evolution  of  the  continents  and  of  the  life  of  the  past. 

Length  of  Geological  Time.  —  Without  an  appreciation  of  the 
vastness  of  geological  time  (p.  417)  as  compared  with  the  brief  span 
of  a  man's  life,  the  work  accomplished  by  the  various  geological 
agents  cannot  be  understood.  This  conception  of  the  length  of 
geological  time  can,  perhaps,  best  be  grasped  by  a  comparison: 
"  Let  a  year  be  represented  by  a  foot ;  the  average  length  of  a  human 
life  is  then  measured  by  the  breadth  of  a  dwelling  house,  and  human 
history  is  limited  approximately  to  a  mile;  but  the  duration  of 
geologic  time  is  comparable  to  the  circumference  of  the  globe." 

Present  Status  of  Geology.  —  Much  of  the  science  of  geology  is 
definitely  known  and  has  been  learned  as  a  result  of  the  accurate 
observation  and  careful  reasoning  of  many  geologists.  It  should, 
however,  be  borne  in  mind  that  many  of  the  theories  are  subject  to 
change,  as  will  be  pointed  out  from  time  to  time  in  the  following 
chapters.  This  is  due  to  the  fact  that  geology  deals  with  many 
problems  concerning  which  our  knowledge  is  as  yet  incomplete, 
notwithstanding  careful  observations  and  deductions.  For  example, 
before  it  was  known  that  the  crust  of  the  earth  is  heated,  to  some 


INTRODUCTION  23 

extent,  as  a  result  of  the  radioactivity  of  certain  minerals,  a  correct 
theory  of  the  earth's  interior  was  impossible.  The  true  theory  has 
probably  not  yet  been  found,  but  every  advance  in  knowledge  brings 
the  solution  nearer.  The  modification  of  geological  theories  from  time 
to  time  should  not  be  a  source  of  annoyance  to  the  student,  but 
should  rather  serve  to  stimulate  him  to  reason  for  himself. 

Fundamental  Terms.  —  There  are  a  few  terms  with  which  the 
student  must  become  familiar  before  a  discussion  of  the  subjects 
taken  up  in  the  following  chapters  can  be  understood.  Of  these 
terms  only  very  elementary  definitions  will  be  given  in  this  place, 
since  more  complete  explanations  will  be  taken  up  later. 

Rocks.  — With  the  exception  of  a  comparatively  thin  layer  of  soil, 
which  varies  greatly  in  thickness  and  is  entirely  absent  in  some 
places,  the  earth  is  composed  of  rock  which  extends  from  the  surface 
downward  for  many  miles  (the  lithosphere),  and  probably  through 
the  central  core  (the  centrosphere).  In  general,  the  rocks  of  the 
earth's  crust  can  be  classified  according  to  their  origin  as  of  three 
kinds  :  (i)  sedimentary,  (2)  igneous,  and  (3)  metamorphic. 

Sedimentary  Rocks.  —  If  one  examines  the  sediment  deposited  by 
a  muddy  rivulet  in  a  temporary  pool  of  water,  he  will  find  that  it 
consists  of  sand  or  clay,  and  that  it  is  in  layers.  This  deposition 


FIG.    i.  — Niagara  limestone,  showing  well-bedded  layers  with  two  sets  of  strong 
joints  at  right  angles  to  each  other.     (U,  S,  Geol.  Surv.) 


24  INTRODUCTION 

represents  on  a  minute  scale  what  is  occurring  in  the  lakes  and  oceans 
of  the  earth.  In  the  pool  the  deposit  may  be  only  a  fraction  of  an 
inch  thick,  while  off  the  shores  of  the  ocean  it  may  be  many  thou- 
sands of  feet  in  depth.  When,  as  has  often  occurred  in  the  past, 
such  an  enormously  thick  deposit  has  been  raised  above  the  sea  and 
streams  have  cut  deep  valleys  into  it,  it  is  seen  to  be  made  up  of  layers, 
and  the  rocks  composing  it  are  consequently  called  stratified  (p.  233) 
or  layered  rocks  (Fig.  i).  The  planes  which  separate  the  layers  from 
one  another  are  called  bedding  planes  or  planes  of  stratification.  If  the 
rock  is  made  of  hardened  mud  it  is  called  shale,  and  is  usually  divided 
into  many  thin  layers  or  lamina,  the  laminae  being  separated  by 
planes  of  bedding.  If  the  rock  is  composed  of  sand  whose  grains  are 
cemented  together  by  lime  or  other  substances,  it  is  called  a  sandstone. 
Sandstone  is  also  stratified,  but  the  bedding  planes  are  usually  farther 
apart  than  in  shale.  Sedimentary  rocks  are  not  always  in  the 
horizontal  position  in  which  they  were  deposited,  but  are  often  folded 
and  tilted. 

Igneous  Rocks.  —  Although  few  persons  have  seen  lava  flowing 
from  a  volcano,  many  have  seen  the  molten  slag  or  glass  of  blast 
furnaces,  which  bears  a  resemblance  to  lava  after  hardening,  and  is,  in 
fact,  not  unlike  the  lava  of  some  volcanoes,  both  in  composition  and 
structure.  Lava  (p.  298)  is  an  igneous  rock  (Latin,  ignis,  fire) ; 
that  is,  a  rock  which  has  been  in  a  molten  condition.  The  majority 
of  igneous  rocks,  however,  are  not  glassy,  but  are  composed  of  dis- 
tinct grains  or  crystals.  This  crystalline  structure,  as  we  shall  learn 
(p.  302),  is  brought  about  when  molten  rock  cools  so  slowly  that 
time  is  given  for  crystals  to  form.  An  igneous  rock  is,  therefore,  one 
which  solidified  from  a  state  of  fusion ;  it  is  either  glassy  or  grained 
(crystalline). 

It  is  apparent,  therefore,  that  igneous  rocks  differ  from  sedimentary 
in  a  number  of  particulars  ;  the  former  are  either  glassy  or  crystalline 
and  are  devoid  of  stratification  planes,  while  sedimentary  rocks  are 
seldom  crystalline  and  are  arranged  in  layers. 

Granite  is  a  typical  crystalline,  igneous  rock  (Fig.  5,  p.  29)  and  is 
composed,  usually,  of  three  minerals,  the  most  conspicuous  of 
which  is  feldspar.  These  feldspar  grains  or  crystals  are  opaque,  and 
white,  pink,  or  gray  in  color.  Mica,  when  present  in  granite,  is  usually 
easily  recognizable  by  its  glistening  leaves  which  split  into  elastic 
scales.  The  third  conspicuous  mineral  of  granite  is  quartz.  It  is 
usually  colorless,  has  the  appearance  of  broken  glass,  and  is  harder 


INTRODUCTION 


than  steel.  A  crystalline  igneous  rock  is,  therefore,  made  up  of  a 
number  of  minerals  differing  in  color,  in  hardness,  and  in  chemical 
composition.  The  importance  of  this  character  will  be  seen  when  the 
effect  of  the  weather  upon  rocks  is  studied. 

Metamorphic  Rocks  are  those  which  have  been  more  or  less  pro- 
foundly changed  from  their  original  condition  by  heat  and  pressure, 
and    are   usually   crystalline   in   texture.     Most   metamorphic   rocks 
possess     a     cleavage     which 
causes  them  to  break  easily 
in  one  direction.     They  are 
derived    both    from    igneous 
and  from  sedimentary  rocks. 
Metamorphic      rocks      have 
parting  planes  like  sedimen- 
tary rocks,  and  a  crystalline 
structure  like  igneous  rocks. 

Divisional  Planes.  —  All 
rocks  are  more  or  less  broken 
by  planes  which  separate 
them  into  blocks.  An  ex- 
amination of  a  sandstone  or 
limestone  quarry  will  show 
that,  in  addition  to  the  bed- 
ding planes,  the  rock  is 
broken  by  two  or  more  sets 
of  fissures  which  run  at  right 
angles  to  the  bedding.  These 
are  called  joints  (Fig.  i). 
Joints  occur  also  in  igneous 
rocks,  some  often  being  ap- 
proximately horizontal  and  others  tending  towards  the  vertical. 

When  beds  are  displaced  along  a  joint  or  other  crack  so  that  the 
strata  on  the  opposite  sides  of  it  do  not  match,  the  beds  are  said  to  be 
faulted  (p.  261)  (Fig.  2). 


FIG.  2.  —  A  fault.     The  thin-bedded   band 
was  once  continuous.     (U.  S.  Geol.  Surv.) 


PLATE  II.  —  Pyramid  Lake,  Nevada.  A  lake  resting  on  the  floor  of  a  steep-sided 
cirque.  Lakes  of  this  origin  are  common  in  the  high  mountains  of  the  United  States 
and  Canada,  where  glaciers  formerly  existed. 

26 


PART    I.     PHYSICAL   GEOLOGY 

CHAPTER  I 
WEATHERING 

NOTHING  endures.  The  most  indestructible  rock  will,  in  time,  dis- 
integrate ;  the  mountain  peaks  will  crumble  away,  and  the  rough 
places  will  be  made  smooth.  The  forces  which  produce  these  results 
are  calleH  the  agents  of  weathering.  They  vary  in  their  effectiveness 
in  different  places  and  at  different  times  in  the  same  place,  but  under 
all  conditions  and  at  all  times  some  agent  is  at  work,  reducing  the 
exposed  rock  to  soil.  The  rate  at  which  rock  weathers  depends 
largely  upon  two  factors:  (i)  the  composition  and  structure  of  the 
rock,  and  (2)  the  physical  conditions  to  which  it  is  exposed.  A 
sandstone  (p.  36)  in  which  the  grains  are  held  loosely  together  will 
disintegrate  rapidly,  while  another,  in  which  the  cementing  material 
is  insoluble  and  abundant,  may  have  a  long  life.  The  effect  of  dif- 
ferent physical  conditions  is  obvious.  In  the  dry  regions  of  Mexico 
and  Arizona  churches  and  houses  built  of  sun-dried  brick  (adobe) 
have  lasted  for  several  centuries ;  houses  made  of  a  similar  material 
would,  in  New  England,  crumble  to  a  mound  of  clay  in  the  course  of 
a  few  years. 

MECHANICAL  AGENCIES 

i.  Frost.  — The  property  possessed  by  water  of  expanding  upon 
freezing  is  of  great  importance  in  the  disintegration  of  rocks  in 
regions  where  the  temperature  falls  below  the  freezing  point,  since 
upon  freezing  it  expands  one  tenth  and  exerts  the  enormous  pressure 
of  150  tons  to  the  square  foot.  This  force  is  well  illustrated  in 
the  bursting  of  water  pipes  in  which  water  has  frozen.  It  is  stated 
that  in  Finland  freezing  water  is  sometimes  used  instead  of  powder, 
and  blocks  of  stone  of  400  tons'  weight  are  broken  out  in  this  way. 

All  rocks,  even  the  most  dense,  contain  pores  and  fissures  in  which 
water  may  accumulate.  Certain  sandstones  when  weighed,  then 

27 


PHYSICAL  GEOLOGY 


FIG.  3.  —  Sandstone  ''set  on  edge,"  showing  the 
scaling  of  the  laminae  by  frost. 


soaked  in  water  for 
twenty-four  hours  and 
again  weighed,  are  found 
to  have  gained  one  eighth 
in  weight.  This  fact 
shows  not  only  that  pores 
are  present,  but  also  that 
they  have  become  filled 
with  water.  If  such  a 
rock,  with  its  pores  full 
of  water,  is  frozen  re- 
peatedly, its  grains  will 
be  forced  apart  until  it 
finally  falls  to  pieces. 
This  test  is  used  in  labo- 


ratories to  determine  the  desirability  of  building  stone  in  temperate 
regions.  The  complete  disintegration  of  the  rock  is  not  attained  by 
one  freezing  and  thawing,  but  if  the  process  is  repeated  many  times, 
the  rock  may  be  completely  re- 
duced to  sand,  as  the  water 
penetrates  farther  into  the  rock 
each  time  it  thaws.  It  is 
readily  seen  that  this  wedge 
work  of  ice  is  usually  more  im- 
portant in  moist,  temperate 
regions  in  early  and  late  winter. 
The  obelisk  which  now  stands 
in  Central  Park,  City  of  New 
York,  stood  for  many  centuries 
in  Egypt  without  apparent 
injury  (although  undoubtedly 
weakened  by  the  extremes  of 
daily  temperature  to  which  it 
had  been  so  long  subjected). 
But  after  one  year's  exposure 
to  the  moist,  changeable  climate 
of  its  new  home,  the  hiero- 
glyphics near  its  base  became 


almost    illegible,    and     it    was 
found    necessary    to    coat    the 


FIG.  4.  —  Much-fractured  limestone  with 
strong  bedding  planes,  illustrating  the  con- 
ditions favorable  for  the  wedge  work  of 
frost  and  roots.  The  beds  are  bent  into  a 
low  anticline. 


WEATHERING 


29 


monument  with  paraffine  dissolved  in  turpentine  to  prevent  further 
decay.  Few,  if  any,  tombstones  in  New  England  which  are  over 
one  hundred  years  old  show  a  polish.  A  marble  slab  at  North 
Adams,  Massachusetts,  for  example,  upon  which  an  inscription  was 
chiseled  in  1865  was  practically  illegible  in  1905  and  had  to  be  recut. 
Such  rapid  weathering  of  marble  as  this  is,  however,  unusual, 
Tombstones  of  red  sandstone  which  were  erected  with  the  bedding 


FIG.  5.  —  Granite  broken  by  two  sets  of  joints.     Animas  River  Canyon,  Colorado. 

(U.  S.  Geol.  Surv.) 

planes  l  on  edge  have  suffered  severely,  because  the  rain  water  has 
soaked  down  the  more  porous  layers,  and  upon  freezing  has  forced 
off  sheets  of  the  rock  (Fig.  3).  Exposed  rock  surfaces  in  polar  regions 
are  pulverized  by  frost,  producing  sand,  and  fine,  dusty  material 
which  is  shifted  by  the  winds. 

Talus.2  —  The   most  conspicuous  effect  of  frost  is  seen  in  rock 
masses  which  are  much  broken  by  cracks  and  joints  (p.  258)  (Figs.  4,  5). 

1  When  rock  is  arranged  in  layers  it  is  said  to  be  bedded,  and  the  planes  separating  the  layers 
are  called  bedding  planes.     For  a  discussion  of  such  rocks  (stratified  rocks)  the  student  is  re- 
ferred to  page  233. 

2  Talus  slopes  produced  in  arid  regions  by  changes  in  daily  temperatures  are  also  important. 


30  PHYSICAL  GEOLOGY 

In  such  cases,  blocks  are  forced  from  cliffs,  building  up  slopes  of  loose 
fragments  at  their  base,  called  talus.  The  formation  of  talus  slopes 
(Fig.  6)  can  best  be  studied  in  the  early  spring,  when  fragments  of  the 
rock,  loosened  as  the  ice  in  the  cracks  melts,  fall  from  the  cliffs. 
These  fragments  are  carried  down  by  their  weight  until  the  declivity 
is  too  feeble  for  them  to  roll  farther.  When  they  come  to  rest  they 
accumulate  to  form  a  slope,  usually  steep,  whose  angle  is  called  "  the 
angle  of  repose."  The  slope  of  talus  (Fig.  14,  p.  39)  varies  from  26  to 
43  degrees,  the  angle  depending  upon  the  size  of  the  fragments  and 
upon  their  shape.  If  the  fragments  are  angular,  the  slope  will  be 


FIG.  6.  —  Diagram  showing  the  formation  of  a  talus  slope  and  the  destruction 
of  a  cliff.  The  successive  faces  of  the  cliff  A,  B,  C,  D  and  of  the  talus  A,  By  C,  D  are 
indicated  by  dotted  lines. 

steeper  than  if  they  are  rounded,  since  with  the  former  an  early  lodg- 
ment is  more  likely.  The  largest  blocks  accumulate  at  the  foot  of  the 
talus  slope,  the  size  diminishing  regularly  to  the  top.  If  talus  accu- 
mulates under  water  the  slope  will  be  steeper,  since  the  fragments 
are,  to  some  extent,  buoyed  up  by  the  water. 

It  is  evident  that  the  alternate  freezing  and  thawing  of  water  in  the 
pores  and  joints  of  a  rock  may  bring  about  its  complete  disintegration 
unless  it  is  protected  by  the  soil  of  its  own  making.  It  should  be 
remembered,  however,  that  this  agent  seldom  acts  alone,  but  usually 
serves  as  an  aid  to  the  chemical  agencies  of  weathering  (p.  35),  by 
breaking  up  the  rock  into  small  fragments  and  thus  furnishing  a  larger 
surface  upon  which  the  latter  may  work. 

Rock  Glaciers.  —  A  striking  example  of  the  above  is  shown  in  the 
formation  under  favorable  conditions  of  rock  glaciers  or  "  stone 
rivers  "  (Fig.  7),  many  hundred  feet  in  length  in  regions  of  severe' 
cold,  when  great  masses  of  talus  are  slowly  moved  some  distance  down 
a  valley,  producing  the  appearance  of  a  glacier.  This  is  accomplished 
by  the  alternate  freezing  and  thawing  of  the  water  in  the  interstices 
of  the  talus. 


WEATHERING 


Creep  of  Soils.  —  Another  important  result  of  frost  action  is  the 
creep  of  soils  on  slopes.  If  soil  contains  water,  each  freezing  slightly 
raises  the  fragments 
at  right  angles  to  the 
surface  of  the  hill, 
and  each  thawing 
permits  gravity  to 
pull  them  down  hill. 
If  the  process  is  often 
repeated,  the  soil 
moves  slowly  down 
the  slope.  In  the 
course  of  many  years, 
many  tons  of  earth 
may  be  thus  carried 
to  a  lower  level. 

2.  Changes  in 
Daily  Temperature. 
—  In  regions  where 
the  air  is  dry  and 
clear  the  radiation 
of  heat  is  rapid  and 
the  range  in  daily 
temperature  is  wide, 
often  varying  80°  F., 
while  in  the  Sahara 
Desert  a  change  of 
131°  F.  within  a  few 


FIG.  7. —  Rock  glacier,  McCarthy  Creek,  Alaska.  The 
talus  forming  the  rock  glacier  is  derived  from  the  high 
cliffs  (cirque).  (U.  S.  Geol.  Surv.) 


hours  has  been  re- 
corded. In  such  re- 
gions, the  naked  rocks  are  heated  to  a  high  temperature  during  the 
day  and  are  cooled  rapidly  at  night.  Since  rocks  are  not  good  con- 
ductors of  heat,  the  side  of  the  rock  exposed  to  the  sun's  rays  is  often 
raised  to  a  temperature  of  120°  F.  or  more  during  the  day,  while  a 
short  distance  beneath  the  surface  the  rock  is  still  cool.  The  result 
is  that  the  outside  shell  is  expanded,  while  the  interior  is  still  con- 
tracted. Strains  are  thus  produced  which  tend  to  break  off  fragments 
of  the  rock,  dark-colored  rocks  being  particularly  affected,  since 
they  absorb  more  heat.  In  the  late  afternoon  and  night,  on  the  other 
hand,  when  the  temperature  falls,  the  interior  which  had  been  grad- 

CLELAND   GEOL. — 3 


PHYSICAL  GEOLOGY 


ually  acquiring  heat  during  the  day  is  still  warm  when  the  surface  is 
cool  and  contracted.  The  contracted  exterior  is  then  too  small  for 
the  still  expanded  interior  and  the  surface  of  the  rocks  tends  to  shell 
off  (Fig.  8),  forming  onion-like,  concentric  layers,  the  process  being 
known  as  exfoliation.  It  is  stated  that  in  certain  parts  of  Africa  the 
rock  temperature  rises  to  a  height  of  137°  F.  during  the  day  and  falls  so 
rapidly  at  night  as  to  throw  off,  by  contraction,  masses  as  much  as 
200  pounds  in  weight.  Slabs  of  granite  8  to  10  inches  thick  and  10 
feet  long  are  known  to  have  been  broken  off  by  changes  in  daily 

temperature.  In  the 
western  part  of  the 
United  States,  where 
the  climate  is  too  dry 
to  afford  much  scope 
for  the  operation  of 
frost,  cliffs  are  slowly 
disintegrated  by 
these  changes  in  tem- 
perature, producing 
talus  slopes  of  large 
size. 

The  alternate  heat- 
ing and  cooling  of  a 
rock  causes  its  disin- 
tegration in  still  an- 
other way.  When  a 
rock  is  composed  of 

minerals  differing  in  color  and  composition,  it  is  especially  liable  to 
disintegration  by  changes  in  daily  temperature.  Since  dark-colored 
minerals  absorb  heat  more  rapidly  than  light-colored  ones  and  also 
radiate  it  more  quickly,  rocks  containing  both  expand  and  contract  at 
different  rates,  with  the  result  that  the  grains  are  gradually  loosened 
until  the  surface  is  reduced  to  sand.  The  fact  that  the  coefficients 
of  expansion  of  the  various  minerals  differ  widely  also  aids  in  the  dis- 
integration of  the  rock.  Igneous  rock,1  composed  of  minerals  of  dif- 
ferent kinds,  is  therefore  more  easily  disintegrated  by  this  process 
than  rocks  made  up  of  one  mineral. 

Changes  in  daily  temperature  are  especially  effective  in  high  alti- 


FIG.  8.  —  Exfoliated  granite.     (U.  S.  Geol.  Surv.) 


1  Igneous  rocks  are  those  which  have  been  formed  from  molten  masses  by  cooling.    See 
page  329  for  a  discussion  of  these  rocks. 


WEATHERING 


33 


tudes,  and  mountain  peaks  often  owe  their  jagged  shapes,  to  some 
degree,  to  this  action,  although  more  largely  to  that  of  frost.  In 
regions  of  deficient  rainfall,  talus  accumulates  at  the  foot  of  cliffs,  the 
fragments  forming  the  slope  having  been  broken  off  by  temperature 
changes.  Mountains  in  desert  regions  are  sometimes  almost  buried 
beneath  rock  fragments  and  sand,  broken  from  their  sides  by  changes 
in  daily  temperature. 

When  the  heated  rocks  of  arid  regions  are  wet  by  a  sudden  down- 
pour, they  cool  quickly  and  are  broken  asunder.  In  western  Texas 
blocks  25  feet  in  diameter  are  reported  to  have  been  rent  into 
several  pieces  in  this  way.  (Hobbs.) 

3.  Mechanical  Action  of  Animals  and  Plants.  —  If  one  observes 
a   cliff  upon  which  vegetation  is  abundant,  he  will  see  that  not 
only  the  large  but  also  the  small  cracks  of  the  rock  are  filled  with 
roots  and  rootlets.     As  these  roots  and  rootlets  grow  larger  they  tend 
to  push  the  blocks  of  rock  apart.     The  root  of  the  garden  pea,  for 
instance,  has  a  wedging  force  equal  to  200  or  300  pounds  a  square 
inch.     Abundant  examples  of  this  wedging  process  can  be  found  in 
fertile  regions,  and  are  also  often  seen  in  cities,  where  the  pavements 
are  frequently  broken  and  tilted  by  the  enlarging  roots  of  trees. 

Plants,  earthworms,  and  burrowing  animals  open  channels  through 
which  water  from  the  surface  can  reach  deep  down  into  the  soil. 
Moreover  the  organic  matter  carried  into  the  tunnels  by  the  animals 
is,  upon  its  decay,  a  source  of  organic  acids  which  actively  attack 
the  rocks  and  thus  hasten  the  decomposition  of  those  otherwise  pro- 
tected by  soil.  It  is  thus  seen  that  the  mechanical  disintegration  of 
rocks  is  accomplished  both  by  the  agents  of  the  weather  and  by 
organisms. 

4.  Rain.  —  The  mechanical  effect  of  rain  consists  in  (i)  the  impact 
of  the  raindrops  upon  the  surface,  which  in  the  aggregate  has  a  con- 
siderable effect,  as,  for  example,  in  gravel  deposits  where  the  larger 
bowlders  protect  the  gravel  underneath  from  the  impact  of  the  rain, 
while  that  which  is  not  so  protected  is  removed.     In  the  process  of 
time,  columns  a  score  or  more  feet  in  height  may  result  (Fig.  9). 
On  a  small  scale,  this  same  result  can  be  seen  in  almost   any  soft 
material  after  a  rain.     (2)  The  mechanical  work  of  rain  is  seen  also  in 
the  softening  of  clay  soils,  which,  on  slopes,  causes  them  to  creep; 
(3)  in  the  washing  and  later  deposition  of  dust  from  the  atmosphere, 
and  (4)  in  the  dissolving  of  some  of  the  atmospheric  gases  which  may 
later  be  used  in  weathering  the  rocks.     Dew  and  hoarfrost,  being 


34 


PHYSICAL  GEOLOGY 


condensed  from  the  lower  layers  of  the  air,  absorb  and  furnish  to  the 
soil  more  gases  and  inorganic  matter  than  rain.     (5)  Rain  water  is 

also  effective  in  causing 
certain  rocks  to  swell.  In 
excavating  the  Panama 
Canal  it  was  found  that 
certain  rocks  which,  when 
first  uncovered,  had  to 
be  blasted  before  they 
could  be  removed,  be- 
came so  soft  after  a  few 
months'  exposure  to  the 
tropical  rains  that  they 
could  be  excavated  with 
the  steam  shovels.  The 
slides  which  have  oc- 
curred in  the  Culebra 
Cut  were  due  both  to  the 
softening  of  the  rock  in 
this  way  and  to  gravity 
FIG.  9.  — The  work  of  rain  water  in  sculpturing  which  tends  to  cause  the 

TiroTk  °f  Carth  C°ntaining  b°wlders'  near  Bogen'     rock  to  move  toward  the 

excavation.  The  soften- 
ing action  is  taken  advantage  of  in  extracting  diamonds  from  the 
inclosing  rock  in  the  South  African  mines. 

5.  Wind.  —  The  mechanical  work  of  the  wind  carrying  sand  is 
very   effective    in   wearing   away   rock,    especially  in    arid  regions, 
and  will  be  discussed  on  another  page  (p.  45). 

6.  Lightning.  —  When   lightning   strikes  the   earth   it   sometimes 
fractures   large   masses   of  rock.     When   it   strikes   sand,   drops  or 
bubbles  of  glass  and  irregular  tubes  or  rods  are  sometimes  formed  by 
the  partial  fusion  of  the  soil.     These  fulgurites,  as  they  are  called, 
are  seldom  more  than  a  few  inches  long,  but  are  sometimes  several 
feet  in  length  and  two  and  a  half  inches  in  diameter.     The  entire 
summit  of  Little  Ararat  in  western  Asia,  where  electrical  storms  are 
extremely  common,  is  said  to  be  drilled  by  lightning.     "  A  piece  of 
rock  about  a  foot  long  may  be  obtained,  perforated  all  over  with  irreg- 
ular tubes,  having  an  average  diameter  of  three  centimeters.     Each 
of  these  is  lined  with  a  blackish-green  glass."      (A.  Geikie.)     This  is, 
however,  unusual,  and  the  total  effect  of  lightning  is  inconsiderable. 


WEATHERING 


35 


CHEMICAL  AGENCIES 


The  chemically  active  gases 
carbon  dioxide.  Un- 
less they  are  dissolved 
in  water,  however, 
their  effect  in  the 
weathering  process  is 
unimportant,  but  in 
the  presence  of  both 
moisture  and  heat 
they  accomplish  a 
great  part  of  the  work 
of  chemical  disinte- 
gration. It  is  evident, 
therefore,  that  the 
chemical  decomposi- 
tion of  rocks  must 
vary  greatly  in  effec- 
tiveness in  different 
places  and  at  different  times  in 


of  the  atmosphere  are  oxygen    and 


FIG.  ii.  —  Joints  in  limestone,  widened  by  solution. 
(Photo.  H.  L.  Fairchild.) 


FIG.  10.  —  Limestone  bowlder  channeled  by  water 
containing  carbon  dioxide. 


the  same  place,  and  we  find  that  it  is 
most  active  in  moist, 
tropical  regions,  less 
rapid  in  temperate 
regions,  and  least  im- 
portant in  the  frigid 
zones  and  in  arid 
regions. 

i.  Solution. — Pure 
water  is  a  poor  sol- 
vent, but  when  it 
contains  a  consider- 
able quantity  of  car- 
bon dioxide  its  sol- 
vent power  becomes 
greatly  increased,  so 
that  limestone,  gyp- 
sum, and  other  easily 
soluble  rocks  are 
slowly  taken  up  by 


36  PHYSICAL  GEOLOGY 

it  and  carried  away.  Water  obtains  its  supply  of  carbon  dioxide 
from  the  air,  from  the  decay  of  plants  and  animals,  and  from 
subterranean  sources  (p.  296).  It  has  been  estimated  that  the 
surfaces  of  certain  limestones  in  England  have  been  lowered  at 
rates  varying  from  one  inch  in  24  years  to  the  same  amount  in  500 

years.   Although  solu- 

,---' — ,---> r--J X-J -jr_i  —  r-l...       tion  is  most  conspicu- 

".'."]._"   .'-~i~'     '."!"  .1."."_V.r"        •--=---     --T--J.-A    ously     exhibited      in 

limestone     regions, 
B    where     the     rock     is 

"i  '         i,,i  C    often  furrowed  by  the 

FIG.  12.  —  The  formation  of  the  residual  soil  B  from  rivulets  (Fig.  lo) 
the  limestone  A  is  shown.  The  soil  was  derived  from  which  flow  over  the 
the  limestone  by  the  removal  of  the  soluble  portions  and  /.  i  i  •  • 

the  concentration  of  the  insoluble.  Large  areas  of  Ken-  surtace>  and  the  joints 
tucky  and  Virginia  owe  their  fertility  to  this  process.  and  other  cracks  are 

widened  by  its  action 

(Fig.  n),  it  is  also  effective  on  feldspar  and  even  on  quartz. 
Sandstones  with  calcareous  cements  are  disintegrated  by  the  solu- 
tion of  the  cement,  causing  the  rock  to  fall  to  pieces  and  form  sand. 
In  regions  of  impure  limestone  the  insoluble  residue,  such  as  clay  and 
flint  nodules  (p.  77),  will  be  left,  covering  the  unweathered  rock 
(Fig.  12).  The  depth  of  this  cover  often  gives  a  basis  for  estimat- 
ing the  thickness  of  limestone  which  has  been  dissolved  and  carried 
away.  Many  caves  are  formed  by  solution  (p.  70). 

2.  Oxidation.  —  Oxygen  is  effective  only  on  rocks  which  contain 
minerals  capable  of  taking  up  further  oxygen  and  thus  forming  new 
compounds.     The  most  important  of  these  are  iron  compounds,  and 
to  them  the  red  and  yellow  coloring,  so  conspicuous  in  rocks,  is  due. 
If  oxygen  alone  is  added  to  the  iron  molecule,  a  red  color  (Fe2Os) 
results;    if  moisture  is  present,  however,  the  brown  or  yellow  rust 
(hydroxide),  common  in  moist  regions,  is  formed.     One  noticeable 
result  of  oxidation  is  an  increase  in  volume ;   this  being  the  case,  the 
newly  formed  and  bulky  minerals  crowd  the  grains  of  the  rock  apart 
and  tend  to  produce  disintegration.     Complex  silicates,  such  as  feld- 
spar, mica,  and  hornblende  (p.  690),  are  attacked  by  oxygen  and  carbon 
dioxide,  and  reduced  to  simpler  and  more  stable  compounds. 

3.  Hydration.  —  The  union  of  water  with  chemical  compounds  is 
known  as  hydration^  and  is  very  important  in  weathering.     An  im- 
portant effect  is  the  increase  of  the  volume  of  the  mineral  acted  upon. 
The  operation  of  hydration  and  oxidation  is  well  illustrated  in  the 


WEATHERING  37 

weathering  of  iron  pyrite  (FeS2)  (p.  686)  which  often  occurs  dis- 
seminated through  rocks.  The  first,  and  usually  most  conspicuous 
effect  is  the  appearance  of  a  yellow  stain  on  the  rock.  If  the  pyrite  is 
abundant,  hydration  may  cause  the  rock  to  fall  to  pieces  as  a  result 
of  the  increase  of  volume  and  of  the  formation  of  sulphuric  acid. 
Building  stones  which  are  uniform  in  color  when  first  quarried  some- 
times become  discolored,  after  an  exposure  of  a  year  or  more,  by 
blotches  of  brown  stain.  Upon  examination,  it  is  usually  found  that 
the  stain  was  formed  from  the  weathering  of  small  crystals  of  pyrite. 
From  these  blotches  the  stain  spreads,  sometimes  covering  an  area 
of  100  or  more  square  inches. 

4.  Carbonation.  —  By  the  union  of  carbon  dioxide,  derived  from 
the  air  and  soil,  with  the  calcium,  magnesium,  or  iron  of  complex 
silicates,  soluble  compounds  are  formed   which  upon  being  carried 
away  in  solution  cause  the  rock  to  crumble.     This  is  an  important 
cause  of  the  disintegration  of  granite,  although  oxidation  and  hydra- 
tion are  also  effective  in  the  same  process.     If  organic  acids  derived 
from  decaying  vegetable  matter  are  present  in  water,  they  tend  to 
decolorize  red  and  yellow  rocks.     Such  decolorization  can  often  be 
seen  where  water  trickles  over  cliffs.     For  example,  the  red  cliffs  of 
the    Vermilion    River    in   northern    Ohio    are    bleached    wherever 
rivulets  trickle  over  them.     This  is  accomplished  by  the  union  of  the 
carbon  dioxide  with  the  oxides  of  iron  which  gave  the  red  and  yellow 
color  to  the  rock. 

5.  Organisms.  —  Although     not     agents     of    the    weather,     the 
chemical  action  of  plants  and  animals  should  be  considered  in  a  dis- 
cussion of  rock  disintegration.     Certain  bacteria  are  found  in  great 
numbers  on  the  surface  of  bare  rock.     They  live  not  only  in  low,  moist 
regions,  but  even  on  mountain  peaks,  where  they  have  been  found 
coating  the  surfaces  and  crevices  of  the  rocks.     They  draw  their 
nourishment  from  the  nitrogen  and  other  compounds  brought  down  in 
snow  and  rain.     Rocks  are  attacked  by  the  nitric  acid  which  these 
bacteria  form  from  the  ammonia  of  the  air  and  water.     The  chemical 
action  of  their  excretions  makes  them  an  important  though  incon- 
spicuous agent  of  disintegration.     Other  organisms,  such  as  lichens, 
mosses,  and  flowering   plants,   contribute  to  the   decomposition  of 
rocks.     The  roots  of  trees  not  only  pry  the  rocks  apart  (p.  33),  but 
they  also  act  chemically  by  producing  carbon  dioxide  and  organic 
acids,  which  dissolve  the  lime  and  transform  the  silicates  into  car- 
bonates and  other  products. 


38  PHYSICAL  GEOLOGY 

Comparison  of  Effects  of  Chemical  and  Mechanical  Weathering. 
—  Chemical  decomposition  of  rocks  is  slow  and  long  continued  as 
compared  with  mechanical  disintegration,  which  is  a  rapid  process. 
By  the  former  the  rocks  are  broken  up  into  fine  particles,  and  by  the 
latter  into  larger  and  smaller  fragments.  Chemical  action  is  not  only 
long  continued,  but  is  also  more  universal  than  mechanical  action, 
being  important  under  all  climates,  except  in  desert  regions  and  on 
mountains  where  mechanical  disintegration  is  so  rapid  that  sufficient 
time  is  not  permitted  for  conspicuous  chemical  action.  Chemical 
decomposition  tends  to  smooth  surfaces,  while  mechanical  disinte- 
gration tends  to  roughen  them.  Where  the  mechanical  predominates, 
the  slopes  are  stronger  and  tend  to  forrn  cliff's. 


RESULTS  OF  WEATHERING 

Some  of  the  most  conspicuous  features  of  scenery  are  produced  by 
weathering.  These  features  are  seldom  due  to  a  single  agent,  but 
more  often  to  two  or  more  acting  in  conjunction. 


FIG.  13.  —  Pinnacle  Peak,  Canadian  Rockies.     The  ragged  outlines  are  due 
largely  to  frost  work.     (Photo.  M.  H.  Smith.) 

The  rough  and  jagged  peaks  so  characteristic  of  high  mountains 
have  been  sculptured  largely  (i)  by  frost  and  (2)  by  changes  in  daily 
temperature  (p.  31)  (Fig.  13).  The  debris  derived  from  such  peaks 


WEATHERING 


39 


may  accumulate  in  the  valleys 
and  on  the  sides  of  the  moun- 
tains to  great  depths,  in  some 
cases  more  than  a  thousand  feet. 
Where  the  supply  of  talus  is  too 
great  for  the  stream  in  the  valley 
to  remove,  the  stream  is  dammed 
and  a  lake  is  formed  (Fig.  14). 
The  form  of  the  crests  and  cliffs 
of  high  mountains  is  determined 
to  a  large  degree  by  cracks  which 
have  a  uniform  direction  (joints), 
as.  when  the  water  which  fills 


FIG.  14.  —  Drawing  showing  a  stream 
so  dammed   by  talus  as  to  form  a  lake. 


them  freezes,  the  rock  is  broken   Note  the  angle  of  the  talus  slope, 
off  along  these  planes.     In  tropi- 
cal regions  the  fractures  of  the  rock  are  also 
important,  since  they  permit  the  access  of  water, 
and  chemical  decomposition  is  therefore  accom- 
plished at  greater  depths  at  these  points. 

It  is  often  possible  to  state  from  the  shape  of 
the  topography  in  any  one  locality  what  the 
nature  of  the  underlying  rock  is,  but  a  general 
rule  is  impossible,  since  the  same  rock  is  differ- 
ently affected  by  the  weather  under  different 
climates.  For  example,  granite  rocks  which 
in  a  cold  climate  may  be  broken  into  jagged 
crests,  may,  in  moist,  tropical  regions,  be 
reduced  to  rounded  forms  through  the  chemical 
agencies. 

Spheroidal  Weathering.  —  Spheroidal  weather- 
ing results  from  chemical  action  and  should  be 
distinguished  from  similar  shapes  which  are  pro- 
FIG  ic  —  Dia  rams  ^uced  by  exfoliation  due  to  changes  in  daily 
showing  the  effect  of  temperature  (p.  31).  When  water  percolates 
weathering  upon  rock  through  the  joints  (p.  2J8)  and  horizontal  planes 
&&(££*£$  ^to  which  all  rocks  are  more  or  less  divided, 
The  corners  and  edges  it  attacks  with  its  dissolved  gases  all  the  rock 
are  most  affected,  and  surfaces  with  which  it  comes  in  contact ;  but 
bhe:ombe°CtS  SphenroidaT  «nce  the  corners  and  edges  of  the  blocks  formed 
(Modified  after  Hobbs.)  by  these  joints  and  planes  have  a  greater 


4o 


PHYSICAL  GEOLOGY 


FIG.  16.  —  Granite  weathering  under  tropi- 
cal conditions.  Rhodes'  Grave,  southern 
Rhodesia.  The  bowlders  are  residual  frag- 
ments of  a  sheet  of  granite  that  once  overlay 
the  hill.  (Photo.  G.  A.  J.  Cole.) 


surface  exposed,  they  are 
more  vigorously  acted  upon. 
Such  places,  too,  encounter 
water  from  two  or  more 
directions  and  are  more 
likely  to  be  affected  by  the 
strongest  solutions.  The 
greater  weathering  of  the 
edges,  and  especially  of  the 
corners,  causes  them  to  dis- 
integrate more  rapidly,  leav- 
ing a  spheroidal  core  of  un- 
weathered  rock,  embedded  in 
less  compact,  weathered  rock. 
When  the  rock  is  exposed  to 
the  action  of  wind  and  water, 
the  unaltered,  spheroidal  core 
is  exposed  (Figs.  15,  16). 

Differential  Weathering.  — 
When  rocks  are  not  uniform 
in  character  but  are  softer  or 

more  soluble  in  some  places  than  in  others,  an  uneven  surface  may 
be  developed  (Figs.  17,  18,  19) ;  in  deserts  by  the  action  of  the  wind 
and  in  moist  regions  by 
solution.  Columns  of 
rock  which  have  been  iso- 
lated in  any  way  show 
the  effect  of  differential 
weathering.  In  arid  re- 
gions the  lower  parts  of 
the  columns  (Fig.  20)  are 
worn  away  more  rapidly 
than  the  upper  parts,  be- 
cause the  drifting  sand  is 
more  abundant  and  effec- 
tive near  the  ground,  and 
the  bases  grow  smaller 

and     smaller     until     the  {    -      IT 

r       I,                .  fie.    17. —  A      bowlder      showing      differential 

monuments  finally  topple  weathering.     the  projecting  portions  are  relatively 

over.  insoluble  silica,  while  the  .main  portion  is  limestone. 


WEATHERING 


FIG.  1 8.  —  Differential  weathering.     The  limestone  has 
been  dissolved,  leaving  the  quartz  veins  projecting. 


Widening  of   Val- 
leys. —  Valleys     are 

widened  by  the  work 

of    streams    (p.    81), 

but   a   large  part  of 

the    width    of   their 

upper  portions  is  due 

to  the  work  of  the 

weather,  which  first 

disintegrates     the 

rocks,     after     which 

rain,    hillside    creep 

(p.  31),  and  other  agents  bring  the  weathered  material  within  reach 

of  the  stream  which  carries  it  away. 

Rock  Mantle  and  Soil.  —  We  have  seen  that  everywhere  on  the 

earth's  surface  the  rock  is  being  broken  to  pieces  by  one  or  more  of 

the  agents  of  weathering.     This   results   in  the   accumulation  of  a 

mantle  of  rock  waste  which 
in  the  process  of  time  would 
cover  the  lands  to  a  great 
depth  if  it  were  not  removed. 
The  thickness  of  the  mantle 
rock  varies  greatly.  In  tropi- 
cal regions  the  solid  rock  may 
not  be  encountered  even  at 
a  depth  of  150  feet,  and  in 
Washington,  D.C.,  granite 
can  be  excavated  with  pick 
and  shovel  at  a  depth  of  80 
feet.  In  the  Valley  of  Vir- 
ginia and  in  the  Blue  Grass 
regions  of  Kentucky  a  thick 
layer  of  soil,  representing  the 
insoluble  portion  of  many 
feet  of  limestone,  covers  the 
underlying  rock.  Under 
normal  conditions  (Fig.  21) 

FIG.  19. —  The  more  rapid   weathering  of  tfoe    SQ{\    {s    thickest    on    the 

a    weak    bed    of   limestone    in    the    cliff   has  ,               i        KOC^C    nf 

formed    the    shelf.     Helderberg    Mountains,  'rests    and    *    the    baSCS    °* 

near  Albany,  New  York.  hills,    and    thinnest    on    the 


42 


PHYSICAL  GEOLOGY 


FIG.  20.  —  An  erosion  pillar,  shaped  largely 
by  the  work  of  wind-blown  sand.  Near 
Adamana,  Arizona. 


slopes,  since  loose  material  has  a  tendency  to  creep  down  hill 
(p.  31).  In  regions  which  have  been  covered  by  glaciers  (p.  168) 
the  soil  has  often  been  removed  from  the  hilltops  by  them. 

The  same  agencies  that 
cause  the  disintegration  of 
the  rock  break  up  the  mantle 
rock  to  finer  and  finer  par- 
ticles and  form  soil.  Soil 
grades  into  the  coarser  sub- 
soil which  has  not  yet  been 
completely  disintegrated. 
This  subsoil  is  gradually 
brought  to  the  surface  by 
earthworms  where  the  soil  is 
clay,  and  by  ants  where  it  is 
sandy.  In  the  aggregate, 
the  work  of  these  animals  is 
important.  It  has  been  esti- 
mated that  in  England  earth- 
worms bring  17  to  1 8  tons  of 
material  an  acre  to  the  surface  each  year,  and  that  in  Massachusetts 
ants  bring  up  one  fourth  inch  of  earth.  Leaves  and  other  organic 
matter  which  are  carried  into  the  soil  and  subsoil  by  earthworms  form 
organic  acids  which  hasten  the  chemical  disintegration  of  the  rock. 
Roots  of  plants  and  overturned  trees  also  help  to  mingle  soil  and  sub- 
soil. The  fertility  of 
soil  is  greatly  increased 
by  the  organic  mat- 
ter, either  animal  or 
vegetable,  which  it 
contains,  but  its  char- 
acter depends  largely  FIG.  21.  —  Section  showing  the  thickness  of  mantle 
.  i  i  r  rock  on  different  parts  of  a  hill.  (Modified  after  Cham- 

upon   the    rock    from  berlin>) 

which  it  was  derived. 

Kinds  of  Soil.  —  Mantle  rock  and  soil  are  moved  by  hillside  creep 
(p.  31),  by  rain  (p.  33),  by  avalanches,  by  landslides  (p.  73),  by 
slumping  (p.  73),  etc. ;  all  of  which  combine  to  remove  it  from  the 
uplands  and  carry  it  to  the  valleys.  There  are  two  kinds  of  soil, 
(i)  residual  soil,  that  derived  from  the  rock  which  it  covers,  such  as 
that  which  overlies  large  areas  where  the  country  has  not  been  affected 


WEATHERING 


43 


by  glaciation,  and  (2)  transported  soil.  Transported  soils  may  be 
further  classified  as  (a)  alluvial,  those  which  have  been  carried  and 
deposited  by  streams,  and  which  vary  greatly  in  composition  from  the 
finest  clay  to  coarse  gravel ;  (b)  glacial  soils  which  in  any  place  may 
vary  greatly,  both  in  the  character  and  the  size  of  their  constituents 
(p.  663) ;  (c)  soils  of  sand  and  clay  deposited  by  the  winds,  such  as  the 
fertile  loess  of  China  (p.  53)  and  of  the  western  part  of  the  United 
States,  and  (d)  talus  soils  of  mountain  regions. 

Removal  of  Soil.  —  When,  by  deforestation,  overgrazing  by 
animals,  or  other  causes,  the  vegetation  which  prevented  the  washing 
of  the  soil  is  removed,  the  soil  may  be  carried  away  rapidly,  and  a 
fertile  region  may  become  almost  a  desert.  This  appears  to  have  been 
true  of  portions  of  China  and  Greece.  When  the  fertile  soil  is  once 
removed,  it  is  difficult  for  plants  ever  again  to  gain  a  foothold,  and  the 
region  may  be  permanently  desolated.  It  is  stated  that  a  single 
lumberman  may  in  fifty  years  deprive  the  human  race  of  soil  that 
required  tens  of  thousands  of  years  to  form. 

REFERENCES  FOR  WEATHERING 

BUCKLEY,  E.  R.,  —  Building  and  Ornamental  Stones  :  Bull.  Wis.  Geol.  and  Nat.  Hist. 

Surv.  No.  4,  1899,  pp.  11-34. 

DANA,  J.  D.,  —  Manual  of  Geology,  pp.  118-129;  I58~I59- 
DE  MARTONNE,  E.,  —  Geographie  Physique,  1909,  pp.  404-411. 
GEIKIE,  A.,  —  Textbook  of  Geology,  4th  ed.,  Vol.  i,  pp.  447-465. 
HAUG,  E., —  Traite'de  Geologic,  1911,  pp.  371-401. 
MERRILL,  G.  P.,  —  Rocks,  Rock- Weathering  and  Soils,  pp.  173-285. 
SHALER,  N.  S.,  —  Aspects  of  the  Earth,  pp.  300-339. 


CHAPTER    II 
WORK  OF  THE  WIND 

THE  conditions  essential  for  the  effective  work  of  the  wind  are 
aridity  and  a  scarcity  of  vegetation.  Since  such  conditions  prevail 
over  more  than  one  fifth  of  the  land  surface  of  the  world,  the  work 
accomplished  by  this  agent  is  of  great  importance. 

WIND  AND  SAND 

Wind  without  Sand.  —  Wind  is  much  less  effective  without  sand 
than  with  it,  but  is  nevertheless  important.  In  semiarid  regions 
and  in  those  which  are  suffering  from  a  long  period  of  drought, 
cultivated  fields  may  be  excavated  disastrously.  In  Wisconsin 
there  are  extensive  regions  of  light  lands  which  almost  every  year 
suffer  from  the  drifting  action  of  the  wind.  In  these  regions  winds  dry 
up  the  soil  and  sometimes  sweep  away  the  crops  of  grain,  even  after 
they  are  four  inches  high,  uncovering  the  roots  by  the  removal  of  one 
to  three  inches  of  surface  soil.  (King.)  During  the  drought  of  1894  *n 
Nebraska,  the  finely  pulverized  soil  of  the  cultivated  fields  was  blown 
out  over  extensive  areas  to  a  depth  of  two  or  more  inches  and  was  piled 
up  in  small  dunes  near  fences  and  buildings.  Blow-outs,  as  the  pits 
excavated  by  the  wind  are  called,  are  often  the  indirect  result  of  the 
close  grazing  of  a  light  soil,  or  are  developed  in  land  which  is  covered 
with  a  sparse  vegetation.  Blow-outs  may  be  excavated  to  a  depth 
^  often  feet  or  more,  and  at  certain  seasons  of  the  year  may  be  occupied 
by  temporary  lakes.  The  work  of  the  wind  in  removing  loose  sand 
is  termed  deflation. 

Besides  these  more  important  effects  of  the  wind,  rocks  are  dis- 
lodged from  cliffs  by  its  force,  as  in  the  Orkney  and  Shetland  Islands, 
where  it  is  common  to  find  pieces  of  flagstone  or  slate  weighing  several 
pounds,  which  have  been  detached  from  the  precipices  and  blown 
upon  the  moors  above  during  high  gales.  (A.  Geikie.)  Trees  are 
blown  down;  water  is  thrown  into  waves;  and  birds,  insects,  and 
seeds  are  carried  about. 

44 


WORK  OF  THE  WIND 


45 


Wind  with  Sand.  —  As  soon  as  the  wind  picks  up  pieces  of  the 
mantle  rock  it  has  tools  with  which  to  work,  and  it  becomes  a  geological 
agent  whose  effect  in  desert 
regions  is  not  easily  over- 
stated. It  is  from  the  man- 
tle of  rock  waste  formed  by 
the  agents  of  the  weather, 
and  from  the  sediment  car- 
ried to  the  deserts  by  the 
mountain  streams  that  the 
sheets  of  sand  which  cover 
the  deserts  are  made.  The 
work  of  wind  laden  with 
sand  is  well  illustrated  in  the 
artificial  sand  blast  by  which 
granite  is  polished  and  glass  is  etched  in  desired  patterns.  Telegraph 
poles  in  arid  regions  are  often  cut  off  near  the  base  by  wind-blown 
sand. 

Pebbles   worn  by  the   wind  (Fig.   22)   usually  have  a  character- 
istic,   brazil-nut    shape    (dreikanter) ,    the    faces    meeting   in    ridges. 


FIG.  22.  —  Pebbles  faceted  by  the  abrasion 
of  wind-blown  sand  (dreikanter) . 


FIG.  23.  —  Surface  eroded  by  wind-blown  sand.     The  small  table  is  the  remnant 
of  a  once  extensive  bed.     (De  Martonne.)     (See  Fig  24.) 

This  shape  is  explained  as  follows  :  the  planing  of  the  exposed  surface 
of  the  pebble  by  the  wind-blown  sand  continues  until  the  pebble  stands 
on  a  narrow  base.  It  is  then  overturned  by  a  slightly  stronger  gust 


46  PHYSICAL  GEOLOGY 

of  wind  and  a  new  surface  is  exposed  which  is,  in  turn,  planed  by  the 
blown  sand.  The  pebble  may  be  turned  over  by  the  undermining 
of  the  sand  on  one  side,  when  the  pebble  falls  into  the  depression  thus 
made,  and  the  wind  is  permitted  to  plane  another  surface.  Pebbles  of 
this  sort  are  sometimes  found  in  ancient  rocks  and  afford  evidence  of 
the  physical  conditions  of  the  time  in  which  they  were  deposited. 

Wind  scour  wears  away  the  softer  strata  of  a  desert  much  more 
rapidly  than  the  harder,  producing  wide  plains  above  which  the 
harder  rocks  stand  as  isolated  hills.  In  areas  composed  of  horizontal 
strata  the  soft  rocks  are  removed,  and  the  region  is  lowered  to  a  harder 
stratum,  which  may,  in  turn,  be  cut  up  and  the  whole  region  reduced 
to  a  still  lower  level.  During  this  gradual  lowering,  the  deserts  are 


ft: 


FIG.  24. —  Diagram  showing  a  table  such  as  that  appearing  in  Figure  23,  formed 
by  the  wearing  away  of  the  beds  A  and  B,  by  wind-blown  sand. 

eroded,  leaving  extensive,  flat-topped  elevations  capped  by  harder 
rock  (Figs.  23,  24),  which  are  later  cut  up  into  conical  hills  and  finally 
destroyed. 

In  desert  regions  where  plains  predominate,  we  sometimes  find 
mountains  whose  bases  are  covered  for  1000  to  2000  feet  with  sand, 
rising  above  the  plains.  (McMahon.)  In  places  we  find  also  bare 
rock  exposed  in  the  basins,  showing  that  the  wind  is  able  to  exca- 
vate the  rock  to  low  levels.  In  fact,  it  seems  probable  that  so  long 
as  the  ocean  is  held  back  from  a  desert,  eolian  excavation  may  go 
below  sea  level.  The  limit  to  eolian  excavation  is  the  level  of  under- 
ground water  (p.  56),  for  when  that  is  encountered,  wind  erosion  is 
ineffective,  since  the  sand  is  then  held  by  the  moisture  and  further 
removal  prevented. 

Sand  Dunes.  —  When  wind  meets  an  obstacle,  its  velocity  is 
lessened  and,  if  it  carries  sand,  some  of  its  burden  is  dropped.  The 
mounds  of  sand  which  are  thus  piled  up  by  the  wind  are  called  dunes. 
Sand  dunes  are  most  abundant  (i)  in  deserts,  being  as  a  rule  more 
numerous  in  low-lying  areas ;  (2)  on  sandy  coasts  where  the  prevailing 
winds  are  on  shore;  and  (3)  near  the  beds  of  rivers  whose  volume 
varies,  leaving  broad  areas  of  sand  exposed  during  the  dry  season. 
Dunes  of  this  origin  are  common  in  Nebraska,  Kansas,  Mexico,  and 
many  other  regions.  Sand  dunes  occur  in  the  above-mentioned 


WORK  OF  THE  WIND  47 

regions  because  there  only  the  wind  finds  sufficiently  thick  accumula- 
tions of  dry  sand  and  sufficiently  extended  flat  surfaces  for  effective 
work.  There  also  the  winds  blow  in  the  same  direction  a  sufficient 
length  of  time.  Frequent  changes  in  the  direction  of  winds  are  as 
unfavorable  to  the  development  of  dunes  as  vegetation  or  a  rough 
topography. 

If  the  direction  of  the  wind  is  constant,  typical  dunes  will  have  a 
gentle  slope  on  the  windward  side  and  a  steep  slope  on  the  leeward  side 


FIG.  25.  —  Sand  dunes.     The  direction  of  the  wind  was  from  the  right  to  the  left. 
(Photo.  D.  T.  MacDougal.) 

(Fig.  25).  This  difference  between  the  windward  and  leeward  slopes 
is  due  to  the  fact  that  the  sand  is  pushed  up  the  former  by  the  wind 
and  dropped  over  the  crest,  where  it  comes  to  rest  at  a  steeper  angle. 
On  the  other  hand,  if  the  prevailing  direction  of  the  wind  changes  from 
season  to  season,  the  difference  between  the  angles  of  the  slopes  will 
be  less  marked.  The  slope  is  generally  steepest  on  high  dunes,  but 
is  never  greater  than  10°  on  the  windward  and  30°  on  the  opposite 
side.  Since  the  winds  vary  greatly  in  velocity  from  time  to  time,  the 
size  of  the  sand  particles  carried  up  the  dunes  differs  and  usually 
produces  distinct  layers,  or  stratification.  The  inclination  or  dip 
(p.  252)  of  the  stratification  also  varies  widely  in  direction  and  steep- 
ness (Fig.  26),  since  it  depends  upon  the  direction  and  the  force  of  the 
wind.  The  formation  of  this  cross-bedding,  as  the  layers  in  one 

CLELAND   GEOL. —  4 


48 


PHYSICAL  GEOLOGY 


deposit  which  vary  in  direction  are  called,  is  clear  when  the  conditions 
of  their  formation  are  considered.     If  the  direction  and  force  of  the 

wind  remain  constant,  the 
sand  will  be  carried  up  the 
gentle  slope  and  will  fall  over 
the  steep  slope,  forming  lay- 
ers of  uniform  inclination.  If, 
however,  as  is  nearly  always 
the  case,  the  force  and  direc- 
tion of  the  wind  vary,  the 
sand  will  be  laid  down  on 
different  sides  of  the  dune  at 
different  times.  In  either 
case  cross-bedding  will  be 
produced,  but  it  will  be  more 
irregular  in  the  second  case 
than  in  the  first. 

Shape  and  Origin  of 
Dunes.  —  When  winds  are 
moderate  and  the  supply  of 
sand  is  small,  crescent-shaped 
dunes  (Fig.  27  A)  are  formed  ; 
if  the  wind  is  moderate  but  the  supply  of  sand  great,  dune  ridges  are 
often  developed  which  are  at  right  angles  to  the  direction  of  the  wind 
(Fig.  27  B) ;  while  in  regions  where  the  prevailing  winds  are  strong 
and  the  sands  abundant,  long  ridges  parallel  to  the  direction  of  the 
wind  usually  result  (Fig.  27  C).  The  crescent  shape  of  dunes  results 
when  wind,  blowing  over  a  sandy  stretch,  heaps  the  sand  into  small 
piles.  The  grains  of  sand  which  are  subsequently  carried  to  the  piles 
are  deviated  to  the  right  and  left,  until  crescents  are  formed.  When 
the  direction  of  the  wind  changes,  the  points  of  the  crescents  dis- 
appear and  then  form  on  the  lee  side  in  a  new  direction. 

Any  obstacle,  such  as  a  bush,  a  rock,  a  fence,  or  even  a  mere  rough- 
ness of  the  land  surface,  may  cause  the  beginning  of  a  dune.  Dunes 
are  also  sometimes  formed  when  the  sand  is  wet  by  slow  springs. 
These  moist  heaps  serve  to  anchor  additional  particles  of  sand  until 
a  mound  some  feet  in  height  is  formed,  which  may  afford  lodgment  for 
shrubs.  The  presence  of  obstacles  is,  however,  not  always  essential 
to  the  formation  of  dunes.  This  can  be  observed  on  a  small  scale  on  a 
smooth  asphalt  street,  where  the  dust  is  seen  to  be  collected  into  small 


FIG.  26.  —  A  quarry  in  eolian  limestone, 
Bermuda  Islands.  The  cross-bedding  was 
formed  by  the  shifting  winds  which  carried  the 
sand. 


WORK  OF  THE  WIND 


49 


ridges,  perpendicular  to  the  direction  of  the  wind.     The  waves  of  the 

sea  have  the  same  origin  as  these  ridges,  but  the  molecules  of  the 

water  are  not  carried    along  by  ^ 

the  wind    as    are   the   grains   of 

sand,  but  after  completing  their 

orbits   (p.   199),  return  to  their 

original  positions. 

Migration  of  Sand  Dunes.  — 
Dunes  are  constantly  migrating 
unless  the  sand  of  which  they  are 
composed  is  prevented  from  blow- 
ing by  grass  or  other  vegetation 
(Fig.  28).  The  forward  move- 
ment is  accomplished  by  the  force 
of  the  wind,  which  shifts  the  sand 
of  the  dunes  to  the  leeward. 
This  can  be  seen  on  a  windy  day, 
when  the  crests  of  the  dunes  ap- 
pear to  smoke.  The  rate  at 
which  dunes  move  varies,  de- 
pending largely  upon  the  velocity 
of  the  wind  and  the  height  of  the 
dune,  small  dunes  migrating  the 
faster.  In  Denmark  the  rate  is 
from  three  to  twenty  feet  a  year ; 
in  France,  on  the  Bay  of  Biscay, 
the  sands  have  advanced  at  a  rate 
estimated  from  15  to  105  feet  a 
year,  burying  in  their  progress 


forests,  farms,  vineyards,  villages, 

and   churches    (Fig.   29).     Some 

of  these,  after  being  buried  for 

years,  have  been  again  uncovered 

by  the  further    advance   of  the 

dunes.      The    church    of    Lege, 

taken  down    at  the  end  of  the 

seventeenth  century  and   rebuilt 

two  and  a  half  miles  inland,  had  again  to  be  removed   160  years 

afterwards,  showing  an  advance  of  the  sands  at  a  rate  of  81  feet 

a  year.     (Wheeler.)     On  the  south  side  of  Lake  Michigan  forests 


FIG.  27.  —  Diagrams  showing  the  form 
of  sand  dunes.  In  A  the  wind  blows  from 
the  upper  left-hand  corner.  The  supply 
of  sand  and  strength  of  wind  are  moder- 
ate. In  B  the  direction  of  the  wind  is 
from  the  upper  right-hand  corner.  The 
supply  of  sand  is  large  and  the  winds 
moderate.  Under  these  conditions  dunes 
transverse  to  the  wind  are  formed.  When 
the  supply  of  sand  (C)  is  large  and  the 
winds  strong,  dune  ridges  parallel  to  the 
direction  of  the  wind  are  formed. 


50  PHYSICAL  GEOLOGY 

which  were  covered  by  sand  dunes  have  been  uncovered  as  the  dunes 
moved  on.  There  are  hundreds,  perhaps  thousands,  of  square  miles 
of  buried  towns  and  cities  in  Central  Asia. 

One  of  the  difficulties  in  connection  with  the  maintenance  of  the 
Suez  Canal  is  the  sand  which  is  constantly  being  blown  into  it  from 


FIG.  28.  —  Dunes  held  by  mesquite  bushes.     (Photo.  D.  T.  MacDougal.) 

the  desert,  necessitating  frequent  dredging.  Many  communities 
in  the  past  which  depended  for  their  existence  upon  irrigation  have 
been  obliged  to  abandon  their  homes,  because  an  unstable  government 
failed  to  keep  the  irrigation  canals  free  from  drifting  sand. 

The  drifting  of  sand  has  often  affected  the  drainage  of  the  land ; 


FIG.  29.  —  A  sand  dune  covering  a  cabin.     The  origin  of  the  sand  is  the  seabeach. 

(Bermuda  Islands.)  . 

the  Grand  Calumet  River  (Fig.  30)  formerly  emptied  into  Lake 
Michigan  in  Indiana,  but  its  mouth  became  so  filled  with  drifting 
sand  that  the  course  of  the  stream  was  reversed  and  it  now  empties 
into  the  lake  at  Chicago,  twenty-four  miles  distant.  Large  lakes 
have  been  formed  in  consequence  of  the  damming  of  rivers  by  dunes, 
where  they  emptied  into  the  sea.  One  such  in  France,  Lake  Cazaux, 
has  a  width  of  nearly  seven  miles  and  a  depth  of  130  feet. 

The  ripples  which  mark  the  surfaces  of  sand  dunes  shift  their  posi- 
tions gradually  and,  in  general,  are  affected  as  are  the  dunes. 

The  movement  of  sand  dunes  can  sometimes  be  prevented  by  plant- 


WORK  OF  THE  WIND  5! 

ing  grasses,  shrubs,  and  trees  on  the  gentle  slopes  in  order  that  they 
may  hold  the  sand  with  their  roots.  This  has  been  done  successfully 
in  San  Francisco,  in  Provincetown  (Massachusetts),  and  elsewhere. 

Beneficial  Effect  of  Dunes.  —  Dunes  are  not,  however,  always  a 
detriment  to  man.  A  writer  states  that  the  people  of  Holland  and 
Denmark  "  deal  as  carefully  with  their  dunes  as  if  dealing  with  eggs, 
and  talk  of  their  fringe  of  sand  hills  as  if  it  were  a  border  set  with 
pearls.  They  regard  these  as  their  best  defense  against  the  sea." 
(Kahl.)  As  this  implies,  Holland  depends  to  a  large  degree  for  its 


FIG.  30.  —  Map  of  the  Grand  Calumet  River.       The  river  formerly  entered  Lake 
Michigan  at  the  east,  but  was  cut  off  by  sand  dunes  and  now  enters  the  lake  at  Chicago. 

protection  from  the  sea  upon  sand  dunes,  which  are  from  one  to  three 
miles  wide  and  from  40  to  50  feet  high.  The  sand  of  certain  dunes  in 
England  (Padstow),  which  consists  largely  of  shell  fragments,  is  used 
to  some  extent  for  a  fertilizer. 

Material  of  Dunes.  —  The  material  of  sand  dunes  varies  but  is 
usually  quartz  sand.  However,  in  the  Bermudas,  Bahamas,  and  por- 
tions of  England,  dunes  are  composed  of  shell  sand  (CaCOs).  In  the 
Bermudas  these  sands  are  cemented  by  the  rain  water  which  dissolves 
the  calcium  carbonate  and  later  redeposits  it,  thus  forming  stratified 
eolian  rock.  When  the  shallow,  alkaline  lakes  of  portions  of  New 
Mexico  (Otero  Basin)  dry  up,  they  leave  on  their  beds  thin  sheets 
of  various  salts,  chiefly  gypsum.  These  soon  curl  up  into  leaves  which, 
when  blown  together,  are  broken  into  gypsum  and  salt  sands.  The 
winds  carry  the  light  gypsum  out  to  the  plains,  where  it  gathers  in  a 


52  PHYSICAL  GEOLOGY 

great  series  of  white  dunes,  60  to  100  feet  in  height,  covering  an  area 
15  miles  by  40  miles  in  extent.  Dunes  are  formed  also  of  fine  clay, 
as  well  as  of  disintegrated  granite  sand. 

Height  of  Dunes.  —  The  height  of  dunes  in  regions  where  the  direc- 
tion of  the  wind  is  fairly  constant  is  seldom  more  than  300  feet,  but 
in  such  deserts  as  the  Sahara,  where  the  wind  varies  from  season  to 
season,  the  height  may  reach  1500  feet.  In  the  latter  case,  the  dunes 
do  not  migrate,  and  their  greater  height  is  due  to  the  piling  up  of  the 
sand  from  different  directions. 

Eolian  Sandstone.  —  Extensive  strata  of  sandstone  of  very  ancient 
date  are  known  to  have  been  formed  of  wind-blown  sand.  Rocks  of 
this  origin  can  often  be  distinguished  from  the  sandstones  laid  down 
on  the  ocean  bottom.  The  following  differences  assist  in  recog- 
nizing the  source  of  the  original  deposits,  (i)  The  former  consist 
chiefly  of  quartz,  the  softer  minerals  having  been  worn  to  dust  and 
carried  away,  while  in  the  latter  the  softer  and  harder  minerals  are 
more  likely  to  occur  together.  (2)  Since  water-laid  sands  are  carried 
in  suspension,  they  are  subjected  to  less  wear  than  eolian  sands,  as 
the  water  between  the  particles  acts  as  a  cushion,  and  the  grains  are 
consequently  less  worn  and  more  angular  than  the  sand  grains  which 
have  been  buffeted  by  the  winds.  (3)  The  stratification  of  eolian 
sand  (Fig.  26,  p.  48)  usually  exhibits  cross  or  false  bedding  (p.  47), 
i.e.,  it  is  not  horizontal  but  varies  greatly  in  inclination  and  direction 
within  short  distances.  (4)  Wind-blown  sands  may  also  be  dis- 
tinguished from  marine  sandstones  by  the  character  of  the  fossils,  if 
such  exist. 

Dust.  —  As  sand  grains  are  borne  to  and  fro  by  the  wind,  striking 
against  each  other  or  against  rock  surfaces,  the  softer  grains  are 
reduced  to  dust,  and  even  the  harder  ones  may  finally  reach  a  similar 
state.  The  dust  thus  formed  is  carried  by  air  currents,  often  to  great 
distances.  In  a  single  storm  in  1901  it  is  estimated  that  1,960,420  tons 
of  dust  were  carried  from  the  Sahara  desert  to  Europe,  reaching  Italy 
on  the  second  day  of  the  storm,  and  Germany  and  Denmark  on  the 
fifth  day.  It  is  probable  that  every  square  mile  of  the  earth's  surface 
has  dust  upon  it  from  every  other  square  mile.  Even  the  snows  of 
mountain  glaciers  and  those  of  the  Arctic  and  Antarctic  regions  con- 
tain dust,  carried  to  them  from  lands  hundreds  of  miles  away. 

Loess.  — One  striking  result  of  the  transportation  of  dust  by  winds  is  that  regions 
to  the  leeward  of  deserts  are  constantly  receiving  dust  which  settles  gradually  upon 
them.  Such  a  deposit  of  fine  dust  is  called  loess.  The  fine  dust  is  carried  by  the 


WORK  OF  THE  WIND 


53 


wind  to  the  edge  of  the  dry  region,  where  it  is  precipitated  by  rain  or  falls  slowly  by 
its  own  weight.  Here  some  of  it  is  held  by  the  grasses  of  the  high  plains,  whose  roots 
have  left,  upon  their  decay,  the  vertical  columns  characteristic  of  loess.  "  But  if 


FIG.  31.  —  Loess  deposits,  Shan-si,  China.  The  canyon-like  depression 
was  excavated  by  the  wind  as  the  loess  was  loosened  by  traffic.  The  two 
levels  on  the  right  are  old  roads.  The  ability  of  loess  to  stand  in  almost 
vertical  walls  is  shown.  (Carnegie  Institution.) 

the  desert  dust  has  ceased  to  be  the  plaything  of  the  wind,  it  has  not  ended  its  jour- 
ney. From  now  on  rills  take  charge  of  it  and  continue  the  work  of  which  the  wind  is 
no  longer  capable."  (De  Martonne.)  In  this  way  loess  is  spread  over  a  large  terri- 
tory. 

In  China  there  are  extensive  areas  which  have  been  built  up  by  the  accumulation 


54  PHYSICAL  GEOLOGY 

of  such  dust,  in  some  regions  to  a  depth  of  1000  to  2000  feet.  The  fertility  of  the 
soil  of  these  regions  is  remarkable.  Although  cultivated  for  many  thousands  of  years 
without  artificial  fertilizer,  it  still  retains  its  fertility.  This  is  due  largely  to  the  con- 
stant supply  of  new  dust  from  the  desert.  It  is  stated  that  the  limit  of  the  loess 
practically  marks  the  extreme  limit  of  the  extension  of  Chinese  agriculture  and  com- 
merce. (Richthofen.)  Large  areas  in  the  United  States  (p.  657)  and  in  Argentina 
are  also  covered  with  loess,  and  in  all  such  regions  grass  and  grains  flourish,  although 
trees  are  usually  few.  The  principal  deposits  of  loess  in  the  United  States  were  de- 
rived from  the  fine  material  of  glacial  deposits  which  were  caught  up  by  the  winds 
during  the  dry  phases  of  the  interglacial  periods  (p.  657). 

Loess  has  the  property  of  maintaining  a  vertical  face  when  cut  through  artificially 
or  by  streams.  In  China  the  roads  of  the  loess  region  are  often  in  nearly  vertical, 
walled  canyons  (Fig.  31),  many  feet  below  the  surface,  having  been  deepened  by  the 
blowing  out  of  the  dust  of  the  traveled  road.  On  either  side  of  these  roads  cave  houses 
have  been  excavated  and  furnish  homes  for  many  thousands  of  people. 

Dust  is  obtained  by  the  winds  from  sources  other  than  desert  sand, 
such  as  fine  volcanic  ash,  solid  particles  of  smoke,  pollen  of  flowers, 
and  spores  of  plants.  The  amount  of  material  thrown  into  the  air 
during  volcanic  eruptions  is  enormous.  The  volcano  Krakatao  in  the 
East  Indies,  for  example,  in  1883  threw  volcanic  dust  to  a  height  of 
several  miles,  which  in  fifteen  days  had  encircled  the  globe.  So 
abundant  was  the  dust  in  the  air  that  for  many  months  after  the 
eruption  the  sunsets  were  remarkably  brilliant.  In  Kansas  and 
Nebraska  there  are  deposits  of  volcanic  dust,  locally  30  feet  thick, 
which  had  their  source  in  ancient  volcanoes  hundreds  of  miles 
away. 

REFERENCES  FOR  THE  WORK  OF  THE  WIND 

BEADNELL,  H.  J.  L.,  —  Sand  Dunes  of  the  Libyan  Desert:  Geog.  Jour.,  Vol.  35,  1910, 

PP-  379-395- 
COBB,  C.,  —  Where  the  Wind  Does  the  Work  :  Nat.  Geog.  Mag.,  Vol.  17,  1906,  pp.  310- 

317. 
DAVIS,  W.  M., —  The  Geographical  Cycle  in  an  Arid  Climate :  Jour.  Geol.,  Vol.  13, 

1905,  PP-  381-407- 

DE  M ARTONNE,  E.,  —  Geographic  Physique,  1909,  pp.  649-672. 
FREE,  E.  E.,—  The  Movement  of  Soil  Material  by  Wind:  Bull.  U.  S.  Bureau  of  Soils, 

No.  68,  1911. 

GEIKIE,  J.,  —  Earth  Sculpture,  1898,  pp.  250-265. 
HAUG,  E.,  —  Traite  de  Geologie,  1911,  pp.  387-403. 
HOBBS,  W.  H.,  —  Earth  Features  and  their  Meaning,  1912,  pp.  197-222. 
HUNTINGTON,  ELLSWORTH,  —  The  Pulse  of  Asia,  1907. 
KEYES,  C.  R.,  —  Relation  of  Present  Profiles  and  Geologic  Structures  in  Desert  Ranges: 

Bull.  Geol.  Soc.  America,  Vol.  21,  1910,  pp.  543-563.     (Dr.  Keyes  holds  extreme 

views  on  wind  erosion.) 


WORK  OF  THE  WIND 


55 


MACDOUGAL,  D.  T.,  —  The  Desert  Basins  of  the  Colorado  Delta  :  Bull.  Am.  Geog.  Soc., 

Vol.  39,  1907,  pp.  705-729- 
WALTHER,  J.,  —  Das  Gesetz  der  Wustenbildung,  1900. 

TOPOGRAPHIC  MAP  SHEETS,  U.  S.  GEOLOGICAL  SURVEY,  ILLUSTRATING  WIND  WORK 

Moses  Lake,  Washington.  Camp  Clark,  Nebraska. 

Lamed,  Kansas.  Norfolk,  Virginia  —  North  Carolina. 

Kinsley,  Kansas.  Sandy  Hook,  New  Jersey  —  New  York. 

Pratt,  Kansas.  Toleston,  Indiana. 

St.  Paul,  Nebraska.  Yuma,  California  —  Arizona. 


CHAPTER  III 
THE  WORK   OF   GROUND   WATER 

TAKING  the  world  as  a  whole,  about  78  per  cent,  of  the  rainfall 
either  soaks  into  the  ground  or  is  evaporated,  the  remainder  —  the 
run-off —  being  carried  directly  into  streams  and  rivers.  The  amount 
of  the  precipitation  which  is  retained  in  the  soil  depends  upon 
(i)  the  climate,  (2)  the  slope  of  the  ground,  (3)  the  porosity  of  the 
soil  and  rock,  and  (4)  the  amount  and  character  of  the  vegetation. 
In  moist  climates  the  run-off  may  amount  to  as  much  as  one  half  of 
the  rainfall,  while  in  arid  regions,  on  account  of  the  excessive  evapo- 
ration and  the  dryness  of  the  soil,  there  may  be  no  run-off.  That 
portion  of  the  rainfall  which  sinks  into  the  soil  is  called  ground  water. 
Once  beneath  the  surface,  it  continues  its  descent  through  the  pores 
and  cracks  of  the  rock  until  it  may  reach  great  depths. 

Quantity  of  Ground  Water.  —  All  rocks  are  more  or  less  porous,  even  granites  con- 
tain some  water;  for  example,  chalk  may  hold  two  gallons  of  water  a  cubic  foot, 
and  sandstones  may  hold  20  to  30  per  cent,  of  their  weight.  The  total  amount  of 
water  in  the  rocks  is  therefore  very  large,  and  it  is  probable  that,  if  the  ground  water 
were  squeezed  from  the  rocks,  there  would  be  enough  to  cover  the  earth  with  a  sheet 
of  fresh  water  one  hundred  or  more  feet  deep.  Locally,  the  quantity  of  underground 
water  may  be  much  greater;  as,  for  example,  in  Wisconsin  and  Minnesota,  where  the 
underlying  sandstone  alone  contains  enough  water  to  form  a  layer  50  to  100  feet 
deep.  The  ground  water  of  any  region  is  not  always  derived  from  the  local  rainfall, 
but  may  have  had  a  long  subterranean  course,  as  is  true  of  the  underground  water 
of  the  Great  Plains,  the  source  of  which  is  in  the  mountains,  many  miles  distant. 

The  Water  Table.  —  The  level  beneath  which  the  rock  is  saturated 
with  water  is  called  the  water  table  or  the  level  of  underground  water.1 
This  varies  greatly  in  different  regions.  In  humid  portions  of  North 
America  it  is  from  one  to  forty  feet  below  the  surface ;  in  limestone 
regions,  where  the  drainage  is  largely  subterranean,  such  as  in  por- 
tions of  Kentucky  and  Tennessee,  it  may  be  two  to  three  hundred 

1  "  In  deep  mines  in  various  parts  of  the  world  water  is  found  only  in  the  upper  levels, 
within  2500  feet  or  less  of  the  surface,  while  below  that  the  mines  are  dry  or  even  dusty." 
(Scott.) 

56 


THE  WORK  OF  GROUND  WATER         57 

feet  deep ;  while  in  the  Colorado  Plateau,  where  the  surface  is  cut 
by  deep  canyons,  it  is  sometimes  3500  feet  beneath  the  surface;  or 
it  may  be  entirely  absent,  except  where  water-bearing  strata  conduct 
water  from  other  areas.  In  the  Navaho  Reservation  in  Arizona,  for 
example,  no  water  except  artesian  (p.  59)  is  encountered  below  a 
depth  of  100  feet.  (H.  E.  Gregory.) 

The  water  table  varies  with  the  slope  of  the  land,  being  farther 
from  the  surface  on  the  hills  than  in  the  valleys  (Fig.  32).  The  greater 
depth  beneath  the  surface  of  the  hills  is  due  to  gravity,  which  between 
rains  and  during  dry  seasons  tends  to  pull  the  water  downward  to 
the  level  of  the  valleys,  but  is  unable  entirely  to  do  so  because  of 
capillarity  and  friction  of  the  water  with  the  grains  of  rock.  As  a 
result,  the  water  table  is  never  flat  in  a  hilly  region,  although  it  is 
more  nearly  so  after  a  prolonged  drought.  It  necessarily  follows 
that  the  depth  of  the  water  table  in  any  place  will  depend  largely 
upon  (i)  the  slope  of  the  land,  (2)  the  porosity  of  the  rock,  and 
(3)  the  frequency  and  character  of  the  precipitation,  slow,  soaking 
rains  accomplishing  more  than  sudden  and  brief  downpours. 

In  forested  areas  it  is  found  that  the  water  table  is  lower  than 
under  similar  conditions  of  moisture,  rock,  and  topography  ,in  other 
regions,  because  of  the  great  quantity  of  water  abstracted  by  the 
roots  of  the  trees  and  lost  by  evaporation  through  the  leaves. 
The  headwaters  of  streams,  however,  should  be  kept  forested, 
since  much  of  the  water  of  excessive  rains  is  retained  in  the  thick 
layer  of  forest  mold,  from  which  it  slowly  drains  away  and  thus 
tends  to  prevent  great  floods. 

Wells.  —  When  wells  are  sunk,  it  is  necessary  that  they  penetrate 
to  a  permeable  rock  or  to  a  much  fractured  one  (Fig.  33)  below 


FIG.  32.  —  Diagram  showing  the  water  table  or  level  of  underground  water  A  A  A  A 
and  the  effect  upon  natural  and  artificial  depressions. 

the  water  table  (Fig.  32),  for  otherwise  they  do  not  afford  a 
perennial  supply  of  water.  The  value  of  wells,  both  for  drink- 
ing purposes  and  for  irrigation,  is  inestimable.  It  is  stated  that 
in  India  more  land  is  irrigated  from  wells  than  from  streams,  and 


PHYSICAL  GEOLOGY 


FIG.  33. —  Diagram  showing  the  source 
of  well  and  spring  water  in  fractured  rock. 
(Modified  after  H.  E.  Gregory.) 


in  southern   California   one  half  of  the  irrigation  water   and   the 
greater  part  of  the  city  supplies   are  drawn  from  the  sands  and 

gravels  that  underlie  the  val- 
leys. It  is  estimated  that 
75  per  cent,  of  the  population 
of  the  United  States  depends 
for  its  water  supply  directly 
upon  underground  water. 

Movement  of  Ground  Water.  — 

Underground  water  seldom  moves 
in  definite  channels,  except  in  lime- 
stone regions,  but  percolates  slowly 
through  the  pores  and  crevices  of 
the  rocks.  Even  in  coarse  sand- 
stone the  rate  of  movement  may 
be  only  one  fifth  of  a  mile  a  year, 
although  in  regions  of  soluble  lime- 
stone it  may  flow  several  miles  a 

day  in  tunnels.     In  such   regions  the   direction  of  the   underground   flow  may  be 
opposite  to  that  of  the  surface  streams,  since  it  is  determined  by  the  dip  of  the  rock. 

Much  of  the  ground  water  eventually  reaches  the  surface  again 
unless  it  enters  into  chemical  combination  with  minerals  of  the  rocks 
with  which  it  comes  in  contact.  A  large  amount  is  taken  up  by  plants 
and  passes  into  the  atmosphere  by  evaporation ;  some  of  it  is  drawn 
out  in  wells ;  some  seeps  out,  or  is  discharged  in  springs,  either  in 
river  valleys  or  in  lakes  and  seas.  Large  springs  of  fresh  water  come 
to  the  surface  of  the  Mediterranean,  the  Gulf  of  Mexico,  and  other 
seas  at  short  distances  from  the  shore,  and  in  certain  places  fresh 
water  is  obtained  from  springs  on  the  ocean  bottom  by  diving. 

The  total  quantity  of  mineral  matter  dissolved  by  the  ground  water 
is  enormous.  The  greater  part  of  the  4,975,000,000  tons  of  mineral 
matter  carried  to  the  ocean  each  year  was  obtained  by  the  streams 
from  the  ground  water  which  escaped  through  springs  and  seepage. 

Depth  of  Ground  Water.  —  We  have  seen  that  the  rocks  of  the 
earth's  surface  are  much  broken  by  cracks  of  various  kinds.  This 
condition  holds  true  of  rocks  below  the  earth's  surface,  down  to  a 
depth  where  the  weight  above  them  is  greater  than  their  strength 
to  resist  pressure.  This  outer  zone  is  called  the  zone  of  fracture. 
The  depth  of  this  zone  varies  with  the  strength  of  the  rock.  In  the 
case  of  soft  rocks,  such  as  shales,  no  crack  may  be  found  at  a  depth 
of  2000  feet,  while  in  the  strongest  rocks  some  cracks  may  possibly 


\ 


. 

THE  WORK  OF  GROUND  WATER         59 

exist  "  at  a  depth  of  at  least  eleven  miles."  (Adams.)  At  depths 
greater  than  eleven  miles  it  does  not  seem  possible  that  a  crevice  can 
open,  and  if  a  fracture  should  occur,  the  parts  would  actually  weld 
together.  It  is  evident  from  the  above  that  water  will  not  descend 
a  greater  distance  than  eleven  or  twelve  miles  under  the  most  favor- 
able conditions,  and  usually  far  less  than  that.  The  temperature  of 
the  rocks,  and  therefore  of  underground  water,  increases  i°  F.  for 
each  60  to  100  feet  of  descent,  a  fact  which  accounts  for  the  warmth 
of  deep  wells  and  springs  coming  from  great  depths  (p.  273). 

Artesian  Wells.  —  Strictly  speaking,  an  artesian  well  is  one  in 
which  the  water  rises  above  the  surface  of  the  ground  as  a  fountain, 
but  the  term  is  now,  unfortunately,  frequently  employed  for  any  deep 
well  from  which  water  is  obtained,  whether  it  flows  to  the  surface  or 
not.  This  change  in  usage  is  doubtless  due  to  the  fact  that  often 


FIG.  34.  —  Block  diagram  showing  the  conditions  favorable  for  artesian  water. 
The  porous  beds  ( dotted  )  receive  water  from  the  rain  which  falls  on  their  outcrops, 
and  from  the  streams  which  lose  somewhat  in  volume  as  they  flow  over  them.  Three 
water-bearing  beds  ( aquifers )  are  shown,  from  two  of  which  water  can  be  obtained 
on  the  barrier  island  which  is  separated  from  the  mainland  by  a  salt-water  lagoon. 

artesian  wells,  after  flowing  for  some  months  or  years,  cease  to  do  so 
and  must  be  pumped  because  of  the  excessive  withdrawal  of  water 
from  the  artesian  basin.  This  was  true  of  the  first  artesian  well  at 
Artois,  France  (from  which  the  name  "  artesian"  was  derived).  Many 
wells  in  the  San  Bernardino  valley,  California,  which  flowed  strongly 
when  first  drilled,  are  now  pumped.  The  conditions  favoring  artesian 
water  (Fig.  34)  are  (i)  a  porous  bed  capable  of  absorbing  and  trans- 
mitting large  quantities  of  water ;  (2)  relatively  impervious  beds  above 
and  below;  (3)  exposure  of  the  porous  stratum  where  it  may  absorb 
water  supplied  either  by  rain  or  by  streams  flowing  over  it;  (4)  an 
inclination  of  the  water-bearing  stratum  so  that  gravity  may  force  the 
water  down;  (5)  a  lack  of  easy  escape  of  the  water  at  lower  points; 
and  (6)  a  sufficient  supply  of  water  to  maintain  the  "  artesian  head." 
The  artesian  water  of  South  Dakota  (Fig.  35),  for  example,  is  derived 
from  a  saturated  sandstone  bed  which  receives  its  water  in  the  Black 


6o  PHYSICAL  GEOLOGY 

Hills  from  the  rain  that  falls  on  it  and  the  streams  that  flow  over  it. 
It  is  covered  by  clays  and  shales  as  it  extends  eastward,  and  when 
borings  are  made  at  elevations  lower  than  its  source  in  the  Black 
Hills,  the  water  rises  and  supplies  wells  even  350  miles  from  this  source. 


FIG.  35.  —  Diagram  showing  the  conditions  favorable  for  artesian  water,  from  the 
Rocky  Mountains  to  eastern  Nebraska.  The  Dakota  sandstone  under  the  imper- 
vious Pierre  clay  carries  water  from  the  Rocky  Mountains  and  supplies  artesian  wells 
on  the  plains.  ( U.  S.  Geol.  Surv. ) 

Artesian  wells  vary  in  depth,  some  being  4000  feet  deep,  while  others 
may  be  less  than  100  feet.  Artesian  water,  both  for  drinking  pur- 
poses and  for  irrigation,  is  of  great  importance.  It  varies  greatly  in 
composition,  some  wells  affording  excellent  water,  while  others  may 
be  so  charged  with  salts  as  to  be  useless  for  drinking  or  irrigation. 

Springs  corresponding  to  artesian  wells  are  formed  if  the  impervious 
bed  overlying  the  porous  bed  is  broken  by  a  fissure  or  fault  (p.  25). 
These  springs  may  be  of  great  volume. 

Chemical  Work  of  Ground  Water.  —  (i)  Solution.  Pure  water  has 
little  power  to  dissolve  the  minerals  of  which  rocks  are  composed,  but 
rain  water  is  seldom  pure  since  it  receives  carbon  dioxide  from  the  air, 
and,  in  passing  through  the  soil,  takes  up  this  and  other  acids  formed 
by  the  decay  of  organic  matter.  It  may  be  heated  in  its  downward 
course  and  is  subjected  to  great  pressure.  Thus  equipped,  its  solvent 
power  is  greatly  increased,  and  in  its  descent  through  the  rocks  it 
carries  away  the  more  soluble  minerals  and  the  cement  of  many  of  the 
rocks,  rendering  them  more  porous  and  causing  their  decay.  At  or 
near  the  surface,  water  is  an  active  agent  in  causing  the  disintegration 
of  the  rocks,  both  by  the  mechanical  work  of  the  frost  and  by  its 
chemical  action. 

(2)  Replacement  and  (3)  Deposition.  — When  ground  water  contains 
much  mineral  matter,  a  slight  change  in  temperature  or  pressure, 
or  a  mingling  with  other  waters  of  a  slightly  different  composition,  may 
cause  the  dissolved  material  to  be  deposited.  This  results  in  replace- 
ment, and  deposition  in  cavities.  (2)  Replacement  results  when  in  its 
descent  ground  water  dissolves  and  carries  away  one  mineral,  deposit- 


THE  WORK  OF  GROUND  WATER 


61 


ing  another  in  its  place.  Shells, 
bones,  and  trees  are  petrified  by 
the  replacement,  molecule  by 
molecule,  of  the  original  substance 
by  mineral  matter.  (3)  Deposi- 
tion occurs  when  minerals  are 
taken  from  the  rock  in  one  place 
and  later  deposited  elsewhere. 
In  this  way  many  metallic  and 
other  veins  (Fig.  36)  are  formed 
(p.  371),  and  loose  sands  and 
clays  are  cemented  into  hard 
rocks.  Besides  this  more  impor- 
tant work,  concretions  (p.  75) 
and  geodes  (p.  78)  are  formed, 
and  in  regions  of  thick  limestone 
cave  deposits  are  built  up  (p.  70). 

Belts  of  Weathering  and  Cem- 
entation. —  The  belt  of  weathering 
extends  from  the  surface  of  the 
ground  to  the  level  of  underground  water  and  is  of  variable  thick- 
ness. In  this  belt  the  greatest  chemical  decomposition  of  rocks 
occurs.  This  work  consists  mainly  in  hydration,  oxidation,  absorp- 
tion of  carbon  dioxide,  and  solution,  and  it  is  here  that  minerals 
with  complex  molecules  are  broken  down  into  simpler  compounds. 
This  belt  is,  therefore,  that  portion  of  the  earth's  crust  which  is  being 
prepared  for  its  ultimate  disintegration  into  soil.  Great  porosity, 
low  temperature,  and  low  pressure  characterize  this  zone. 

The  belt  of  cementation  is  beneath  the  level  of  underground  water. 
In  this  belt,  as  the  name  implies,  deposition  rather  than  solution 
plays  the  leading  part.  The  consolidation  of  sands  and  clays  into 
hard  rock  is  brought  about  here,  both  by  the  deposition  of  minerals 
obtained  by  solution  from  the  belt  of  weathering  and  also  by  the 
pressure  of  the  overlying  rocks.  The  rocks  of  this  deeper  zone  are 
more  or  less  porous  and  fractured,  and  the  temperature  is  compara- 
tively low. 

As  the  surface  of  the  land  is  lowered  by  erosion,  the  belt  of 
weathering  invades  the  belt  of  cementation,  and  the  minerals  which 
were  deposited  in  the  pores  and  cracks  of  the  latter  may  again  be 
dissolved  out. 


\ 


62  PHYSICAL  GEOLOGY 

Desert  Limestone.  —  In  arid  regions  the  underground  water  may,  by  capillarity, 
bring  to  the  surface  large  quantities  of  lime  which,  upon  evaporation,  is  deposited 
as  desert  limestone.  About  Valencia,  Venezuela,  for  example,  the  underlying  rock  is 
almost  entirely  hidden  by  thick  layers  of  this  deposit,  and  extensive  areas  of  New 
Mexico,  Arizona,  and  other  states  are  covered  by  this  limy  incrustation. 

Mechanical  Work  of  Ground  Water. — The  mechanical  work  of 
underground  water  is  important  in  producing  landslides  (p.  73),  but 
aside  from  this  the  effect  is  usually  slight,  since  its  movement  is, 
for  the  most  part,  extremely  slow. 

An  interesting  result  of  the  drying  out  of  underground  water  was  observed  in 
England  at  the  end  of  a  prolonged  drought  in  the  summer  of  1911.  It  was  found  that 
the  foundations  of  hundreds  of  houses  which  rested  on  clay  began  to  settle  after  the 
return  of  the  rains.  In  ordinary  summers  the  clay  is  quite  moist  at  a  depth  of  2.5  to 
3  feet  below  the  surface,  but  during  the  summer  mentioned  it  was  often  dry  at  depths 
of  5  to  6  feet.  The  dry  clay  became  powdery,  and  when  the  autumn  rains  began  the 
water  found  its  way  into  the  fissures  and  washed  out  the  clay,  causing  sliding  and 
lateral  movements. 

SPRINGS 


> 


The  rain  water  which  sinks  into  the  soil  and  rocks  through  joints, 
fissures,  and  pores  usually  issues  once  more  to  the  surface  through 
seepage  and  springs  (Fig.  37). 

Origin  of  Springs.  —  (i)  Springs  commonly  owe  their  existence  to 
the  presence  of  a  stratum  of  pervious  material  overlying  an  impervious 


FIG.  37. — Thousand  Springs,  Snake  River,  Idaho.     (U.  S.  Geol.  Surv.) 


THE  WORK  OF  GROUND  WATER 


one.  The  water  penetrat- 
ing the  pervious  or  frac- 
tured stratum  (Fig.  38, 
A,  By  C)  is  prevented 
from  moving  downward 
through  the  impervious 
layer  whose  slope  it  fol- 
lows until  it  emerges  at 
the  contact  of  the  two 
layers.  (2)  A  second 
class  of  springs  rise 
through  cracks  or  fissures 
(Fig.  40).  These  are 
often  of  great  volume 
and  .may  have  a  tem- 
perature higher  than  the 
springs  of  the  first  type. 
(3)  When  the  surface  of 
a  limestone  region  is 
lowered  by  streams  (Fig. 
39),  an  underground 
stream  is  often  encoun- 
tered and  gives  rise  to 
the  springs  of  great  vol- 
ume which  are  so  fre- 
quent in  such  districts. 
Silver  Spring  in  Florida 
forms  a  navigable  stream 
valleys  at,  or  above,  the 
lowest  part  of  the  valley. 


FIG.  38.  —  Diagrams  showing  the  origin  of  springs. 
In  A  the  porous  stratum  is  indicated  by  dots,  the 
saturated  zone  being  shaded.  B  is  an  impervious 
stratum.  A  spring  (sp)  appears  at  the  left,  and 
during  wet  seasons,  when  the  water  table  is  high,  a 
spring  will  flow  also  from  the  right  of  the  hill. 

In  B  the  impervious  stratum  is  horizontal,  and 
springs  will  flow  from  both  sides  of  the  hill.  If  the 
surface  of  the  saturated  zone  (shaded)  becomes  so 
low  that  it  does  not  reach  the  surface,  the  springs 
will  cease  to  flow. 

C  shows  a  porous  stratum  B  overlain  by  an 
impervious  stratum.  In  such  a  case  the  water  is 
derived  from  the  surface  at  B  and  appears  as  a 
spring  (sp)  at  the  left. 

from  its  source.     Springs  may  flow  into 
thalweg,  which   is    a   line   following   the 


FIG.  39.  —  Large  springs  often  issue  from  the  base  of  limestone  cliffs.     Such  springs 
are  frequently  contaminated,  since  their  water  enters  through  wide  joints  and  sinks 
without  being  filtered  by  soil. 
CLELAND   GEOL.  —  $ 


PHYSICAL  GEOLOGY 


FIG.  40.  —  A  fissure  spring. 


The  oases  of  deserts  often  owe  their  existence  to  springs.     The 
oases  of  Kerid  in  the  northern  Sahara  desert  contain   about  6000 

acres,  which  support  nearly  1,000,000 
date  palms.  They  lie  at  the  foot  of  an 
escarpment  which  forms  the  northern 
boundary  of  the  desert.  From  the  base 
of  this  escarpment  or  cliff,  numerous 
springs  gush  forth  and  furnish  a  constant 
supply  of  water  for  irrigation.  The 
water  of  the  springs  falls  as  rain  in  the  highlands  many  miles  distant. 
After  flowing  as  streams  for  a  short  distance,  the  water  disappears 
in  the  sand.  It  then  follows  underground  courses  until  the  escarp- 
ment is  reached. 

Constant  and  Intermittent  Springs.  —  Whether  springs  are  constant 
or  intermittent  depends  upon  a  number  of  factors :  if  the  rainfall  is 
not  uniformly  distributed  throughout  the  year,  if  the  region  is  not 
forested,  or  if  the  porous  rock  is  too  limited  to  hold  a  sufficient  supjsly 
of  water,  an  intermittent  spring  may  result.  In  such  a  hill^s  that 
shown  in  Fig.  38  A  the  glacial  deposit  (till)  and  sand  allow  the 
water  to  be  absorbed  in  large 
amounts  and  to  sink  to  the 
impervious  stratum  along 
which  ground  water  flows  to 
S^>.  When  the  water  stands 
at  A)  a  spring  may  flow  which 
will  cease  when  the  water  is 
below  that  level.  In  unusu- 
ally dry  seasons  all  may  dis- 
appear. It  is  seldom,  per- 
haps never,  that  a  siphon 
operates  to  form  an  inter- 


FIG.  41.  —  Diagram  illustrating  the  possi- 
bility of  the  occurrence  of  a  siphon  spring 
in  nature.  If  the  vertical  joints  B  and  C  do 
not  reach  the  surface,  the  water  filling  the 
joints  Ay  Ay  A  will  continue  to  flow  as  a  spring 
(Sp)  until  the  joints  are  emptied,  because  the 


not  begin  to  flow  until  the  joints  are  filled  above 
BC.     (Modified  after  De  Martonne.) 


mittent  spring,  but  in  Such  a     weight  of  the  water  in  the  arm  CD  is  greater 

case  as  that  shown  in  Fig.  41     than  in  B.     When  once  emptied  the  water  will 

it  will  be  seen  that  the  water 

will    not    flow    until    it    has 

reached   BC,   after   which   it  will  discharge  until   the   reservoir   is 

empty.     This  is  due  to  the  fact  that  the  weight  of  the  water  in  the 

arm  CD  is  greater  than  that  of  the  arm  B. 

Mineral  Matter  in  Spring  Water.  —  Since  springs  are  derived  from 
underground  water  which  has  been  in  close  contact  with  various  rocks, 


THE  WORK  OF  GROUND  WATER 


they  usually  contain  a  much  greater  quantity  of  dissolved  minerals 
than  do  streams.  Silver  Spring  in  Florida  is  carrying  to  the  sea  in 
solution  340  pounds  of  mineral  matter  a  minute,  or  600  tons  a  day,  and 
it  is  estimated  that,  in  central  Florida,  a  little  more  than  400  tons  of 
rock  a  square  mile  are  annually  carried  away  in  solution.  This  would 
be  equivalent  to  a  lowering  of  the  surface  of  the  central  peninsular 
section  of  Florida  by  solution  alone  at  a  rate  of  one  foot  in  five  or  six 
thousand  years. 

Falls  Creek,  Oklahoma  (Fig.  71),  receives  water  from  springs  which 
contain  much  lime  carbonate.  In  the  immediate  vicinity  of  the  springs, 
however,  no  deposits  are  formed,  as  there  is  a  sufficient  amount  of 
carbon  dioxide  present  in  the  water  to  hold  the  lime  in  solution,  but  by 
the  time  the  stream  has  flowed  a  quarter  of  a  mile  large  quantities 
of  carbon  dioxide 
have  been  given  off, 
and  travertine  is  de- 
posited in  the  bed  of 
the  stream  in  the 
form  of  dams  which 
vary  in  height  from  a 
few  inches  to  15  feet, 
and  are  being  built 
up  faster  than  the 
stream  can  cut  them 


away. 

The     great     lime- 
stone     deposits      at 


FIG.  42.  —  Block  diagram  showing  the  formation  of  a 
travertine  terrace  and  natural  bridge.  Water  containing 
much  lime  carbonate  emerges  from  springs  in  the  lime- 
stone at  the  right.  Travertine  has  been  rapidly  de- 
Tivoll  in  Italy,  from  posited,  forming  the  terrace  and  natural  bridge, 
which  was  quarried 

much  of  the  stone  used  in  the  construction  of  the  Coliseum  and 
St.  Peter's  at  Rome  and  the  interior  of  the  Pennsylvania  railroad 
station  in  the  City  of  New  York,  were  laid  down  by  springs.  The 
quantity  and  rapidity  of  the  deposition  of  limestone  under  excep- 
tionally favorable  conditions  is  well  shown  in  the  great  travertine 
natural  bridge  at  Pine,  Arizona,  more  than  125  feet  high,  which, 
together  with  a  terrace  of  25  acres,  was  formed  by  such  a  deposit 
(Fig.  42).  Springs  containing  lime  carbonate  or  gypsum  in  solution 
are  called  "  hard,"  since,  in  washing,  the  fatty  acids  of  the  soap  unite 
with  the  dissolved  minerals  to  form  the  insoluble  "  curd." 

By  abstracting  carbon  dioxide  from  the  water  in  which  they  grow 


66  PHYSICAL  GEOLOGY 

algae  may  cause  lime  to  deposit.  In  this  way  beds  of  so-called 
"  petrified  moss,"  more  than  ten  feet  thick,  have  been  formed. 
In  the  Yellowstone  National  Park  the  deposits  about  the  geysers 
were  built  up  both  by  the  evaporation  of  the  water  and  by  algae 
(p.  65).  A  reduction  in  temperature  and  pressure  may  also  cause 
minerals  in  solution  to  be  deposited,  as  may  also  the  mingling 
of  waters  carrying  in  solution  substances  of  different  composition 

(p.  372). 

Mineral  Springs.  —  Mineral  springs  contain  various  salts  or 
gases.  Such  springs  are  often  called  "  medicinal  "  because  of  their 
supposed  curative  properties.  The  total  value  of  mineral  waters 
is  large,  amounting  to  $5,631,391,  in  1913,  in  the  United  States 
alone. 

Temperature  of  Springs.  —  The  temperature  of  springs  is  usually 
much  lower  than  that  of  the  air  in  summer,  being  about  47°  F.  in 
Connecticut;  and  the  water  is  often  described  as  being  "  icy  cold/' 
The  temperature  of  such  springs  in  middle  latitudes  is  fairly  constant 
if  they  come  from  a  depth  greater  than  50  or  60  feet,  since  at  this 
depth  the  water  is  not  affected  by  daily  or  seasonal  changes  and  has, 
consequently,  about  the  average  temperature  of  the  region. 

Thermal  Springs.  —  The  temperature  of  many  so-called  hot 
springs  varies  from  lukewarm  to  boiling,  (i)  The  heat  is  sometimes 
due  to  the  presence  of  deep  fissures  through  which  the  surface  water 
has  percolated  until  it  has  reached  great  depths,  where  its  temperature 
has  been  raised  by  the  interior  heat  of  the  earth.  After  being  thus 
heated,  the  water  is  forced  by  hydrostatic  pressure  to  a  point  on  the 
surface  which  is  lower  than  the  point  of  ingress.  The  depth  from 
which  come  springs  like  those  of  Bath,  England,  which  have  a  tem- 
perature of  120°  F.,  may  be  approximately  told  from  well  borings,  such 
as  that  of  a  well  at  Berlin,  Germany,  the  water  of  which  has  a  tem- 
perature of  110.5°  F.  at  a  depth  of  3390  feet.  Springs  located  along 
fissures  in  the  earth's  crust  occur  in  Virginia,  Arkansas,  Colorado, 
Nevada,  and  South  Dakota,  and  are  often  the  seat  of  popular  health 
resorts.  (2)  The  water  of  some  springs  is  heated  by  chemical  action. 
(3)  Water  in  volcanic  regions  may  be  heated  at  comparatively  shallow 
depths  by  the  presence  of  uncooled  lava.  Of  this  class  there  are  more 
than  3000  in  the  Yellowstone  National  Park,  some  of  which  deposit 
limestone  (travertine)  and  others  silica  (geyserite).  Hot  springs  may 
bring  about  a  considerable  change  in  the  character  of  the  rocks  in  the 
regions  in  which  they  occur,  by  causing  the  disintegration  of  some  and 


THE  WORK  OF  GROUND  WATER 


by  adding  new  material  to  others.     This  is  due  to  the  fact  that  hot 
water  is  a  more  powerful  solvent  than  cold. 

Geysers.  —  Geysers  are  springs  which  intermittently  erupt  col- 
umns of  hot  water  and  steam  (Fig.  43).  They  occur  in  regions  of 
comparatively  recent  volcanic  activity,  where  the  lava  is  hot  at  a 
relatively  shallow  depth.  They  are  well  developed  in  but  three  lo- 
calities in  the  world,  and  the  total  area  occupied  by  them  is  probably 
less  than  ten  square  miles.  The  most  notable  geysers  occur  in  Ice- 
land, New  Zealand,  and  the  United  States,  although  smaller  ones  are 
to  be  seen  in  Mexico,  Tibet, 
the  Azores,  and  the  island 
of  Formosa.  Some  of  them 
throw  water  to  a  great  height. 
The  Monarch  Geyser  in  New 
Zealand  became  active  in 
1903  and  is  said  to  have 
thrown  mud  and  stones  to  a 
height  of  1000  feet.  Such  a 
height,  however,  is  unique. 
In  the  Yellowstone  National 
Park  an  eruption  throwing 
water  300  feet  vertically  is 
rare. 

The  quantity  of  water 
flowing  from  geysers  varies 
greatly :  in  the  smaller  ones 
it  may  be  only  a  few  gallons 


FIG.  43.  —  Lone  Star  Geyser,  Yellowstone 
National  Park. 


an  hour,  while  in  others,  as 
in  Old  Faithful  in  the  Yellowstone  National  Park,  the  discharge  may 
be  as  great  as  750,000  gallons  an  hour,  a  quantity  sufficient  to  supply 
a  city  of  150,000  inhabitants.  The  water  of  geysers  is  rain  water 
which  has  percolated  through  porous  lava,  and  under  normal  condi- 
tions would  be  discharged  as  springs.  Consequently,  if  the  climate 
of  the  Yellowstone  National  Park  should  become  arid,  the  geysers 
would  disappear.  This  water,  heated  by  its  passage  through  the 
lavas,  dissolves  soda  and  potash,  becoming  alkaline  and  thus  capable 
of  dissolving  silica  from  the  silicates  of  the  lavas.  Accordingly, 
the  waters  erupted  by  geysers  contain  much  mineral  matter  in  solu- 
tion, the  chief  of  which  is  silica.  This  silica  is  deposited  about  the 
openings  of  the  springs  as  siliceous  sinter,  or  geyserite,  forming  a 


68 


PHYSICAL  GEOLOGY 


Observed 


mound  both  by  evaporation  and  also  through  the  action  of  minute 
plants  (algae)  which  are  capable  of  living  in  hot  water  and  of  secreting 
silica.  It  is  stated  that  by  evaporation  alone  a  geyser  can  produce  a 
maximum  thickness  of  geyserite  of  one  twentieth  of  an  inch  a  year, 
while  the  increase  from  algae  deposition  under  favorable  conditions 
may  be  as  much  as  eight  inches  during  the  same  period. 

A  geyser  usually  originates  as  a  spring  in  a  fissure,  the  opening  of 
which  is  gradually  built  up  by  the  deposition  of  siliceous  sinter  until 
a  considerable  mound  or  terrace  is  formed.  As  long  as  the  tube 
through  which  the  water  reaches  the  surface  is  short  or  the  circulation 
of  the  water  unimpeded,  a  siliceous  spring  will  flow.  When,  as  a 
result  of  the  building  up  of  the  mound  or  for  other  reasons,  the  tube 

becomes  so  long  that  the  water  can- 
not circulate  with  rapidity  (Fig.  44), 
the  water  at  some  distance  below  the 
top  of  the  tube  will  increase  in  tem- 
perature more  rapidly  than  that  at 
the  surface.     Eventually  water  at  a 
depth  of  a  number  of  feet  will  reach 
its   boiling  point  with  the  resultant 
formation  of  bubbles  of  steam  which, 
in  turn,  will  cause  the  .water  to  spill 
FIG.  44.  — Cross  section  of  a  geyser,    over  the  edge  of  the  opening.     This 
showing  the   boiling   temperature  at     overflOw  promotes   boiling  by  reduc- 
the  right  and  the  recorded  tempera-     .          .  . 

ture  at  the  left.    (After  Campbell.)       »ng  the  pressure  upon  the  water  deep 

in    the   tube.     As    a    consequence    a 

large  quantity  of  water,  which  was  not  quite  at  the  boiling  point 
because  of  the  weight  of  the  overlying  column  of  water,  will  instantly 
burst  into  steam  and  will  eject  the  overlying  water  from  the  tube, 
sometimes  to  a  great  height.  Usually  the  eruptions  are  not  regular, 
but  in  Old  Faithful  an  eruption  can  be  predicted  at  intervals  of 
about  sixty  minutes.  When  a  quantity  of  soap  or  lye  is  thrown 
into  a  geyser,  the  viscosity  of  the  water  is  increased  and  its  circula- 
tion correspondingly  lessened.  In  this  way  an  eruption  may  be 
hastened.  As  the  lavas  cool,  the  geysers  must  necessarily  disappear. 
However,  the  loss  of  heat  is  very  slow,  as  is  shown  by  the  fact  that, 
although  careful  records  have  been  kept  since  the  Yellowstone  basin 
was  discovered,  the  Yellowstone  geysers  have  shown  little  sign  of 
change  since  they  were  first  studied.  The  eruptions  of  Old  Faithful, 
for  example,  continue  to  be  regular. 


THE  WORK  OF  GROUND  WATER 


69 


STRIKING  EFFECTS  OF  GROUND  WATER 

Swallow  Holes.  —  In  limestone  regions  it  is  not  unusual  to  find 
many  funnel-shaped  depressions  in  the  surface  of  the  ground  into 
which  water  may  flow.  These  are  called  "  sink "  or  "  swallow 
holes "  and  may  be  very  conspicuous  features  of  the  landscape 
(Fig.  45).  They  are  formed  either  (i)  through  direct  solution 
by  surface  waters  along  joints,  in  which  case  they  are  usually 
more  or  less  circular  in  outline;  or  (2)  by  the  falling  in  of  the 

roof    of     a     cavern, 

when  they  are  often 

irregular   in   outline. 

Those  formed  in  the 

first   way   are    much 

more   common    than 

the    latter,    but    are 

usually  smaller.     An 

example  of  sink  holes 

formed  by  the  falling 

in   of  a   cavern   roof 

occurred  in  the  city 

of  Staunton,  Virginia, 

in    1910,   when    four 

"cave-ins"  occurred 

within    three  weeks, 

the  largest  of  which 

was  60  by  90  feet.     During  the  formation  of  this  largest  one  three 

trees  and  portions  of  a  dwelling  house  were  engulfed.     (3)  In  regions 

underlain  by  salt,  local  sinkings  result  from  the  solution  and  removal 

of  the  salt  by  underground  water. 

After  a  swallow  hole  is  formed,  more  or  less  of  the  material  imme- 
diately around  the  hole  will  be  carried  in  by  surface  wash.  More- 
over, a  large  amount  of  water  entering  through  the  sink  may  cause  a 
rapid  solution  of  the  limestone  in  its  immediate  vicinity,  resulting  in 
the  formation  of  large  basins  locally  called  "  prairies  "  or  "  coves."  In 
the  United  States  these  are  well  developed  in  Kentucky  and  Florida. 

If  the  bottoms  of  swallow  holes  become  choked,  small  lakes  or  pools 
come  into  existence.     A  striking  example  is  shown  in  the  history  of 
Alachua  Lake,1  Florida  (Fig.  46).     Previous  to  1871  the  waters  of 
1  Florida  Geol.  Surv.,  Third  Annual  Report,  1910,  pp.  62-67. 


FIG.  45.  —  Small  swallow  or  sink  holes  in  the  Juras, 
Switzerland. 


70  PHYSICAL  GEOLOGY 

the  principal  stream  of  this  region  emptied  into  a  sink  or  swallow 
hole  in  the  Alachua  prairie.  By  the  choking  of  this  outlet  a  lake  was 
formed  which,  at  its  greatest  extent,  was  eight  miles  long  and  in  one 
place  four  miles  wide  and  of  sufficient  depth  to  permit  a  number  of 


FIG.  46. — Diagrammatic  section  from  Devil's  Mill  Hopper  (northwest  of  Gainesville) 
to  Alachua  sink,  Florida.  The  Devil's  Mill  Hopper  is  115  feet  deep  but  does  not 
quite  reach  the  level  of  underground  water,  D— E.  B  is  Alachua  sink,  whose  bottom  is 
filled  with  water  to  or  a  little  above  the  level  of  underground  water.  C  is  a  small  sink 
above  the  water  table,  which  does  not  contain  water.  If  the  opening  at  the  bottom 
of  the  Devil's  Mill  Hopper  becomes  clogged,  the  sink  will  fill  up  to  the  surface  and 
become  one  of  the  deep,  small,  circular  lakes  frequently  found  in  the  region.  (Modified 
after  Sellards.) 

freight  steamers  to  ply  upon  it.  After  existing  for  about  twenty 
years  the  underground  passage  from  the  swallow  hole  was  opened 
again,  and  the  lake  gradually  disappeared. 

Caverns.  —  Caverns  occur  in  limestone  regions  and  are  usually 
connected  with  swallow  holes  by  more  or  less  distinct  passages. 
They  have  been  formed,  with  few  exceptions,  by  the  solvent  power 
of  the  water  which  poured  through  the  swallow  holes  and  joints,  or 
seeped  through  the  rocks  from  the  surface,  and,  to  some  extent, 
through  abrasion  by  the  sediment  carried  by  the  subterranean 
streams.  Since  water  circulates  most  rapidly  along  joints  and  bedding 
planes,  it  is  in  such  positions  that  most  rapid  solution  takes  place,  and 
it  is  here  that  caverns  occur.  In  certain  spots,  owing  to  the  presence 
of  numerous  open  joints  or  to  the  solubility  of  the  rock,  large  domes 
are  formed.  Solution  is  usually  most  effective  in.  forested  regions, 
since  the  humus  affords  a  large  and  constant  supply  of  carbon  dioxide, 
without  which  water  is  but  slightly  solvent.  Caves  may,  however,  be 
formed  by  carbonated  waters  ascending  from  below;  an  example  of 
which  is  the  interesting  and  extensive  Wind  Cave  in  the  Black  Hills 
of  South  Dakota,  which  was  formed  by  hot  water  coming  up  from  a 
great  depth  and  gradually  enlarging  the  joints  and  fissures  in  its 
ascent. 

In   regions  of  thick   limestone,  caves   at   different   levels,   called 


THE  WORK  OF  GROUND  WATER  71 

"  galleries,"  occur  (Fig.  47) ;  as,  for  example,  in  Mammoth  Cave, 
Kentucky.  These  galleries  are  the  result  (i)  of  the  presence  of  layers 
of  relatively  insoluble  rock  upon  which  the  underground  streams  flow 
until  they  dissolve  and  erode  out  a  wide  passage.  If  this  layer  is 
worn  through  after  a  time,  or  a  joint  is  enlarged,  permitting  the  water 
to  reach  a  lower  soluble  layer,  it  may  descend  until  a  second  relatively 


FIG.  47.  —  Diagram  showing  the  formation  of  the  galleries  of  limestone  caves  by 
the  lowering  of  the  valley  (indicated  by  dotted  lines)  to  which  the  underground  water 
dissolving  them  flowed. 

insoluble  layer  is  encountered.  If  this  process  is  repeated,  several 
galleries  will  result.  The  lowest  level  at  which  caves  may  be  formed 
is  that  of  the  lowest  surface  stream  into  which  the  underground  water 
is  discharged.  (2)  Migration  from  one  level  to  another  may  also 
result  from  the  intermittent  lowering  of  the  valleys  (Fig.  47)  of  the 
surface  streams  into  which  the  underground  waters  of  the  caverns 
flow.  The  galleries  of  caves  may  divide  and  reunite,  forming  a 
network  of  channels  at  the  different  levels.  It  has  been  estimated 
that  in  Kentucky  alone  there  are  100,000  miles  of  underground 
passages. 

Natural  Bridges  may  be  formed  by  the  partial  caving  in  of  the  roofs 
of  caverns,  or  by  the  enlarging  of  two  swallow  holes  opening  to  the 
same  underground  stream.  Natural  bridges  are  also  formed  in  other 
ways  (pp.  91,  112). 

^--  Cave  Deposits.  —  After  a  cave  has  been  abandoned  by  the  stream 
which  formed  it,  the  water  entering  is  confined  chiefly  to  small  seepage. 
At  this  stage  much  of  the  water  is  removed  by  evaporation  so  that 
solution  gives  place  to  deposition.  The  deposits  in  caves  are  usually 
in  the  form  of  stalactites  and  stalagmites.  The  former  begin  as  a  thin 
film  of  lime  around  the  outside  of  a  drop  of  water  which  evaporates 
on  the  roof  of  a  cavern.  Upon  this  additional  lime  is  left  by  other 
drops  until  a  stalactite,  resembling  an  icicle,  is  suspended  from  the 
roof  of  the  cavern.  The  accumulations  of  lime  which  form  where  the 


PHYSICAL  GEOLOGY 


FIG.  48.  —  Stalactites  and  stalagmites  in  Marengo  Cave,  Indiana.     (D.  Appleton 

and  Company.) 

water  evaporates  on  the  floor  of  the  cavern  are  known  as  stalagmites 
(Fig.  48).  By  the  union  of  the  stalactites  and  stalagmites  pillars  are 
formed. 

Caverns  are  also  formed  in  other  ways  (pp.  209,  298),  but  the  great 

majority  are  formed 
from  solution. 

Karst.  —  Karst  is 
used  as  a  descriptive 
term  for  any  lime- 
stone region  which 
has  been  etched  and 

eroded  by  water  into 
FIG.  49.— Block  diagram  of  a  karst  (limestone)  region,  ,  f         ,F; 

illustrating  the  effect  of  solution.     Sink  holes  AAA  drain  a  rOUSh  surtace  (*  !f 

the  surface  and   discharge  their  water  through   under-  49)-        The     name    IS 

ground  channels  to  an  open  valley.     Surface  streams  are  derived    from    Karst 
lacking,  and  the  main  valley  has  steep  sides.     The  spring  ,  'A         f 

at  C  may  be  very  large.     A  fault  is  also  shown  at  C. 

(Modified  after  De  Martonne.)  the    Adriatic,    where 


THE   WORK  OF  GROUND  WATER 


73 


such  a  surface  is  developed  upon  a  nearly  pure  limestone.  It  is  a 
desolate  region  in  which  vegetation  is  scanty,  except  in  swallow 
holes  (dolines),  where  the  small  amount  of  insoluble  matter  yielded 
by  the  rock  accumulates  and  furnishes  a  soil  for  plants.  The  drain- 
age is,  for  the  most  part,  subterranean;  and  the  surface  is  etched 
out  into  a  network  of  narrow  channels  between  which  blade-like 
masses  of  rock  rise.  It  is  pitted  with  swallow  holes  and,  where 
important  streams  cross  the  karst  land,  they  flow  in  deep  gorges, 
rather  than  in  ordinary  valleys. 

Landslides.  —  Landslides  may  result  from  a  number  of  conditions, 
one  of  which  is  often  associated  with  underground  water.  Soil  and 
subsoil  tend  to  move 
down  a  hillside  when 
they  become  charged 
with  water.  If  this 
movement  is  insensible 
it  is  called  "  creep  "  ;  if  FIG.  50.  —  Conditions  favoring  landslides.  The 

sensible      "  slumping  "    strata  AC  and  BD  are  clay  or  shale  wnich>  wnen  wet> 

!  .  are  slippery,  so  that  sliding  is  likely  to  occur. 

or     sliding.        Railroad 

tracks  may  be  gradually  moved  down  hill  and  trees  be  tilted  by  the 
slow  movement  of  hillside  creep. 

The  conditions  favorable  for  a  landslide  are  a  steep  slope  upon 
which  soil  rests,  or  steeply  dipping  rock  which  has  been  undercut 
at  the  base,  artificially  or  by  streams,  so  that  the  upper  layers  are 
unsupported  (Fig.  50).  When,  under  either  of  these  conditions,  the 

soil  or  rock  becomes 
saturated  with  water, 
its  weight  is  increased, 
and,  moreover,  the 
water,  acting  as  a 
lubricant,  lessens  the 
friction  which  pre- 
viously prevented 

FIG.  51.  —  Diagram  showing  a  valley  which  has  been  the  soil  or  rock  from 
deepened  by  glacial  erosion,  leaving  steep  slopes  unsup-  sliding.  Such  was 


the  cause  of  the  Mt. 


ported  on  each  side.     Fractures  may  develop  at  AB,  and 
a  portion  of  the  side  may  slide  into  the  valley. 

Greylock,  Massachu- 
setts, landslide,  in  which  a  great  mass  of  soil  and  glacial  debris  slid 
down  the  steep  mountain  side  after  a  period  of  excessive  rainfall; 
and  of  the  landslide  in  Quebec,  where  the  rock  hillside  slipped 


74 


PHYSICAL  GEOLOGY 


FIG.  52.  —  Landslide,  Turtle  Mountain,  British 
Columbia.     (Photo.  Hopkins.) 


along  a  plane  of 
steeply  dipping  slate 
which  had  been  lubri- 
cated by  underground 
water. 

In  mountainous  re- 
gions, where  the  val- 
leys are  deep  and  the 
slopes  steep,  condi- 
tions are  extremely 
favorable  for  land- 
slides (Figs.  51,  52). 
One  of  the  most  de- 
structive of  such  slides 
occurred  on  the  Ross- 
berg  in  Switzerland 

in  1806.  Here  the  rocks  high  up  on  the  mountain  slid  suddenly  into 
the  valley,  burying  the  village  of  Goldau  and  causing  the  death  of 
several  hundred  people.  Masses  of  rock,  some  of  which  were  as 
large  as  houses,  were  spread  over  the  valley  for  two  or  three  miles. 
Evidences  of  many  prehistoric  landslides  are  to  be  seen  in  Switzer- 
land, as  well  as  in  other  mountainous  regions.  At  Siders  a  land- 
slide is  spread  out  for  several  miles  across  the  Rhone  valley,  and 
some  of  the  hills  formed  from  the  material  of  the  slide  are  almost 
200  feet  high.  So 
marked  is  this  land- 
slide topography  that 
it  forms  the  boundary 
between  the  French 
and  German-speak- 
ing people  in  the 
Rhone  valley. 

Conditions  favor- 
able for  landslides 
were  created  artifici- 
ally in  the  excavation 
of  the  Culebra  Cut 
in  the  Panama  Canal, 


where  the   rock  will 
continue  to  slide  peri- 


FIG.  53.  —  A  lake  in  eastern  France  formed  by  a 
landslide.  The  character  of  the  material  of  the  dam  is 
shown  in  the  foreground. 


THE   WORK  OF  GROUND  WATER 


75 


odically  until  a  gentle  slope  is  formed.  During  a  single  year  (1911) 
nearly  36  per  cent,  of  the  total  material  excavated  had  been  brought 
in  by  landslides. 

Landslides  may  dam  streams,  forming  lakes  (Fig.  53)  or  rapids. 
Lake  Oechenen,  in  the  Kandersteg  valley  of  Switzerland,  and  the  Cas- 
cades of  the  Columbia  River  were  formed  by  landslides  broken  from 
the  high  mountains  a  few  centuries  ago.  The  rounded  hills  and  basins 


FIG.  54.  —  Landslide  topography  which  has  much  the  appearance  of  a  moraine. 
Kandersteg  valley,  Switzerland. 

sometimes  produced  by  landslides  are  very  similar  in  appearance  to 
those  formed  by  glaciers  (Fig.  54).  Moreover,  the  heterogeneous  clays 
and  angular  bowlders  of  which  they  are  composed  resemble  glacial 
debris.  The  rocks  of  landslides,  however,  instead  of  being  scratched, 
as  is  true  of  glacial  bowlders,  often  show  impact  marks ,  formed  by  the 
striking  of  one  rock  against  another  in  their  violent  descent  down  the 
mountain  side. 

CONCRETIONS 

Although  concretions  are  usually  of  little  geologic  importance, 
they  occur  so  frequently  in  the  rocks  and  sediments  of  the  earth 
and  excite  so  much  interest  that  they  deserve  some  attention  (Fig.  55). 


76 


PHYSICAL  GEOLOGY 


Concretions  are  masses  varying  greatly  in  shape  and  in  size  from  less 
than  a  pinhead  to  more  than  10  feet  in  diameter,  and  are  formed  by 
the  gradual  segregation  of  mineral  matter.  The  shape,  as  has  been 
said,  varies  greatly.  Some  concretions  are  spherical,  some  are  flat, 
and  others  curved.  The  odd  shapes  which  resemble  animals  (Fig.  55) 
are  usually  produced  by  the  growth  of  two  or  more  concretions  until 
they  join.  The  center  of  attraction  may  be  a  fossil  or  a  bit  of  mineral, 


FIG.  55.  —  Clay-stone  concretions  of  various  shapes.     They  are  composed  largely 
of  lime  carbonate  and  occur  in  clay. 

but  in  the  majority  of  specimens  no  nucleus  can  be  detected.  In 
some  formations  (for  example,  the  Arikaree,  Miocene,  in  Nebraska) 
they  may,  by  their  abundance,  so  strengthen  the  loose  sands  and  clays 
containing  them  as  to  form  a  resistant  bed  which  stands  as  cliffs 
wherever  cut  by  streams. 

Concretions  usually  occur  in  definite  beds  in  a  formation,  and  it 
is  sometimes  possible  to  trace  such  beds  for  several  miles.  They 
occur  in  rocks  of  every  age,  from  the  most  ancient  to  those  now 
forming  on  the  bottoms  of  lakes  and  seas. 


THE  WORK  OF  GROUND  WATER 


77 


Composition  of  Concretions.  —  Concretions  are  seldom  of  the  same  composition  as 
the  containing  rock;  those  occurring  in  limestone  are  apt  to  be  of  silica;  in  clays  and 
shales,  of  lime  or  iron  carbonate;  in  sand  and  sandstones,  of  iron  oxide  or  lime  carbonate. 
Lime  concretions,  or  clay  stones,  are  probably  more  abundant  than  any  others. 

When  concretions  of  limestone  and  iron  carbonate  (clay  ironstones)  are  much 
cracked  in  the  interior  and  the  cracks  filled  with  calcite  or  quartz,  they  are  called 
septaria  (Fig.  56).  In  sandstone  iron  concretions  of  two  kinds  may  occur:  "  spherical," 
in  which  a  spherical  shell  surrounds  a  core  of  sand,  and  "  pipestem,"  which,  as  the 
name  implies,  are  cylindrical.  The  former 
are  probably  formed  as  the  result  of  the 
chemical  change  of  some  iron  mineral  in 
the  rock,  such  as  pyrite,  which  renders 
the  latter  soluble.  After  being  thus 
changed,  "  it  spreads  outward  as  a  drop 
of  ink  does  on  blotting  paper.  Evapora- 
tion takes  place  around  the  outer  margin 
of  the  solution,  iron  oxide  is  precipitated, 
and  the  first  ring  or  shell  is  formed." 
(J.  Geikie.)  Pipestem  concretions  are 
formed  where  soluble  iron  compounds  are 
oxidized  about  the  tubes  produced  by 
the  roots  of  plants. 

It  is  probable  that  certain  masses  of 
gravel  in  southern  California  which  now 


stand  up  as  hills  have  been  cemented 
together  by  a  kind  of  concretionary 
i 


FIG.  56.  —  A  polished  section  of  a  sep- 
tarium.     The  white  veins  are  calcite,  the 
darker  portions  chiefly  lime  carbonate. 
action.1 

The  flint  nodules  that  are  so  abundant  in  the  chalk  of  southeastern  England  some- 
times had  their  beginnings  in  sponges  which  secreted  a  siliceous  skeleton,  and  in  other 
fossils.  Upon  this  small  quantity  of  silica  as  a  center,  other  silica  taken  from  the  sea 
water  was  added  to  form  the  nodular  flints.  Since  by  a  microscopic  examination  the 
structure  of  the  chalk  in  which  the  nodules  lie  can  be  traced,  it  is  evident  that  the 
flint  nodules  were  formed  in  the  chalk  mud  of  the  ocean  floor,  rather  than  on  top  of 
these  sediments. 

Time  of  Formation.  —  Lime  concretions  or  clay  stones  are  formed  by  the  gradual 
accumulation  of  lime  carbonate,  and  during  their  growth  they  inclose  portions  of  the 
sediments  in  which  they  lie.  They  are  often  formed  before  the  rock  containing  them 
is  hardened  (indurated),  as  is  shown  by  the  facts  that  (i)  they  are  often  cut  by  joints 
and  (2)  when  they  contain  fossils,  these  remains  are  seldom  flattened  by  the  pressure 
of  the  overlying  rocks  as  are  those  in  the  surrounding  shale.  Although  many  of  the 
concretions  which  occur  in  sedimentary  rocks  were  formed  while  they  were  in  an  un- 
consolidated  state  and  before  they  were  deeply  buried,  there  is  no  doubt  that  some  were 
formed  after  the  sediments  had  been  consolidated  into  rock. 

Oolitic  Limestone  (Greek,  oon,  egg,  and  lithos,  a  stone),  so-called  be- 
cause of  its  resemblance  to  fish  roe,  may  be  almost  completely  corn- 


Arnold,  R., — Jour.  Geol.,  1907,  Vol.  15,  pp.  560-570. 


PHYSICAL  GEOLOGY 


posed  of  minute  concretions  (Fig.  57).    Limestone  of  this  origin  (p.  249) 
is  often  widespread  and  many  feet  in  thickness.     It  is,  however,  held 

by  some  investigators  that  the 
most  of  the  oolitic  limestone  is 
the  product  of  microscopically 
small  algae  (plants)  capable  of 
secreting  lime. 

Geodes.  —  Geodes  differ  from 
concretions  in  that  they  are 
formed  in  cavities  of  the  rock 
and  from  without  inward  (Fig. 
58).  When  lava  contains  steam 
cavities,  silica  may  be  deposited 
on  the  walls  of  the  cavities  and, 
by  slow  addition,  may  in  time 
fill  them.  In  this  way  agates 
are  formed,  the  colored  layers 
of  which  are  due  to  coloring 
matter  carried  in  and  deposited 
with  the  silica.  Other  geodes 
are  formed  by  the  force  of 
crystallization  in  the  following 
way :  if  silica  begins  to  crystallize  in  the  cracks  of  a  crushed  fossil 
embedded  in  a  rock,  —  a  shell,  for  example,  —  the  fragments  of  the 
shell  may  be  forced  farther  and  farther  apart  by  the  force  of  crys- 


FIG.   57.  —  A  hand   specimen  of  oolitic 
limestone.     (U.  S.  National  Museum.) 


FIG.  58,  —  A  geode  broken  in  two.     Cheyenne  River,  South  Dakota. 


THE  WORK  OF  GROUND  WATER 


79 


tallization  until  a  hollow  sphere,  many  times  larger  than  the  original 
fossil,  may  result,  lined  with  crystals.  In  some  geodes  of  this  sort 
the  fragments  of  the  fossil  may  be  entirely  dissolved  away. 

REFERENCES  FOR  UNDERGROUND  WATER 

SPRINGS 

BOWMAN,  I.,  —  Forest  Physiography,  pp.  41-61. 

DE  MARTONNE,  E.,  —  Geographic  Physique,  pp.  342-347. 

FULLER,  M.  L., —  Occurrence  of  Underground  Waters:    Water-Supply   Paper,  U.S. 

Geol.  Surv.  No.  114,  1905,  pp.  18-40. 
GREGORY,    H.    E., —  Underground  Water   Resources  of    Connecticut:    Water-Supply 

Paper,  U.  S.  Geol.  Surv.  No.  232,  1909,  pp.  60-76. 
SLIGHTER,  C.  S.,—  The  Motions  of  Underground  Water:    Water-Supply  Paper,  U.  S. 

Geol.  Surv.  No.  67,  1902. 
WEED,  W.  H.,  —  Formation  of  Travertine  and  Siliceous  Sinter  by   Fe  gelation  of  Hot 

Springs:  Ninth  Ann.  Rept.,  U.  S.  Geol.  Surv.,  1889,  pp.  613-676. 
WOODWARD,  H.  B.,  —  The  Geology  of  Water  Supply,  pp.  79-95. 

ARTESIAN  WELLS 

• 

CHAMBERLIN,  T.  C.,  —  Artesian  Wells:  Geology  of  Wisconsin,  Vol.  I,  1883,  pp.  689- 

701. 
CHAMBERLIN,  T.  C.,  —  The  Requisite  and  Qualifying  Conditions  of  Artesian  Wells: 

Fifth  Ann.  Rept.,  U.  S.  Geol.  Surv.,  1885,  pp.  125-173. 
DARTON,  N.  H., — Geology  and  Underground  Water  Resources  of  the  Central  Great  Plains : 

Professional  Paper  No.  32,  U.  S.  Geol.  Surv.,  1905,  pp.  190-372. 

KARST 

DE  MARTONNE,  E.,  —  Geographie  Physique,  pp.  462-472. 

GEIKIE,  J.,  —  Earth  Sculpture,  pp.  266-277. 

KATZER,  F.,  —  Karst  und  Karsthydrographie,  1909. 

KNEBEL,  W.  V.,  —  Hohlenkunde  mil  Beriicksichtigung  der  Karstphdnomene,  1906. 

CAVES 

BLATCHLEY,  W.  S.,  —  Indiana  Caves   and  their  Fauna:    Twenty-first  Ann.   Rept., 

Ind.  Geol.  Surv.,  1897,  pp.  121-212. 
HOVEY,  H.  C.,  —  Celebrated  American  Caverns. 
M  ARTEL,  E.  A.,  —  Les  Abimes. 
MATSON,  G.  C.,  —  Water  Resources  of  the  Blue  Grass  Region:    Water-Supply  Paper, 

U.  S.  Geol.  Surv.  No.  233,  1909. 
SHALER,  N.  S.,  —  Aspects  of  the  Earth,  pp.  98-142. 

CONCRETIONS  AND  GEODES 

ARNOLD,  R.,  —  Dome  Structure  in  Conglomerate:  Jour.  Geol.,  Vol.  15,  1907,  pp.  560- 

570. 
GEIKIE,  J.,  —  Structural  and  Field  Geology,  pp.  120-124. 

CLELAND   GEOL. — 6 


8o  PHYSICAL  GEOLOGY 

GRABAU,  A.  W.,  —  Principles  of  Stratigraphy,  pp.  467-475;  718-721. 

GRATACAP,   L.   P.,  —  Opinions  upon    Clay  Stones  and   Concretion :  Am.  Naturalist, 

Vol.  18,  1884,  pp.  882-892. 

MERRILL,  G.  P.,  —  Rocks,  Rock- feathering,  and  Soils,  pp.  35-37. 
NICHOLS,  H.  W.,  —  New  Forms  of    Concretions:    Field  Columbian  Museum,  Geol. 

Series,  Vol.  3,  No.  3,  1906,  pp.  25-54. 
SHELDON,  J.  M.  C.,  —  Concretions  from  the  Clay  Stones  of  the  Connecticut  Valley. 

GENERAL 

HOBBS,  W.  H.,  —  Earth  Features  and  their  Meaning,  pp.  180-194. 
RIES  AND  WATSON,  —  Engineering  Geology,  pp.  295-357. 

TOPOGRAPHIC  MAP  SHEETS,  U.  S.  GEOLOGICAL  SURVEY,   ILLUSTRATING  THE  WORK 

OF  GROUND  WATER 

Arredondo,  Florida.  Greenville,  Tennessee— North  Carolina. 

Bristol,  Virginia^Teni^essee.  Williston,  Florida. 

Weingarten,  MissourT*4llinois.  Standingstone,  Tennessee. 

Princeton,  Kentucky.  Lockport,  Kentucky. 

Kingston,  Tennessee.  Waterloo,  Illinois. 


CHAPTER  IV 

THE   WORK   OF   STREAMS 

IT  is  difficult  to  over-emphasize  the  importance  of  streams,  since 
they  carry  off  the  excess  of  rainfall  above  evaporation,  with  the  excep- 
tion of  the  ground  water  which  enters  into  chemical  composition  with 
rocks  or  is  discharged  in  underground  courses  directly  to  the  seas 
(p.  56).  The  quantity  of  water  carried  in  streams  is  therefore 
enormous.  It  has  been  estimated  that  the  rivers  of  the  world 
annually  discharge  6500  cubic  miles  of  water;  a  volume  which,  if 
spread  over  Massachusetts,  would  cover  it  three  quarters  of  a  mile 
deep.  The  water  of  flooded  streams  is  derived  largely  from -rainfall, 
while  the  chief  source  is  spring  water,  when  they  are  low. 

FACTORS  IN  STREAM  EROSION 

Material  Carried  by  Streams.  —  In  walking  up  a  small  valley 
one  can  readily  discover  the  sources  of  the  gravel  and  sand  in  the 
stream  bed,  and  of  the  mud  which  renders  the  water  turbid.  The 
small  particles  which  have  been  broken  from  the  rocks  of  the  banks 
by  the  various  agents  of  the  weather,  and  the  larger  fragments  which 
have  been  loosened  by  frost  are  continually  being  carried  down  into 
the  bottom  of  the  valley  by  gravity  (hillside  creep,  p.  73)  and  washed 
down  by  rains ;  deposits  of  sand  and  clay  through  which  the  valley 
is  cut  in  places  furnish  an  easy  supply  during  floods ;  the  solid  rock 
of  the  valley  sides,  when  undercut  by  the  stream,  falls  into  the  water; 
and  some  sediment  is  obtained  from  the  bed  over  which  the  stream 
flows. 

How  the  Sediment  is  Moved.  —  Streams  accomplish  their  work  of 
removing  this  load  of  sediment  (i)  by  pushing  along  the  larger  of 
angular  rocks,  (2)  by  rolling  the  rounded  and  smaller  pebbles,  and 
(3)  by  carrying  in  suspension  the  finer  sand  and  clay,  as  well  as  such 
thin,  flat  particles  as  mica  flakes.  This  ability  of  running  water  to 
carry  fine  particles  in  suspension  is  due  to  the  fact  that  the  smaller 
the  volume  of  an  object,  the  larger  in  proportion  is  its  surface.  This 

81 


82  PHYSICAL  GEOLOGY 

being  the  case,  a  slight  upward  current,  formed  by  the  deflection  of 
the  water  from  the  irregularities  of  the  stream  bed  or  side,  will  lift 

small  particles  and   carry 
them   onward    until    they 

>-  again  fall  to  the  bottom, 

or  are  caught   up   by  an- 
^r _j    *~  ^^  — *•*  other    current    (Fig.    59). 

^  / "X^ )         "^*V^T~Ul. )  ^e  ec^*es  anc^  cross  cur~ 

rents  of  a  river  are  espe- 

FIG.    CO.  —  Diagram  showing  how  upward   cur-       •    n       rr       •        •       i  •  i 

rents  are  produced  by  irregularities  on  the  bed  Cially  effective  in  this  work 
of  a  stream.  during  high  water.  In 

this  way,  sand   and   clay, 

after  many  short  journeys,  are  ultimately  carried  to  the  ocean.  The 
quantity  of  sediment  carried  by  a  stream  depends  upon  its  volume 
and  velocity  and  on  the  amount  and  nature  of  the  accessible  material. 

Factors  Determining  the  Velocity  of  Streams. — The  velocity 
of  a  stream  depends  upon  (i)  the  slope  of  its  valley,  (2)  its  volume  of 
water,  (3.)  the  amount  of  its  load,  and  (4)  the  shape  of  its  channel.  It 
is  greatest  in  the  middle  of  the  stream,  and  some  distance  below  the 
surface.  If  the  volume  of  a  stream  is  increased  eight  times,  its  velocity 
is  doubled,  since  the  velocity  varies  as  the  cube  root  of  the  volume ;  if 
the  amount  of  the  sediment  is  decreased,  the  velocity  is  increased ; 
and,  other  things  being  equal,  a  stream  following  a  straight  channel 
flows  faster  than  one  in  a  winding  course,  because  it  loses  less  energy  in 
friction  with  its  sides.  A  stream  which  is  ordinarily  clear  is  often 
muddy  when  swollen,  both  because  of  the  greater  run-off  which  enters 
it,  and  because  of  the  large  amount  of  sediment  which  it  is  enabled  to 
tear  from  its  bed  and  banks  on  account  of  its  greater  velocity. 

If  the  velocity  of  a  stream  is  increased  several  times,  its  power 
becomes  almost  incredible.  It  has  been  shown  that  a  current  moving 
six  inches  a  second  will  carry  fine  sand;  one  moving  12  inches  a 
second  will  carry  gravel;  four  feet  a  second,  stones  of  about  two 
pounds  weight;  eight  feet  a  second,  stones  of  128  pounds;  30  feet 
a  second,  blocks  of  320  tons  ;  if  a  stream  can  ordinarily  move  a  pebble 
of  one  ounce,  it  can  move  a  stone  of  four  pounds  when  doubled  by  a 
flood.  This  fact  is  expressed  in  the  law  that  the  transporting  power  of 
a  stream  varies  as  the  sixth  power  of  its  velocity.  Keeping  the  above 
law  in  mind  and  remembering  that  a  heavy  object  loses  about  one 
third  of  its  weight  in  water,  it  is  easy  to  understand  the  cause  of  the 
destructiveness  of  such  floods  as  that  which  overwhelmed  Johnstown, 


THE   WORK  OF   STREAMS  83 

Pennsylvania,  in  1889,  and  swept  away  large  rocks,  twenty-ton  loco- 
motives, and  massive  iron  bridges  as  easily  as,  under  ordinary  cir- 
cumstances, the  river  could  move  sand.  A  fall  of  one  foot  in  a  mile 
is  quite  sufficient  to  carry  a  river  steadily  onward ;  one  foot  in  a 
thousand  feet  will  make  a  fairly  rapid  river;  one  in  two  hundred,  a 
torrent. 

Water  Wear.  —  The  pebbles  and  sand  carried  by  the  streams  are 
worn  away  by  their  impact  against  the  bed  rock  and  by  striking 
against  each  other.  The  result  of  such  wear  is  the  production  of 
rounded  stones.  In  mountain  streams  the  angular  fragments  from 
the  talus  are  rounded  before  they  have  been  carried  a  mile. 

Solution.  —  In  addition  to  the  sediment  carried  by  the  force  of  the 
current,  the  waters  of  every  river  contain  a  large  amount  of  mineral 
matter  in  solution.  This  is  largely  obtained  from  springs,  but  also 
from  the  run-off  and  by  the  solution  of  the  stream  bed.  The  amount 
in  any  stream  varies  with  the  season,  being  greater  in  proportion  to 
the  volume  of  water  in  dry  than  in  wet  seasons,  since  in  the  former 
the  water  is  largely  underground  water.  The  small  river  Thames, 
England,  carries  to  the  sea  about  348,230  tons  of  dissolved  minerals 
a  year,  and  the  Mississippi  River  carries  113,000,00x3  tons.  "  The 
Rhine  carries  enough  carbonate  of  lime  to  the  sea  each  year  for  the 
annual  formation  of  3,320,000,000  oyster  shells  of  the  usual  size." 
(A.  Geikie.)  It  is  estimated  that  in  every  5000  years  rivers  carry  their 
own  weight  of  minerals  in  solution  to  the  sea.  The  weight  of  the 
dissolved  matter  carried  to  tidewater  by  the  streams  of  the  United 
States  (270,000,000  tons)  is  more  than  half  that  of  the  sediment 
(513,000,000  tons).1  "  The  tons  per  square  mile  per  year  removed 
from  different  basins  show  interesting  comparisons.  In  respect 
to  dissolved  matter  the  southern  Pacific  basin  heads  the  list  with 
177  tons,  the  northern  Atlantic  basin  being  next  with  130  tons. 
The  rate  for  the  Hudson  Bay  basin,  28  tons,  is  lowest;  that  for  the 
Colorado  and  western  Gulf  of  Mexico  basins  is  somewhat  higher. 
The  denudation  estimates  for  the  southern  Atlantic  basin  correspond 
very  closely  to  those  for  the  entire  United  States."  (Dole  and 
Stabler.) 

Vertical  Erosion  (Corrasion).2  —  By  erosion  (Latin,  erodere,  to  gnaw 
away)  streams  are  able  to  cut  down  their  valleys.  This  may  be 

1  Water-Supply  Paper,  U.  S.  Geol.  Surv.  No.  234. 

2  The  terms  corrasion,  abrasion,  corrosion,  erosion,  and  denudation  have  sometimes  been  used 
rather  loosely  in  geological  and  geographical  literature.     In  this  work  corrasion  (Latin,  corra- 


84 


PHYSICAL  GEOLOGY 


accomplished  by  (i)  the  mere  impact  of  the  water,  especially  if  the 
rock  is  easily  disintegrated  (Fig.  60).  The  effect  of  clear  water  upon 
striking  loose  sediment  with  great  force  is  well  shown  in  hydraulic 
mining.  This  principle  was  also  employed  in  the  leveling  of  a  portion 
of  Seattle,  where  a  high  hill  was  cut  down  by  means  of  a  power- 
ful stream  of  water.  (2)  In  thinly  bedded  rocks,  such  as  shales 


FIG.  60.  —  A  bank  undercut  by  clear  water.     (U.  S.  Geol.  Surv.) 

(p.  250),  the  stream  bed  may  be  deepened  by  "  lifting  " ;  that  is,  the 
shale,  broken  by  joints,  is  separated  by  the  water  along  the  bedding 
planes  (p.  234) ;  and  the  fragments  are  thus  floated  off.  The 
effect  of  this  process  alone  in  regions  underlain  by  shales  may  be 
of  the  greatest  importance.  "Lifting"  is  especially  effective  when 
the  stream  beds  have  been  exposed  to  the  weather  at  low  water. 
At  such  times,  temperature  changes  or  frost  may  loosen  much 
material  in  the  bed,  which  is  picked  up  and  removed  during  high 
water.  Water  without  sediment  has  little  effect  in  eroding  thick 

dere,  to  rub)  and  abrasion  are  used  as  synonyms,  meaning  the  detachment  of  rock  particles  as 
a  result  of  wear ;  corrosion  (Latin,  corrodere,  to  gnaw)  is  used  for  the  work  done  by  solution ; 
erosion  is  used  to  include  both  corrasion  and  corrosion,  as  when  we  say  a  river  erodes  its  valley , 
or  a  sea  erodes  its  shores.  The  term  denudation  is  reserved  for  the  lowering  of  a  land  surface  by 
any  agency. 


THE  WORK  OF  STREAMS 


bedded  rocks,  as  is  apparent  on  the  brink  of  Niagara  Falls  where 
the  thousands  of  tons  of  water  which  pour  over  them  hourly  are 
unable  to  remove  the  

r— — ^     ..     

soft  algae  which  cover 
the  rocks,  as  the 
water  is  filtered  by 
Lake  Erie.  (3)  When 
swift  streams  are 
supplied  with  tools 
(Fig.  61)  in  the  form 
of  sand  and  pebbles, 
their  erosive  power 
becomes  greatly  in- 
creased. 

Weathering       and 
Vertical    Erosion.  - 

*.  j  FIG.  61. —  Bowlders  in  a  stream  bed.  Here  the  bowlders 
valleys  are  usually  form  a  pavement  which  hinders  the  erosion  of  the  valley. 
wider  at  the  top  than 

at  the  bottom.  This  is  due  to  the  fact  that  while  the  valley  is 
being  deepened  by  erosion  it  is  also  being  widened  in  several  ways. 
The  rock  is  loosened  by  the  various  agents  of  the  weather  and 
carried  to  the  stream  by  rainwash  and  wind.  Normally,  valleys 
are  most  rapidly  widened  in  temperate  regions,  since  there  the 
soil  freezes  and  thaws  frequently  so  that  "  creep  "  (p.  73)  plays 
an  important  role.  Valleys  cut  in  sand  or  clay  are  often  widened 
to  a  considerable  degree  as  a  result  of  the  pressure  of  the  overlying 
sediment,  which  forces  the  unconsolidated  sand  or  clay  at  the  base 
to  "  flow  out,"  causing  a  slumping  of  the  upper  portion.  Animals 
walking  on  the  slopes,  falling  trees,  the  cutting  of  the  stream  against 
its  sides  are  among  the  agents  which  help  to  loosen  the  material 
of  the  valley  sides  and  thus  tend  to  widen  the  valley.  If  erosion 
is  very  rapid  as  compared  with  the  work  of  the  agents  of  the 
weather,  steep-sided  gorges  barely  wide  enough  to  accommodate  the 
stream  will  result :  such  are  the  gorge  of  the  Aar  at  Meirengen, 
Switzerland,  the  picturesque  gorges  of  Watkins  Glen  and  Ausable 
Chasm,  New  York,  and  the  canyon  of  the  Virgin  River  in  Arizona. 
Usually,  however,  young  valleys  are  V-shaped,  the  wearing  back 
of  the  sides  more  than  keeping  pace  with  the  deepening  of  the 
valley. 


86  PHYSICAL  GEOLOGY 

Base  Level  of  Erosion.1  — If  a  stream  is  swift,  it  continues  to  deepen 
its  valley  as  it  flows  from  the  higher  lands  to  the  sea,  until  at  or  near 
the  mouth,  the  bed  will  be  at,  or  even  slightly  below,  sea  level.  (The 
bed  of  the  Mississippi  River  is  locally  as  much  as  100  feet  below  sea 
level.)  The  entire  length  of  the  valley,  however,  will  not  be  deepened 
to  the  level  of  the  sea,  since  as  its  slope  (gradient)  is  diminished,  the 
ability  of  the  stream  to  erode  its  bed  also  decreases,  and  before  sea 
level  is  reached  the  stream  will  have  ceased  to  deepen  its  valley  in 
its  upper  course.  When  this  condition  is  attained,  the  stream  is 
said  to  be  at  base  level;  that  is,  it  has  reached  the  lowest  level  to 
which  a  stream  can  wear  a  land  surface.  As  the  stream  approaches 
base  level,  its  current  becomes  less  and  less  rapid,  so  that  the  deepening 
of  the  last  few  feet  of  the  valley  may  take  longer  than  all  the  rest. 

If  the  land  is  raised  and  the  gradients  of  the  streams  are  increased, 
they  will  again  cut  until  a  new  base  level  is  reached.  If  on  the  other 
hand  the  land  is  lowered,  base  level  will  be  reached  more  quickly. 

During  their  histories  streams  usually  reach  a  number  of  tem- 
porary base  levels.  If,  for  example,  a  stream  flows  into  a  lake,  it 
cannot  cut  lower  than  that  level;  and  if  the  lake  remains  in  existence 
for  a  long  time,  the  stream  will  excavate  a  broad  valley  where  it  enters 
the  lake.  Again,  if  a  stream  flows  over  a  stratum  of  hard  rock  in  its 
lower  course  while  its  upper  course  is  in  less  resistant  rock,  the  depth 
to  which  it  cuts  in  the  hard  rock  will  be  the  temporary  base  level,  and 
a  broad  valley  will  be  developed  above  the  resistant  rock,  while  the 
latter  will  constitute  the  steep-sided  narrows  so  characteristic  of 
the  scenery  of  eastern  Pennsylvania. 

Effect  of  Load.  —  Whether  a  stream  carrying  sediment  will  erode 
or  deposit  depends  upon  its  velocity  and  upon  the  amount  of  mate- 
rial. If  its  velocity  is  great,  the  sand  and  gravel  will  be  used  as  tools 
with  which  to  cut  down  the  stream  bed,  or  widen  it.  If,  however, 
the  velocity  is  sufficiently  decreased,  as  frequently  occurs  when  a 
side  stream  with  a  steep  gradient  flows  into  a  master  stream  with  a 

1  The  term  base  level  has  been  used  in  several  senses,  the  difficulty  arising  because  of  the 
fact  that  as  commonly  used  the  surface  described  is  a  slope  and  not  a  level  plain.  It  has  been 
suggested  that  "base  level"  be  limited  to  the  level  base  with  respect  to  which  normal  sub- 
aerial  erosion  proceeds ;  to  employ  the  term  grade  for  the  balanced  condition  of  a  mature  or 
old  river ;  and  to  name  the  geographical  surface  that  is  developed  near  or  very  near  the  close  of 
a  cycle  a  "peneplain"  or  "plain  of  gradation."  (Davis,  Wm.  M., — Geographical  Essays, 
P-  387.) 

As  used  in  this  volume  a  base  level  is  the  lowest  possible  slope  to  which  a  region  can  be  cut 
by  running  water.  Thus  a  stream  in  a  canyon  may  cut  its  channel  to  base  level  ages  before  it 
develops  a  plain  at  that  level.  A  peneplain  is  any  extensive  tract  of  land  reduced  to  essential 
planeness  (base  level)  by  the  erosion  of  running  water. 


THE  WORK  OF  STREAMS 


gentle  grade,  it  may  drop  its  load.  Decrease  of  volume  due  to  evap- 
oration and  to  the  absorption  of  the  water  by  the  soil,  such  as  takes 
place  when  a  river  flows  through  a  dry  region,  may  reduce  the  stream's 
velocity  to  such  an  extent  that  it  is  unable  to  carry  its  load  of 
sediment.  The  Platte  River  of  Nebraska  is  a  typical  example  of 
such  a  river.  Its 
headwaters  have  a 
small  amount  of  sedi- 
ment in  proportion 
to  the  volume  of 
water,  and  it  is  there- 
fore able  to  cut  a 
deep  canyon  in  its 
upper  course ;  but  in 
passing  over  the  dry 
and  thirsty  plains  it 
loses  so  much  water 
that  it  is  not  only 
unable  to  degrade  its 
bed,  but  even  de- 
posits much  of  its 
load  during  the  dry 
season.  A  stream 
which  flows  over 
such  sandy  plains 
has  shallow,  crooked 
channels  and  is  con- 
stantly shifting  its 
course  by  cutting 
away  the  banks  in 
some  places  and 

forming  bars  in  others.  When  the  load  is  so  great  that  it  is  deposited 
in  the  channel,  the  latter  may  become  too  small  for  the  water  of  the 
stream,  in  which  case  the  water  will  break  out  and  follow  a  new 
course.  If  this  is  repeated  many  times  a  network  of  small,  shallow 
streams,  called  a  braided  stream  (Fig.  62),  may  result.  The  Colorado 
and  Platte  rivers  have  about  the  same  gradient,  but  the  former 
receives  less  sediment  in  proportion  to  its  volume  and  consequently 
is  able  to  cut  a  great  canyon,  while  the  lower  Platte  flows  in  a  broad 
and  shallow  valley. 


FIG.  62.  —  Braided  stream,  Kandersteg  valley, 
Switzerland. 


88  PHYSICAL  GEOLOGY 

When  a  stream  has  developed  a  slope  which  gives  it  just  sufficient 
velocity  to  carry  its  load,  leaving  no  energy  for  deepening  its  bed, 
it  is  said  to  be  graded.  If  a  stream  has  less  sediment  than  it  can 
carry,  it  will  remove  material  from  its  bed.  If  it  is  unable  to  trans- 
port all  the  sediment  brought  to  it,  part  of  this  will  be  left  as  a  deposit, 
the  channel  will  be  raised,  and  the  gradient  will  be  increased  until  the 
stream  becomes  swift  enough  to  carry  away  its  load.  When  a  stream 
is  at  a  temporary  base  level  (p.  86)  above  a  fall  or  rapid,  there  are 
often  smooth  reaches  where  the  stream  is  at  grade.  If  the  land 
through  which  a  river  flows  has  not  been  elevated  or  depressed  for  a 
long  period  of  time,  few  falls  will  exist,  and  it  will  be  at  grade  for  long 
stretches.  If,  however,  a  long-continued  uplift  or  several  uplifts 
have  occurred,  even  large  rivers  may  be  unable  to  erode  their  beds 
to  grade.  Even  when  the  last  uplift  was  so  remote  that  the  large 
rivers  have  been  able  to  develop  well-graded  courses,  the  tributaries 
may,  and  usually  do,  have  a  steep  slope. 

Factors  Affecting  the  Rate  of  Erosion.  —  The  rate  of  erosion  of  a 
stream  depends  upon  a  large  number  of  factors,  (i)  Loosely  com- 
pacted rocks,  or  rocks  with  a  soluble  cement,  are  easily  eroded.  If, 
for  example,  the  grains  of  a  sandstone  are  held  together  with  lime, 
the  solution  of  the  cement  will  cause  the  grains  to  fall  apart  and  thus 
render  the  work  of  the  stream  easier.  (2)  Rapid  erosion  is  further 
favored  if  the  rock  has  numerous  joints  and  is  thin-bedded  (p.  24). 
Usually  sedimentary  rocks  are  more  readily  eroded  than  massive, 
crystalline  rocks  (p.  330)  such  as  granite.  (3)  The  greater  the  velocity 
of  a  stream,  the  greater,  other  conditions  remaining  the  same,  will  be 
the  erosion.  Since  the  velocity  of  a  stream  depends  upon  the  volume 
of  water  as  well  as  upon  the  slope  of  its  bed,  the  cutting  power  will 
be  greater  during  floods  (p.  82).  (4)  Under  any  of  the  above  con- 
ditions erosion  will  be  favored  if  the  stream  has  sufficient  sand  and 
gravel  with  which  to  cut  its  bed  but  not  so  much  that  a  large  part 
of  its  energy  is  expended  in  carrying  it.  When  the  amount  of  sedi- 
ment is  increased  without  an  increase  in  the  volume  of  water  (p.  86), 
or  when  the  quantity  of  sediment  remains  constant  but  the  volume 
of  water  decreases,  erosion  may  cease  and  deposition  take  place. 
Rapid  erosion  by  abrasion  requires  some  sediment,  but  not  too  much, 
a  steep  slope,  and  a  considerable  volume  of  water. 

Scour  and  Fill.  —  A  stream  at  flood  may  be  deepening  (degrading) 
its  channel  where  its  velocity  is  great,  at  the  same  time  that  it  is  build- 
ing up  (aggrading)  its  flood  plain  where  the  velocity  is  slight.  After 


THE  WORK  OF   STREAMS  89 

the  flood  has  subsided  the  channel  thus  deepened  may  be  entirely 
filled  with  sediment.  This  process  is  called  scour  and  fill.  The 
Missouri  River  sometimes  scours  out  its  channel  to  depths  of  from 
70  to  90  feet  and  later  fills  it  again.  It  is  evident  that  the  deepening 
of  the  beds  of  such  rivers  is  largely  confined  to  high  water.  A  fail- 
ure to  understand  scour  and  fill  has  led  some  observers  to  assign  a 
great  age  to  stone  implements  found  deeply  buried  in  river  gravels 
(p.  680). 

Lateral  Erosion.  —  When  a  young  river  is  deepening  its  valley  it 
flows  in  a  narrow  channel  between  steep  banks,  but  since  its  course 
is  seldom  straight  it  tends  in  places  to  cut  more  on  one  bank  than 
on  the  other,  with  the  result  that  as  it  cuts  downward  it  also  cuts 
sidewise,  thus  widen- 
ing its  valley.  By  the 
time  grade  is  reached 
the  valley  walls  will 

have  flared  open,  but       FJG  ^  _  Unsymmetrical  valley  formed  as  a 
will  be  steeper  on  the  of  the  dip  of  the  rock, 

outside  of  each  curve. 

Unsymmetrical  valleys  are  formed  (i)  in  this  way  and  also  (2)  by  the 
greater  hardness  of  the  rock  on  one  side  of  the  stream  than  on 
the  other  (Fig.  63)  (where  the  strike  of  the  rock  parallels  the  course 
of  the  stream). 

When  two  neighboring  streams  have  ceased  to  degrade  their  beds, 
they  will  cut  laterally  and  may  in  time  wear  away  the  divide  which 
separates  them,  thus  causing  one  to  flow  into  the  other. 

FEATURES  DUE  TO  STREAM  EROSION 

Falls  and  Rapids.  —  Falls  and  rapids  result  from  a  number  of  causes, 
(i)  Regions  in  which  a  harder  layer  of  rock  overlies  a  softer  one  fur- 
nish most  favorable  conditions  for  the  formation  of  falls  (Fig.  64). 
When  a  stream,  in  deepening  its  valley,  encounters  a  harder  bed  of  rock 
lying  in  the  position  shown  in  the  diagram  (Fig.  65),  the  less  resistant 
beds  are  worn  more  rapidly  than  the  harder  ones,  and  a  rapid  will 
result  first,  which  upon  further  erosion  will  become  a  fall.  Falls 
become  lower  and  lower  in  the  course  of  time,  until  the  resistant  beds 
form  mere  ledges  in  the  stream  bed  and  the  falls  cease  to  exist  (Fig. 
65).  Niagara  Falls  (Fig.  66)  have  gradually  cut  back  until  now  they 
are  seven  miles  from  their  original  position.  The  recession  of  these 


90  PHYSICAL  GEOLOGY 

falls  and  their  verticality  are  due  to  the  fact  that  the  strata  which 
compose  the  higher  land  consist  of  massive  limestone,  about  80  feet 
thick  at  the  falls,  which  are  underlain  by  soft  and  easily  weathered 


FIG.  64.  —  Falls  of  the  Genesee  River,  Rochester,  New  York. 
(Photo.  C.  R.  Dryer.) 

and  eroded  shale.  When  the  water  plunges  over  the  limestone,  it 
wears  away  the  soft  rock  beneath  more  rapidly  than  the  hard  capping 
stratum,  leaving  the  latter  projecting.  Fragments  are  continually 
falling  from  this  overhanging  ledge  and  are  used  by  the  water  as  tools 


FIG.  65.  —  Diagram  illustrating  the  recession  of  a  waterfall  formed  by  a  resistant 
bed  that  dips  up  the  stream.     (Modified  after  Salisbury.) 

to  excavate  the  shale  further.  This  erosion  is  also  aided  materially  by 
blocks  of  ice  in  winter.  The  height  of  the  falls  is  about  165  feet,  and 
the  gorge  which  has  been  excavated  is  from  200  to  400  yards  wide  and 


THE  WORK  OF   STREAMS 


FIG.  66.  —  An  ideal  section  of  Niagara 
Falls,  showing  how  the  soft  shales  are 
being  worn  away,  leaving  the  limestone 
above  unsupported.  (Gilbert.) 


about    300    feet    deep    in    places.       The    rate    of   cutting   of    the 

Canadian    Falls    has    been    about    4.5    feet    a    year    since    1842, 

while  that  of  the  American  Falls, 

because    of    the    smaller    volume 

of  water,  is  as  small  as  0.2  foot  a 

year. 

A  natural  bridge  may  be  formed 
when  the  water  above  a  fall  per- 
colates through  a  joint  or  crack 
athwart  the  stream  and  thence 
along  a  bedding  plane  or  approxi- 
mately horizontal  crack,  emerging 
under  the  fall  as  a  spring.  If  the 
cracks  are  enlarged  by  solution 
and  erosion,  a  tunnel  large  enough 
to  accommodate  the  entire  volume 
of  the  stream  may  be  formed,  and  a  natural  bridge  result  (Fig.  67). 

(2)  When  the  fall  of   a   river  in  working   up  stream   passes  the 
mouths  of  tributaries  falls  develop  in  them  also.     The  beautiful  Min- 
nehaha  Falls  of  Minnesota  are  an  example  of  falls  formed  in  this  way. 

(3)  In    mountainous    regions,    where    the    main    streams    have 
deepened  their  valleys  rapidly  their  tributaries   are  often   unable, 
A  B  because   of  their 

smaller  volume,  to 
keep  pace  with  them 
and  therefore  flow 
into  them  over  falls 
or  rapids.  To  this 
cause  the  "  roaring 
brooks "  of  New 
England  are  for  the 
most  part  due. 

(4)  The  Atlantic 
coast,  from  New 
York  southward,  is 
bordered  by  a  low- 
lying  plain  (Coastal 
Plain,  p.  224)  composed  of  soft,  unconsolidated  sands  and  clays. 
To  the  westward,  this  belt  joins  a  belt  of  older  and  harder  rocks 
(Piedmont  Plateau)  along  a  line  roughly  parallel  with  the  coast. 


II            1       \c 

II      1     1    )» 

•:":'.••*'.•.  •';•'•'-  :•''.•/.•:.•.•.'.••'  '."•:•.'-.'-•':•  :';-;':'•'•".'."'.•  •>'•;-'  .'  :  ."  •'-'•'•'•'•'.••''  '-'.-,   '•'•':•'':• 

FIG.  67.  —  Diagrams  illustrating  the  formation  of  a 
natural  bridge  by  the  widening  of  a  joint  or  other  crack 
B  athwart  the  stream,  through  the  solution  of  the  lime- 
stone by  water  which  reappeared  as  a  spring  under  the 
fall  at  C.  In  the  process  of  time  a  tunnel  sufficient  to 
carry  a  large  part  of  the  volume  of  the  stream  was  ex- 
cavated, and  finally  the  entire  volume  of  the  stream. 
When  this  was  accomplished  a  natural  bridge  (shown  in 
the  cross  section)  spanned  the  valley. 


92 


PHYSICAL  GEOLOGY 


When  streams  on  their  way  to  the  sea  pass  from  the  hard  to  the 
soft  rocks  they  flow  over  rapids  or  falls,  because  the  less  resistant 
rocks  are  cut  down  more  easily  than  the  hard.  The  boundary 
between  the  Coastal  and  Piedmont  plains  is  for  this  reason  called 
the  "  Fall  Line,"  and  it  is  here  that  many  cities  are  located,  both 

because  the  falls  fur- 
nish water  power  and 
because  they  deter- 
mine the  head  of 
navigation. 

(5)  When  a  stream 
in  cutting  its  bed  en- 
counters a  hard  rock 
mass,  the  erosion  of 

FIG.    68.  —  A  fall   formed  when  resistant  rock  is  en-  {-fog  valley  is  retarded 
countered  by  a  stream.     The  rock  CD  is  hard  gneiss,  u  .  , 

while  that  represented  by  lines  is  softer  schist.     The  line  at     tftat     Polnt>      bu* 

AB  is  the  course  of  the  stream.  may  continue  farther 

down  the  valley.     A 

fall  or  rapids  (Fig.  68)  will  naturally  be  formed  at  such  a  place,  and 
the  hard  rock  mass  will  constitute  a  temporary  base  level  which  will 
prevent  the  stream  from  deepening  its  bed  above  the  fall.  As  a 
result  of  the  lateral  erosion  of  the  stream  and  of  the  action  of  the 
weather,  the  valley  above  the  fall  may  be  greatly  widened,  forming 
arable  land.  A  case  somewhat  similar  to  the  above  is  that  of  the 
falls  of  the  Yellowstone,  which  are  the  result  of  the  presence  of  lava, 
made  more  resistant  by  thermal  action  (Fig.  69).  When  rocks  are 


FIG.  69.  —  The  falls  of  the  Yellowstone  River.     The  rock  is  lava,  and  the  falls 
at  A  and  B  are  due  to  the  superior  hardness  of  the  lava  at  these  points. 

^ 

less  jointed  or  fractured,  in  one  portion  of  a  valley  than  in  another, 
they  are  less  affected  by  erosion  and  may  produce  a  fall  or  rapids. 

(6)  Falls  also  result  where  rocks  have  strongly  vertical  joints,  as 
vertical  joints  in  homogeneous  rocks  have  the  effect  of  vertically 
inclined  beds. 


THE  WORK  OF  STREAMS 


93 


(7)  The  numerous  falls  of  Switzerland  were  formed  much  as  in 
(3),  but  are  largely  due  to  the  erosion  of  the  main  valleys  by  glaciers 
so  that  the  tributary  streams  enter  their  mains  over  falls.  These 
side  valleys  are  called  "  hanging  valleys." 

Exceptions  —  Falls  not  the  Result  of  Erosion.  —  (i)  A  lava  stream 
(Fig.  70)  may  dam  a  valley  and  thus  produce  a  fall.  Many  examples 
of  this  sort  might  be 
cited.  (2)  Limestone 
(travertine)  may  be 
deposited  in  streams 
in  such  quantities  as 
to  dam  them,  form- 
ing falls  (Fig.  71)  and 
even  ponding  back 
the  water  to  produce 
lakes.  Topolic  Falls 
in  Dalmatia,  east  of 
the  Adriatic,  afford 
an  illustration  of  the 
construction  of  a 
travertine  dam. 
These  falls  are  70 
feet  high  and  are 
advancing  down- 
stream. (3)  When 
tributary  streams 
with  steep  gradients 
carry  a  large  quantity  of  coarse  debris,  they  may  deposit  their  loads 
in  the  main  stream  in  such  amounts  as  to  form  temporary  rapids. 
Landslides  also  accomplish  the  same  result.  The  Cascades  of  the 
Columbia  River  were  formed  thus.  (4)  When  a  stream  is  forced 
out  of  its  valley  by  landslides  (p.  73),  glacial  deposits  (p.  155),  or  in 
any  other  way,  falls  may  result. 

Potholes.  —  When  for  any  reason  a  strong,  permanent  eddy 
is  produced  in  a  stream,  as  at  falls  or  rapids,  pebbles  and  stones  are 
given  a  rotary  motion  as  they  are  carried  through  the  eddy  and  wear 
down  the  stream  bed  in  this  place,  tending  to  produce  circular  holes. 
These  "potholes"  (Fig.  72),  "  washtubs,"  "giant's  caldrons," 
or  "  kettles,"  as  they  are  called,  occur  in  hard  granites  as  well  as  in 
shales  and  limestone,  and  may  be  seen  in  the  bed  of  almost  any  rapid 


FIG.  70.  —  Falls  formed  as  a  result  of  the  damming  of  a 
river  channel  by  lava.    (Modified  after  H.  E.  Gregory.) 


94 


PHYSICAL  GEOLOGY 


FIG.  71.  —  Travertine  Falls  near  Davis,  Oklahoma.  The  travertine  which  has 
been  deposited  to  form  these  dams  comes  from  springs  containing  large  quantities  of 
calcium  carbonate  in  solution.  The  lime  carbonate  is  deposited  as  travertine  when 
the  carbon  dioxide  escapes.  Forty-four  dams  occur  in  this  creek  within  a  mile. 
(Oklahoma  Geol.  Surv.) 


stream.  They  vary  in  diameter  from  a  few  inches  to  ten  feet  or  more 
and  in  depth  to  forty  or  more  feet.  The  size  of  a  pothole  depends 
upon  the  velocity  and  volume  of  the  current  and  the  length  of  time 
during  which  the  eddy  remains  at  the  same  point.  By  the  deepen- 
ing and  coalescing  of  potholes  the  channels  of  streams  may  be  mate- 
rially deepened,  streams  sometimes  accomplishing  their  greatest  work 
of  erosion  in  this  way.  In  the  Alps  there  is  scarcely  a  gorge  through- 


THE  WORK  OF  STREAMS 


95 


out  the  length  of 
which  one  cannot  see 
the  polished  surfaces 
and  regular  curves 
which  are  the  traces 
of  more  or  less  com- 
plete potholes. 

Canyons.  —  Can- 
yons are  deep  valleys 
with  steep  sides. 
They  are  formed 
where  the  down-cut- 
ting of  a  stream 
(corrasion)  greatly 
exceeds  the  weather- 
ing back  of  the  slopes. 

The  conditions  favoring  the  formation  of  such  valleys  are  (i)  a  rock 
capable  of  maintaining  a  steep  face,  such  as  resistant  rock  on  which 
the  trickling  water  cannot  act  quickly,  or  a  firm,  permeable  rock 
into  which  a  large  part  of  the  water  soaks,  leaving  little  for  erosion ; 
and  (2)  a  rapidly  cutting  stream.  (3)  An  arid  climate  is  more 
favorable  than  a  moist  one,  since  the  work  of  the  weather  will  be 
at  a  minimum  in  the  former.  Canyons  are  nevertheless  formed  in 
regions  of  heavy  rainfall  (Fig.  64).  When  a  stream  approaches  base 
level  and  ceases  to  corrade  its  bed  rapidly,  the  walls  of  its  canyon 
will  be  weathered  back  until  in  time  they  form  a  broad,  open  valley. 


FIG.   72.  —  Potholes  in  gneiss,  Shelburne  Falls, 
Massachusetts. 


FIG.  73. —  A  generalized  block  diagram  of  the  Grand  Canyon  of  the  Colorado. 
The  youthful  stage  of  the  region  is  shown  in  the  fact  that  the  streams  have  as  yet 
accomplished  but  a  small  part  of  the  work  to  be  done.  The  Colorado  valley  is  a  young 
valley.  The  cliffs  of  the  canyon  are  formed  of  resistant  beds,  while  the  slopes  are  of 
weaker  beds. 

One  of  the  grandest  canyons  in  the  world  is  the  Grand  Canyon 
of  the  Colorado  in  Arizona  (Fig.  73),  which  was  formed  under  condi- 
tions most  favorable  for  steep-sided  valleys.  The  river  flows  through 

CLELAND   GEOL.  —  ^ 


PHYSICAL  GEOLOGY 


a  high  plateau,  6000  to  8000  feet  above  the  sea,  in  which  it  has  cut 
a  trench  a  mile  deep  in  certain  places.  The  climate  is  arid ;  the 
gradient  of  the  valley  is  steep ;  the  amount  of  sediment  is  sufficient 
to  furnish  tools  for  cutting,  but  not  so  great  as  to  overload  the  stream ; 
the  rocks  are  sandstones,  limestones,  and  shales,  overlying  granite. 
The  Grand  Canyon  in  Arizona  is  about  220  miles  long  and  may  be 
described  as  a  valley  within  a  valley,  since,  in  certain  localities,  the 
upper  portion,  cut  in  the  softer,  sedimentary  rocks,  is  eight  to  ten  miles 

wide,  while  the  lowest 
part  cut  in  the  hard 
granite  is  barely  wide 
enough  to  hold  the 
river.  The  total 
depth  of  the  canyon  is 
almost  a  mile.  The 
canyons  of  the  tribu- 
tary streams  branch 
again  and  again  as 
theyare followed  back, 
and  are  miniatures  of 
the  Grand  Canyon. 

The  gorge  of  the 
Niagara  River,  Au- 
sable  Chasm,  and 
Watkins  Glen,  in  New 
York,  are  examples  of 
canyons  developed  in  a  moist  region.  In  these  cases  the  valleys 
are  all  postglacial  and  have  been  cut  so  rapidly  that  their  sides 
have  been  but  little  widened  by  the  weather.  In  Ausable  Chasm 
(Fig.  74)  the  verticality  of  the  walls  has  been  maintained  in  places  by 
vertical  joints. 

Instances  of  Rapid  Erosion.  —  The  Duna,  a  river  of  eastern  Prussia,  blocked  by 
an  ice  jam  in  1901,  was  forced  to  take  a  new  course.  In  thirty-four  hours  it  was  able 
to  cut  a  gorge  one  meter  to  three  and  a  half  meters  deep  and  four  meters  to  eight  meters 
wide,  representing  an  excavation  of  2250  cubic  meters  of  material.  The  bottom  of 
the  Sill  tunnel  in  Austria  was  provided  with  a  pavement  of  granite  slabs  more  than  a 
yard  thick.  Great  quantities  of  debris  were  swept  over  this  pavement  at  a  high 
velocity,  and  so  rapid  was  the  abrasion  that  it  was  found  necessary  to  renew  the  granite 
slabs  after  a  single  year. 

Effect  of  Deforestation  on  Rivers.  —  When  forests  are  cut  down 
or  the  vegetation  on  the  hills  is  killed,  the  latter  being  sometimes  the 


FIG.  74.  —  Ausable  Chasm,  Chazy,  New  York.     This 
is  a  young  valley.     (U.  S.  Geol.  Surv.) 


THE  WORK  OF  STREAMS 


97 


case  in  the  vicinity  of  smelters,  erosion  may  be  very  rapid.     This  is 
well  shown  in  Potato  Creek  (Figs.  75,  76)  in  the  Ducktown  copper 


FIG.  75.  —  Potato  Creek,  Tennessee,  a  stream  overburdened  with  waste  and 
aggrading.   (See  also  Fig.  76.)    (U.  S.  Geol.  Surv.) 

region  of  Tennessee,  where  the  waste  from  the  bare  slopes  is  too  great 
for  the  stream  to  remove  and  is  piled  up  along  its  course  as  a  flood 
plain  (p.  1 19).  In  this  creek  the  waste  has  accumulated  for  a  number 
of  years  at  the  rate  of  a  footer  more  a  year  (Fig.  76,  A,  B),  and  has 


FIG.  76.  —  Generalized  diagram  showing  the  effect  of  deforestation  on  Potato  Creek, 
Tennessee.  A  shows  the  former  condition  of  the  valley,  and  B  the  condition  after  the 
timber  had  been  killed  and  the  stream  loaded  with  sediment.  The  telephone  poles 
were  buried  to  their  cross  arms. 

built  up  a  flood  plain  in  which  telephone  poles  are  buried  almost  to 
their  cross  arms,  while  highway  bridges  and  roadbeds  have  been 
either  buried  or  swept  away  by  floods. 


98 


PHYSICAL  GEOLOGY 


The  effect  on 
stream  flow  of  for- 
ested and  deforested 
(Fig.  77)  areas  is  well 
illustrated  near  Bilt- 
more,  North  Caro- 
lina. The  David- 
son River  has  its 
upper  drainage  basin 
in  the  Pisgah  for- 
est; the  Tuckasegee 
River  in  a  defor- 
ested land  that  has 
been  logged,  burned 
over,  pastured  and 
farmed.  The  two 
areas  drained  are  of 
geologically  the  same 
age  and  structure ; 
the  headwaters  of  the 
streams  are  found 
within  the  same 
range  of  mountains ; 

the  rainfall  of  the  two  areas  is  the  same ;  the  steepness  of  slope  of 

the  two  watersheds  is  about  the  same.     Yet  the  Tuckasegee,  though 

the   larger  river,  shows  greater  fluctuation   in   discharge  than  the 

Davidson;    and   the    Davidson   is   practically    free    from   sediment, 

while  the  Tuckasegee 

bears      gravel      and 

sand  which  it  often 

spreads   over    fertile 

lands. 

Growth  of  Valleys. 

—  It    is    possible    to 

study  the  growth  of  a 

valley  in  almost  any 

region.     Water  does 

not     flow     down     a 

slope    in    sheets    for 

long    distances,    but 


FIG.    77.  —  Rapid    erosion    of  deforested    land    and    one 
method  of  preventing  further  erosion.    (U.  S.  Geol.  Surv.) 


FIG.  78.  —  A  young  valley,  western  Nebraska. 
work  of  the  stream  has  only  begun. 


The 


THE  WORK  OF   STREAMS 


99 


FIG.  79.  —  Block  diagram  showing  the  manner  in  which 
the  divide  between  two  streams  is  narrowed. 


soon  finds  depressions  where  it  accumulates  into  streams.  Even 
though  a  slope  were  perfectly  uniform,  a  slight  heterogeneity  of  soil 
or  rock  would  permit  the  water  to  remove  more  material  in  one 

place  than  in  another  ,  

and  thus  begin  the 
excavation  first  of  a 
gully,  and  later,  by 
prolonged  erosion,  of 
a  ravine  which  still 
later  would  develop 
into  a  broad  valley. 

A  valley  is  length- 
ened at  its  upper  end 
and  is  cut  back  by  the 
water  which  flows  in 
at  its  head  (Fig.  78), 

the  direction  being  determined  by  the  greatest  volume  of  water 
which  enters  it.  This  is  called  headward  erosion.  A  valley  is 
widened  by  rainwash,  lateral  erosion  (Fig.  79),  and  in  other  ways  (p. 
89).  Its  length  depends  upon  the  distance  to  which  its  stream  can 
cut  inland.  At  the  beginning  a  valley  has  running  water  only 

during  and  immediately  after  rains, 
but  later,  when  it  has  cut  below  the 
water  table  (p.  56),  a  permanent 
stream  flows  through  it  (Fig.  32,  p. 
57).  Tributary  streams  tend  to  turn 
in  the  direction  of  their  main  (Fig. 
80),  a  feature  which  is  often  most 
pronounced  late  in  their  history. 

Valleys    Formed    in    Ways    Other 
than  by  Stream  Erosion.  —  Although 
the  great  majority  of  valleys  are  de- 
veloped by  stream  erosion,  some  were 
already  formed  for  the  streams  which 
FIG.  80.  —  Map  showing  the  usual     flow    through    them.      The    popular 
relation  of  tributary  streams  to  the     notion  tnat  canyons,  such,  as   that  of 
main  stream  into  which  they  flow.  .      „ .  .       A    . 

the  Colorado  River  in  Arizona,  were 

formed  by  great  cataclysms  which  rent  the  earth  and  produced 
the  deep  fissures  now  occupied  by  streams,  is  without  foundation. 
Streams,  however,  do  occasionally  flow  into  fissures  formed  by 


100 


PHYSICAL  GEOLOGY 


the  fracturing  of  the  surface  during  earthquakes,  but  they  are  so 
few  as  to  be  unimportant.  Some  great  valleys,  nevertheless,  were 
ready  made  for  the  rivers  which  flow  through  them.  The  Great 
Valley  of  California,  through  which  the  San  Joaquin  and  Sacra- 
mento rivers  flow,  was  formed,  not  by  stream  erosion,  but  by 


BLACK  FOREST 


FIG.  81.  —  Section  across  the  Vosges  and  Black  Forest,  Germany,  showing  the 
graben  in  which  the  Rhine  flows.      (Penck.) 

the  sinking  of  the  land  along  a  valley-like  depression,  or  by  the 
uplift  of  parallel  mountain  folds,  and  is  called  a  structural  valley. 
Into  such  a  depression  streams  may  flow  from  the  high  lands 
on  the  sides  and  unite  (unless  the  region  is  arid)  to  form  a 
river  system.  The  Great  Basin  region  of  Utah  is  also  a  structural 
valley,  but  because  of  the  aridity  of  the  climate  no  streams  flow 
through  it.  The  River  Jordan  and  the  Dead  Sea  are  in  a  valley 


R 


FIG.  82.  —  Diagram  A  illustrates  the  development  of  parallel  consequent  streams 
on  a  sloping  surface.  Diagram  R  is  the  same  region  after  the  streams  have  become  ad- 
justed to  the  structure  of  the  underlying  rocks.  The  streams  entering  the  main  at 
right  angles  are  subsequent  streams.  The  main  stream  flows  through  its  water  gap  in 
the  hard  ridge.  The  gaps  on  either  side  were  eroded  by  former  streams  but  no  longer 
have  streams  in  them  and  are  called  wind  gaps. 

formed  by  the  sinking  (faulting,  p.  261)  of  a  long  and  comparatively 
narrow  block  of  the  earth's  crust.  Such  a  valley  is  called  a  rift 
valley.  Owen's  valley  in  California  and  a  portion  of  the  Rhine 
valley  in  Germany  (Fig.  81)  are  other  examples  of  valleys  due  to 
faulting.  Glaciers  excavate  valleys  in  the  solid  rock,  which  may 
afterwards  become  occupied  by  streams,  but  these  are  usually  merely 


THE  WORK  OF  STREAMS 


IOI 


ancient  river  valleys  which 
have  been  widened  and 
deepened  by  the  ice  (p. 
129). 

The  Direction  of  Val- 
leys. —  The  direction  of 
stream  valleys  depends 
upon  a  number  of  condi- 
tions, some  of  which  can 
be  illustrated  by  a  hypo- 
thetical case.  If  a  portion 
of  the  bottom  of  a  shallow 
sea  is  raised  above  sea 
level,  the  land,  under  these 
conditions,  will  have  no 
established  stream  valleys. 
When,  then,  the  rain  falls 
first  upon  such  new  land, 
it  will  gather  in  places 
where  there  are  depres- 
sions and  form  large  or 
small  lakes.  Elsewhere, 
rivulets  will  flow  down  the 
slope,  joining  here  and 
there  in  their  descent  until 
a  stream  of  considerable 
length  develops.  Streams 
of  this  sort,  whose  position 
and  direction  are  deter- 
mined by  the  slope  of  the 
original  land  surface,  are 
called  (i)  consequent  (Fig. 
82  A),  since  their  direction 
is  a  consequence  of  the 
topography  of  the  country, 
without  regard  to  the  char- 
acter of  the  rock  through 
which  they  pass.  The 
streams  on  the  Atlantic 
Coastal  Plain  are  chiefly 


FIG.  83.  —  Diagram  A  shows  a  region  in  which 
a  stream  flows  at  grade.  Diagram  B  shows  the 
same  region  after  it  has  been  slowly  upwarped 
athwart  the  course  of  the  stream.  The  river  is 
shown  as  having  been  able  to  deepen  its  valley  as 
rapidly  as  the  elevation  occurred.  A  stream  with 
such  a  history  is  an  antecedent  stream,  since  it 
was  able  to  maintain  the  course  it  had  prior 
(antecedent)  to  the  deformation  of  the  surface. 


102 


PHYSICAL  GEOLOGY 


consequent  streams.  As  streams  deepen  and  lengthen  their  valleys, 
their  tributaries  may  encounter  new  kinds  of  material  and  find  that 
some  are  more  easily  eroded  than  others,  with  the  result  that  they 
gradually  develop  valleys  in  the  less  resistant  rocks.  In  such  case, 
the  position  and  size  of  the  branch  streams  are  determined  by  the 
nature  of  the  underlying  rock  and  not  by  the  original  slope  of  the 
surface;  the  valleys  being  cut  in  the  weaker  strata,  while  the  harder 

strata  stand  up  as  ridges 
or  even  mountains.  The 
Shenandoah  valley  of 
Virginia,  the  Lehigh  val- 
ley of  Pennsylvania,  and 
the  Hoosic  and  Hoosa- 
tonic  valleys  of  Massa- 
chusetts and  Connecticut 
are  examples  of  valleys 
of  this  type.  Valleys 
formed  in  this  way  are 
called  (2)  subsequent  (Fig. 
82  B),  the  process  being 
known  as  structural  ad- 
justment. 

It  will  readily  be  seen 
that  if  streams  drain 
adjoining  regions,  the  one 
whose  course  is  most 
generally  confined  to  the 
more  easily  eroded  beds 
will  grow  more  rapidly 
and  so  may  cut  headward 
until  it  captures  branches 

or  even  the  entire  upper  courses  of  streams  less  favorably  situated. 
Such  a  process  is  called  stream  piracy  (p.  107).  If  the  land  is 
warped  up  athwart  the  course  of  a  consequent  stream  whose  direc- 
tion is  so  well  established  that  it  is  able  to  degrade  its  bed  as  rapidly 
as  the  elevation  takes  place,  thus  keeping  its  old  course,  the  stream 
is  called  antecedent  (Fig.  83  A  and  B}. 

(3)  Another  factor  which  sometimes  determines  the  direction  of 
a  stream  is  faulting  (p.  261)  (Fig.  84).  In  regions  of  pronounced 
faulting,  such  as  the  Adirondacks,  the  courses  of  many  streams  may, 


FIG.  84. —  Direction  of  drainage  determined  chiefly 
by  faulting  and  jointing,  near  Lake  Temiskaming, 
Ontario.  (After  Hobbs.) 


THE  WORK  OF  STREAMS 


103 


for  considerable  stretches,  follow  lines  of  dislocations.  (4)  Where  the 
rock  over  which  a  stream  flows  is  strongly  jointed  (Fig.  85),  the 
joints  are  sometimes  followed  to  some  extent  by  the  smaller  tribu- 
taries. Larger  streams,  however,  are  less  affected,  usually  showing 
little  evidence  of  this  influence.  (5)  When  streams  flow  through 
structural  valleys  (p.  100),  their  direction  is  necessarily  predeter- 
mined. (6)  In  a  region  underlain  by  horizontally  bedded  rock,  the 


FIG.  85.  —  Fall  Creek,  South  Dakota.     Showing  the  effect  of  jointing  on  the 
course  of  a  stream. 

valleys  extend  in  many  directions  without  systematic  arrangement 
and  are  described  as  dendritic  (treelike).  Such  a  river  system  is  in 
striking  contrast  to  one  developed  in  a  region  of  tilted  strata  in  which 
the  beds  vary  in  their  resistance  to  erosion.  In  such  a  region  the 
tributaries  have  a  trellised  appearance  (Fig.  92,  p.  107). 

Basins  and  Divides.  —  All  the  land  surface  which  is  drained  by  a 
river  and  its  tributaries  is  called  its  hydro  graphical  or  drainage 
basin,  and  the  boundary  between  two  river  basins  is  termed 
the  divide,  since  the  water  falling  on  it  is  divided,  part  flowing  into 
one  river  system  and  part  into  the  other.  A  part  of  the  Great  Conti- 


PHYSICAL  GEOLOGY 


nental  Divide,  which  separates  the  basin  of  the  Mississippi  River 
which  empties  into  the  Gulf  of  Mexico  and  that  of  the  Snake  River 
which  finally  discharges  its  waters  into  the  Pacific  Ocean,  is  in  the 
Yellowstone  National  Park.  A  divide  may  be  a  sharp  ridge,  as,  for 


FIG.  86.  —  Block  diagram  illustrating  the  formation  of  outliers  and  the  erosion  of  a 
plateau.  The  fronts  of  the  High  Plains  in  Nebraska  and  elsewhere  are  being  cut  back  in 
this  way. 

example,  in  the  Kicking  Horse  River  basin  of  British  Columbia, 
where  the  divides  between  the  tributaries  have  been  worn  down  to 
knifelike  ridges  which  in  many  places  are  not  a  foot  in  width;  or 
a  flat  plain,  so  level  that  the  location  of  the  divide  is  uncertain.  Such 
a  divide  is  the  height  of  land  between  the  Great  Lakes  and  Hud- 
son Bay,  where  the  same  swamp  often  drains  both  north  and  south. 
The  position  of  the  divide  between  the  Orinoco  and  Amazon  rivers 
in  South  America  is,  perhaps,  even  more  uncertain.  Divides  are 

seldom  stationary, 
since  the  streams  on 
the  opposite  sides  do 
not  usually  cut  head- 
ward  or  laterally  with 
equal  rapidity.  The 
divide  between  two 
tributaries  of  the 
same  river  may  also 
be  narrowed  by  the 
lateral  erosion  of  the 
streams  until  it  dis- 
appears (Fig.  79). 

By  an  increase  in  the  number  of  tributaries,  ridges  are  cut  into 
hills.  In  this  way  the  Seven  Hills  of  Rome  were  sculptured,  and 
many  of  the  conspicuous  buttes  of  the  western  United  States 
(p.  328)  were  separated  from  the  higher  plains  (Figs.  86,  87). 


FIG.  87.  —  Eagle  Rock,  Nebraska.     (U.  S.  Geol.  Surv.) 


THE  WORK  OF  STREAMS 


105 


Elevations  Due  to  Unequal  Hardness. — The  term  hogback  is 
given  to  narrow  ridges  which  stand  above  the  general  level  of  a 
region,  because  of  the  greater  resistance  of  a  steeply  dipping  layer 
of  rock  and  of  the  greater  erosion  of  the  softer  rock  (Fig.  88  A,  B). 
They  are  especially  conspicuous  on  the  flanks  of  mountains.  When 
regions  in  which  the  rocks  are  folded  have  been  subjected  to  erosion 
the  harder  beds  stand  up  as  mountain  ridges.  In  this  way  the  Appa- 


w    s  w    s   w 


FIG.  88.  —  Photograph  and  section  of  a  hogback  near  Canon  City,  Colorado.  The 
ridge  is  due  to  the  superior  strength  of  one  main  and  two  subordinate  strata.  SS  are 
strong  and  WW  weak  rocks.  (After  Brigham.) 

lachian  Mountains  (p.  477)  were  formed.  The  difference  between 
a  hogback  and  such  mountains  is  largely  one  of  height,  width,  and 
extent. 

Where  sheets  of  lava  cover  softer  beds,  as  is  not  uncommon  in 
the  southwestern  portion  of  the  United  States,  flat-topped,  isolated 
hills,  called  mesas  or  tables  (Fig.  89),  are  formed  by  the  headward 
cutting  of  tributary  streams.  Any  harder  bed  of  horizontal  rock 


io6 


PHYSICAL   GEOLOGY 


FIG.  89.  —  Black  Mesa.     (U.  S.  Geol.  Surv.) 

overlying  softer  beds  will  produce  such  hills,  the  name  "  mesa  " 
being  used  to  designate  the  shape  of  the  hill,  not  the  kind  of  rock. 
In  the  western  United  States  the  word  butte  is  used  for  any  steep- 
sided  hill  and  is  also  loosely  used  for  any  conspicuous  elevation. 

Outliers.  —  When  a  part  of  a  formation  is  separated  from  the  main 
body  by  erosion  (or,  occasionally,  by  faulting),  it  is  called  an 
outlier  (Fig.  90).  It  is,  therefore,  simply  a  remnant  of  a  more  ex- 
tensive bed  or  series 
of  beds.  Outliers  are 
usually  short-lived, 
since  they  are  objects 
of  attack  on  all  sides 
by  erosion.  Outliers 


FIG.  90.  —  In  the  diagram  outlier  A  was  formerly  united 
to  By  but  was  separated  from  it  by  erosion. 


often  occur  scattered 
along  the  front  of 
prominent  escarpments;  as,  for  example,  near  the  border  of  the 
High  Plains  in  the  Middle  West  (Fig.  86). 

Rock  Terraces.  —  In  a  region  underlain  by  alternate  hard  and 
soft  beds,  such  as  in  the  Colorado  Plateau,  the  resistant  rocks  may 
form  rock  terraces  and  the  softer  rocks  slopes  in  the  river  valley 
or  canyon.  The  "steps"  or  "benches"  of  the  walls  of  the  Grand 
Canyon  of  the  Colorado  (Fig.  73,  p.  95)  are  among  the  most  striking 
features  of  this  remarkable  valley.  Rock  terraces  may  also  result 


THE  WORK  OF  STREAMS 


107 


from  the  elevation  of  the  land,  since  when  the  gradient  of  a  river  is 
increased  it  is  able  to  cut  a  gorge  in  its  old  valley  floor,  leaving  rock 
terraces  on  the  two  sides. 

Stream  Piracy.  —  Because  of  the  more  rapid  headward  cutting  of 
one  stream  than  another  there  is  a  continual  though  usually  slow 


FIG.  91.  —  One  of  three  diagrams  showing  the  development  of  topography  in  a 
region  where  the  underlying  strata  are  inclined  (dip)  and  vary  greatly  in  their  resistance 
to  erosion.  The  region  is  conceived  to  be  reduced  to  a  peneplain  with  low  ridges  of 
harder  strata.  (Modified  after  Davis.) 

absorption  of  the  tributaries  of  one  river  system  by  another  and 
also  a  struggle  for  existence  among  the  tributaries  of  each  river 
system.  A  stream  which  has  cut  headward  so  rapidly  as  to  divert  the 
headwaters  of  another  stream  to  itself  is  said  to  behead  the  latter, 
and  the  act  is  spoken  of  as  stream  piracy  (Figs.  91,  92,  93). 


FIG.  92.  —  In  this  (second)  diagram  the  peneplain  has  been  elevated  and  the  streams 
have  cut  deep  valleys  and  picturesque  water  gaps.  The  direction  of  the  tributary 
streams  is  determined  by  the  strata,  and  a  "trellised"  drainage  system  results. 

The  result  of  stream  piracy  is  well  shown  in  the  difficulties  experi- 
enced by  a  commission  appointed  by  the  Argentine  and  Chilean  gov- 
ernments to  determine  a  disputed  boundary  in  the  Andes  between 


io8 


PHYSICAL  GEOLOGY 


the  two  republics.  Since  no  map  was  available  when  the  boundary 
was  first  fixed,  this  was  stated  as  following  the  crest  of  the  mountains, 
as  it  was  believed  that  this  was  permanent  and  was  identical  with 
the  divides  between  the  Atlantic  and  Pacific  rivers.  Later  a  dispute 
arose  as  to  the  exact  boundary,  and  the  survey  made  to  settle  the  ques- 
tion showed  how  inaccurate  this  belief  was.  It  was  found  that  the 
Chilean  rivers,  with  their  short,  steep  routes  to  the  Pacific  Ocean, 
had  captured  the  upper  courses  of  nearly  all  of  the  Argentine  rivers, 
obliging  them  to  make  sudden  turns  and  to  flow  through  the  deep 
gorges  which  lead  to  the  Chilean  coast.  Another  example  of  stream 


FIG.  93.  —  In  this  (third)  diagram  some  of  the  subsequent  streams  are  seen  to 
have  cut  back  until  they  have  captured  part  of  the  drainage  of  the  parallel,  consequent 
streams,  leaving  wind  gaps.  When  this  region  is  reduced  to  base  level  it  may  again 
have  the  appearance  shown  in  Fig.  91.  (Modified  after  Davis.) 

piracy  is  to  be  seen  in  the  Kaaterskill  Creek  of  New  York,  which, 
because  of  its  shorter  course  to  the  Hudson,  has  captured  the  lakes 
at  the  headwaters  of  the  Schoharie  Creek  which  flows  on  a  gentle 
gradient  by  a  circuitous  route  to  the  Mohawk  River.  Many  of 
the  "  wind  gaps  "  (passes  without  streams  flowing  through  them) 
of  the  Blue  Ridge  in  Virginia  were  eroded  by  streams  flowing  to 
the  sea,  whose  headwaters  were  captured  by  other  streams  which, 
although  following  longer  courses,  were  able,  because  of  their  greater 
volume  of  water,  to  deepen  their  valleys  more  rapidly  than  those 
flowing  through  these  gaps.  The  Cumberland  Gap,  through  which 
passed  many  thousands  of  the  early  immigrants  to  Kentucky,  has 
such  a  history. 

Conditions  favorable  for  river  capture  occur  in  regions  of  tilted  beds 
(Figs.  91-93)  in  which  there  is  a  marked  difference  in  the  strength  of 
the  rocks.  The  larger  branches  follow  the  outcrops  of  the  weaker  beds, 


THE  WORK  OF  STREAMS 


109 


and  their  tributaries  join  them  at  right  angles,  because  all  except 
the  master  streams  are  subsequent  rivers  (p.  102).  In  such  regions, 
the  larger  streams  cut  rapidly  in  the  weaker  rocks  and  often  behead 
the  streams  that  flow  across  the  hard  beds.  After  a  stream  has  been 
captured  its  new  grade  will  be  steeper  than  before,  and  it  is  likely  to 
cut  a  trench  in  its  old  valley,  leaving  the  remnants  of  the  latter 
as  terraces.  In  regions  of  horizontal  rocks  stream  capture  is  also 
common.  If  one  of  two  streams  heading  toward  the  same  point  has 
a  straighter  and  steeper  course,  or  a  greater  volume  of  water,  or  a 
load  of  sediment  sufficient  for  rapid  cutting  but  not  so  great  as  to 
cause  deposition,  it  may  cut  back  more  rapidly  than  the  other  and 
in  time  capture  the  headwaters  of  the  latter. 


THE  EROSION  CYCLE 

The  terms  youth,  maturity,  and  old  age  are  used  to  express 
the  characteristics  of  valleys,  and  are  helpful  since  they  are  as  descrip- 
tive of  them  as  the  same  terms  applied  to  human  beings.  "  They 
have  reference  not  so  much  to  the  length  of  their  history  in  years  as 
to  the  amount  of  work  which  streams  have  accomplished  in  compari- 
son with  what  they  have  before  them." 

Youth.  —  Young  valleys  are  V-shaped,  with  steep  sides,  and  are 
occupied  by  rapid  streams  unless  the  land  is  low.     Since  they  have 
had    but   a   short   life, 
rapids  and    waterfalls 
are    often   numerous ; 
the   divides    are   wide 
and  ill-drained,  as  the 
frequent  occurrence  of 
marshes  and  lakes  usu- 
ally    indicates.      The 

Grand  Canyon  of  the 

^,  .  ,  FIG.  94.  —  Block  diagram  showing  a   region  in  the 

V^olorado,     the     steep    youthful  stage  of  its  erosion  cycle.     Sea  level  is  repre- 

gorge  of  the   Niagara    sented  by  the  bottom  of  the  diagram. 

River,  and  all  narrow, 

steep-sided,  or  V-shaped  valleys  are  in  youth.     A  region  is  said  to  be 

youthful  (Fig.  94)  when  sufficient  time  has  not  elapsed  for  streams 

thoroughly  to  dissect  and  drain  it ;  in  other  words,  the  streams  have  the 

larger  part  of  their  task  before  them.     The  Red  River  valley  of  North 

Dakota  and  Minnesota  is  such  a  region,  since  it  has  not  long  been 


no 


PHYSICAL  GEOLOGY 


subjected    to    stream   erosion.    It  was    formerly  the  site  of   a  lake 
(p.  656)  whose  bed  was  covered  evenly  with  sediment.      After  the 

lake  was  drained  the 
bed  was  exposed   to 
erosion,  and  a  drain- 
age system  was  de- 
veloped whose  stream 
courses    were    deter- 
mined    by    the    in- 
equalities   of   the 
FIG.  95.  —  Block    diagram    showing    a    region    in   the     bottom.      Later  in  its 
mature  stage  of  its  erosion  cycle.     The  bottom  of  the     history     new      tribu- 
block  is  sea  level.  .  ... 

tanes  will  erode  side 

valleys,  the  main  valleys  will  be  widened,  and  a  mature  topography 
will  result. 

Maturity.  —  A  mature  valley  is  deep,  but  has  flaring  sides  and 
gently  rounded  upper  slopes.  A  region  in  full  maturity  (Figs.  95,  96) 
is  in  decided  contrast  to  a  youthful  region.  Instead  of  few  tribu- 
taries and  consequently  wide 
divides,  the  land  is  thoroughly 
"dissected  by  valleys,  the 
divides  are  narrow,  the  valley 
sides  are  less  steep  than  in 
youth,  and  the  streams  are 
accomplishing  their  greatest 
work  both  in  erosion  and 
transportation.  In  such  a 
region  the  rainfall  runs  al- 
most immediately  into  the 
streams;  lakes  have  practi- 
cally disappeared,  having 
been  drained  by  the  cutting 
down  of  their  outlets  or  filled 
by  stream  sediment  and 
organic  matter.  In  this  stage 
the  relief  is  greatest,  and 
arable  land  is  at  a  minimum; 
roads  are  difficult  and  must 

follow    either    the    valleys    or       FlQ<  96>_Map  showing  the  stream  courses 

the  narrow  divides,  and  the  in  a  mature  region. 


THE  WORK  OF  STREAMS 


III 


inhabitants  are  isolated.  There  are  many  such  regions  in  the  United 
States;  for  example,  large  portions  of  West  Virginia,  southeastern 
Ohio,  eastern  Kentucky,  and  Tennessee  are  in  maturity.  As  a  rule 
a  master  stream  reaches  maturity  earlier  than  its  tributaries,  and  in 
its  lower  course  earlier  than  in  its  upper  course.  A  region  in  maturity 
may  be  traversed  by  a  stream  which  flows  through  a  broad,  old  val- 
ley, and  a  youthful  region  may  be  traversed  by  a  mature  stream. 

Old  Age.  —  Continued  erosion  will  gradually  cut  down  the  valley 
sides  (Fig.  97)  to  gentle  slopes,  lower  the  divides,  and  thus  tend  to 
reduce  the  surface  to  an  undulat- 
ing plain.  The  sluggish  streams 
will  meander  (p.  121)  in  wide 
valleys.  The  region  is  then  in 
old  age  (Figs.  98,  91).  An  abso- 
lute plain  may,  perhaps,  never 
be  reached,  since  elevations  will 


FIG.   97.  —  Diagram  showing  the  profile 
of  young,  mature,  and  old  valleys. 


be  left  here  and  there  because  of  some  favoring  condition,  such  as 
(i)  hardness  of  rock  or  (2)  a  favoring  position  with  reference  to 
the  drainage  of  the  plain.  Such  hills  or  mountains  rising  above  the 
general  level  of  the  surface  are  called  monadnocks,  from  a  mountain 
of  that  type  in  New  Hampshire.  Portions  of  Kansas  have  passed 

through  youth  and 
maturity  and  are  now 
in  the  stage  of  old 
age. 

The  time  required 
for  the  production  of 
a  base-leveled  condi- 
tion or  for  "  pene- 
planation "  is  called 
the  cycle  of  erosion. 
It  will  take,  perhaps,  one  hundred  thousand  times  as  long  to  pass 
from  maturity  to  old  age  as  from  youth  to  maturity.  It  will  be 
seen  from  the  above  that  the  age  of  a  region  is  not  recorded  in  years 
but  in  the  work  accomplished  or  to  be  accomplished. 

Effect  of  Elevation  and  Depression  on  Streams.  —  If  a  region 
is  elevated  after  it  has  been  reduced  to  base  level  (peneplain),  the 
streams  will  be  quickened  and  will  again  be  enabled  to  deepen  their 
valleys.  If  the  streams  meandered  (p.  121)  on  the  peneplains,  they 
may  intrench  themselves  in  their  old  courses  until  they  flow  through 

CLELAND    GEOL.  —  8 


FIG.  98.  —  Block  diagram  showing  a  region  in  old  age. 
Sea  level  is  represented  by  the  bottom  of  the  block. 


112 


PHYSICAL  GEOLOGY 


deep,  meandering  rock  gorges.  When  a  stream  has  thus  intrenched 
its  meanders,  the  evidence  is  strong  that  it  has  been  rejuvenated. 
Many  examples  of  intrenched  meanders  are  to  be  seen  in  Europe  and 
in  the  United  States.  In  the  latter,  Pennsylvania,  Kentucky,  and 
Utah  furnish  excellent  and  striking  examples.  The  great  natural 
bridges  of  Utah,  one  of  which  has  a  height  of  305  feet  and  a  span  of 


1 


FIG.  99.  —  Map  showing  the  course  of  the  Ardeche  River,  France.     The  origin  of 
the  natural  bridge  by  the  perforation  of  the  neck  of  the  meander  is  evident. 

273  feet,  were  formed  by  the  perforation  of  the  necks  of  intrenched 
meanders,  as  was  also  that  of  the  Ardeche  River,  France  (Fig.  99). 

If  a  region  is  uplifted  before  the  erosion  cycle  is  completed,  the 
rivers  will  deepen  their  courses,  leaving  their  former  broad  flood 
plains  (p.  128)  standing  as  terraces  or  "  benches."  A  section  of  such 
a  valley  will  show  a  valley  within  a  valley.  If  a  region  is  more  ele- 
vated near  the  ocean  than  further  inland,  the  upper  courses  of  the 
streams  will  be  "  ponded,"  unless  they  are  able  to  deepen  their  valleys 
as  rapidly  as  the  land  is  elevated.  This  differential  movement  of 
the  earth's  surface  is  called  warping.  Streams  which  hold  their 


THE  WORK  OF   STREAMS 


courses  in  spite  of  differential  elevations,  as  has  been  seen  (p.  102), 
are  called  antecedent  streams. 

If  a  region  underlain  by  tilted  rocks  which  vary  in  composition, 
some  resisting  erosion  more  than  others,  is  reduced  to  base  level  and 
then  raised,  the  sub- 
sequent erosion  is 
such  as  to  give  cer- 
tain proof  of  its 
earlier  history.  An 
interesting  example, 
in  which,  however, 
the  river  encountered 
granite  (Fig.  100  A 
and  B)  rather  than 
tilted  rock,  is  found 
in  the  history  of  the 
Gunnison  River  in 
Colorado.  When  the 
Rocky  Mountains 
were  being  uplifted 
to  their  present  posi- 
tion, the  streams 
which  now  drain 
them  began  to  cut 
their  valleys.  Among 
them  the  Gunnison 
River  followed  along 
the  depression  of  the 
plateau  and  began  to 
deepen  its  bed.  Its 
course  happened  to 
lie  over  a  great  mass 
of  granite,  buried  be- 
neath softer  strata. 
The  river,  having 
a  steep  gradient, 
rapidly  cut  its  way  through 
encountered  the  granite 


FIG.  100.  —  Two  block  diagrams  showing  the  effect  of 
erosion  upon  resistant  and  weak  rocks.  The  streams  in  A 
have  approximately  the  same  slope  and  are  deepening 
their  valleys  in  strata  of  the  same  kind.  B  shows  that 
the  stream  on  the  right  encountered  resistant  granite 
which  was  both  eroded  and  weathered  more  slowly  than 
the  weaker  rock.  As  a  consequence,  the  stream  on  the 
right  has  cut  a  deep  and  steep-sided  gorge,  while  that  on 
the  left  has  cut  a  broad  valley  with  gently  sloping  sides. 


the  soft  surface  rocks  and  finally 
Since  its  valley  was  already  deep  when 
this  occurred,  it  was  unable  to  turn  aside  from  the  hard  rock  and 
continued  to  cut  its  way  through  it  until  the  picturesque  Black 


n4 


PHYSICAL  GEOLOGY 


FIG.  101.  —  Drowned  river  valleys. 
Chesapeake  and  Delaware  bays  and  Albe- 
marle  Sound  were  formed  by  a  lowering  of 
the  land  which  permitted  the  sea  to  fill  the 
valleys. 


Canyon,  more  than  2000  feet 
deep,  was  excavated.  The  Un- 
compahgre  River,  which  joins 
the  Gunnison  after  flowing  in 
approximately  the  same  direc- 
tion for  some  distance,  was  born 
at  the  same  time.  It  has  flowed, 
however,  over  soft  material  which 
could  be  readily  eroded,  and  has 
been  able  to  excavate  a  valley 
several  miles  in  width  which  in 
one  place  is  separated  from  the 
narrow  Black  Canyon  by  a 
narrow  ridge  of  granite. 

If  a  region  is  depressed,  the 
velocity  of  the  streams  will  be 
lessened,  and  the  condition  of  old 
age  will  be  hastened.  Drowned 
river  valleys  (p.  227),  such  as 
the  Delaware,  the  St.  Lawrence, 
and  Chesapeake  Bay  (Fig.  101),. 
are  the  result  of  the  sinking  of 
the  land  in  the  lower  courses  of 
the  rivers. 


PENEPLANATION 


When  a  base-leveled  region 
(peneplain)  has  later  been  up- 
lifted and  dissected  by  erosion, 

the  evidence  of  the  former  base-leveled  condition  is  to  be  seen  in 
the  horizontal  sky  line  presented  by  the  higher  hills  (Fig.  102).  The 
effect  of  erosion  on  an  elevated  peneplain  under  different  conditions 
of  rock  structure  is  shown  by  a  study  of  (i)  southern  New  England, 
(2)  the  Appalachian  region,  and  (3)  eastern  Canada. 

(i)  The  Peneplain  of  Southern  New  England.  —  Southern  New 
England  is  underlain  on  each  side  of  the  broad  Connecticut  valley 
by  hard,  crystalline  rocks ;  the  valley  is  in  part  composed  of  sand- 
stone and  in  part  of  lava.  Before  the  present  elevation  took  place, 
erosion  had  been  active  for  so  long  that  even  the  lavas,  granites, 


THE  WORK  OF   STREAMS 


FIG.   102.  —  The  peneplain  of  the  Rhine  district  near  St.  Goar,  in  which  the  Rhine 
has  cut  a  shallow  valley.     (Photo.  D.  W.  Johnson.) 

gneisses,  and  schists  had  been  cut  down  to  a  comparatively  level 
plain,  above  which  stood  some  hills  a  few  hundred  feet  high,  such  as 
Mt.  Monadnock,  Mt.  Greylock,  and  Mt.  Wachusett.  When  this 
peneplain  was  raised  and  the  streams  again  began  to  erode,  the  weak 
sandstones  were  quickly  cut  away,  leaving  the  trap  rocks  standing 
as  the  Holyoke  and  other  trap  ranges  of  the  Connecticut  valley, 
and  the  old  crystalline  rocks  bounding  the  "  valley  "  as  the  high- 


FIG.    103.  —  Peneplain  with  several  monadnocks  in  the  distance. 
Camp  Douglas,  Wisconsin.     (Sankowsky.) 


Near 


lands.  In  the  highlands,  streams  have  cut  deep  and  usually  narrow 
valleys.  The  higher  hills  have  about  the  same  altitude,  except  that 
the  surface  of  the  ancient  peneplain  rises  to  the  northwest ;  on  Long 
Island  it  is  at  sea  level,  but  its  height  increases  to  an  altitude  of 
about  1500  to  2000  feet  in  Vermont  and  New  Hampshire.  Mt. 


Il6  PHYSICAL  GEOLOGY 

Monadnock,  Mt.  Greylock,  Mt.  Wachusett,  and  some  others,  as  has 
been  stated,  rise  as  monadnocks  (Fig.  103)  several  hundred  feet 
above  the  ancient  plain. 

(2)  The  Appalachian  Peneplain.  —  The  Appalachian  Mountain 
region,  from  the  Hudson  River  to  Alabama,  is  underlain  by  rocks 
differing  in  their  resistance  to  erosion,  which  have  been  bent  into 
broad  folds  and,  in  places,  broken  by  faults  (p.  25)  (Fig.  104).  In 
ancient  times  (Cretaceous,  p.  516)  the  folds  were  planed  off  by  ero- 
sion, leaving  the  outcropping  strata  in  long,  more  or  less  parallel 
lines,  resistant  beds  alternating  with  weaker  ones.  The  surface  of 
this  (Cretaceous)  peneplain  is  now  seen  in  the  approximately  level 
crests  of  the  ridges,  showing  that  the  base-leveling  of  the  region  had 
been  almost  completed.  Upon  this  plain  the  rivers  took  their 
courses  to  the  sea  :  the  Delaware,  Susquehanna,  and  Potomac  flowing 
to  the  Atlantic  across  the  strata  without  regard  to  their  structure; 
the  New  River  of  Virginia  and  the  French  Broad  of  North  Carolina 


FIG.   104.  —  A  generalized  section  across  the  southern  Appalachian  Mountains. 
Peneplains  are  shown  by  the  dotted  lines. 

flowing  to  the  west;  while  the  southern  part  of  the  region  was 
drained  to  the  south  by  the  large  Appalachian  River.  An  up- 
warping  along  a  north-south  axis  occurred  which  diminished  the  ve- 
locity of  some  streams  and  increased  that  of  others,  thus  favoring 
stream  capture  (p.  108).  The  Potomac,  Susquehanna,  and  Dela- 
ware rivers,  continuing  to  flow  in  approximately  their  old  channels, 
cut  the  deep  gorges  or  water  gaps  at  Harpers  Ferry,  the  Delaware 
Water  Gap,  and  near  Harrisburg.  The  tributaries  of  these  rivers, 
such  as  the  Shenandoah  and  Lehigh,  cutting  more  rapidly  in  the 
weaker  limestone  beds,  have  excavated  broad,  subsequent  valleys, 
more  or  less  at  right  angles  to  their  mains,  leaving  the  resistant  strata 
standing  up  as  mountains  (Fig.  105).  The  gradient  of  the  south- 
westward,  as  well  as  that  of  the  eastward-flowing  streams  was  in- 
creased and  resulted  in  the  headward  cutting  of  one  of  these  until  it 
captured  the  headwaters  of  the  southward-flowing  Appalachian 
River.  A  later  warping  in  northern  Alabama  and  Mississippi  along 
an  east-west  line  caused  a  tributary  of  the  Ohio  to  cut  headward 
and  capture  the  stream  which  had  formerly  robbed  the  Appalachian 


THE  WORK  OF  STREAMS 


117 


River.     In  this  way  the  Tennessee  originated,  made  up  of  parts  of 
three  rivers  which  formerly  had  different  courses. 

After  the  first  elevation,  the  region  remained  at  approximately  the 
same  level  for  a  long  time,  as  is  shown  by  the  accordant  altitudes  of 


FIG.  105.  —  The  Water  Gap  near  Harrisburg,  Pennsylvania.     The  horizontal  sky 
line  shows  the  surface  of  the  ancient  peneplain.     (Maryland  Geol.  Surv.) 

the  plainlike  valleys  between  the  mountains,  but  long  before  the 
ridges  could  be  reduced,  another  uplift  (Fig.  104)  occurred  which 
caused  the  streams  to  deepen  their  beds  to  the  present  level.  The 
last  uplift  must  have  been  relatively  recent,  since  the  new  valleys 
are  as  yet  comparatively  narrow. 

(3)  The  Laurentian  Peneplain.  —  The  great  hunting  and  fishing 
region  of  North  America  is  that  vast  area  almost  surrounding  Hud- 
son Bay,  which  stretches  from  Lake  Superior  and  the  St.  Lawrence 
River  on  the  south 
to  the  Arctic  Ocean 
on  the  north,  and 
from  the  shores  of 
Labrador  on  the 
east  to  Lake  Win- 
nipeg on  the  west 
(Fig.  106).  This  is 
known  as  the  "  Lau- 
rentian shield  "  (p. 
389).  When  one 
stands  on  almost 
any  eminence  in  this 
region,  he  finds  that 


he   is    on    a    great 


FIG.    106.  —  The  horizontal   sky  line  of  the  Laurentian 
peneplain  with  an  incised  valley.     (Photo.  T.  C.  Brown.) 


n8  PHYSICAL  GEOLOGY 

plain  dotted  with  lakes,  in  which,  especially  along  the  margins, 
the  streams  flow  through  deep  valleys  over  falls  and  rapids.  The 
ruggedness  of  the  southeastern  margin  of  the  peneplain,  where 
it  borders  the  St.  Lawrence,  is  due  to  the  many  valleys  which 
have  been  cut  in  it  and  which  have  given  rise  to  the  rough  region 
known  as  the  Laurentian  Mountains.  The  complicated  and  dis- 
torted rocks  of  the  region  vary  greatly  in  composition,  but  have, 
nevertheless,  been  reduced  to  a  common  level,  with  the  exception  of 
the  residual  domes  and  ridges  (monadnocks)  of  the  peneplain.  The 
interior  peninsula  of  Labrador  is  so  level  that  in  an  area  of  200,000 
square  miles  there  is  not  a  difference  of  general  level  of  more  than 
300  or  400  feet.  "  The  Canadian  shield  can  be  described  as  an  ancient 
peneplain  which  has  undergone  differential  elevation;  has  been 
denuded,  and  subsequently  slightly  incised  around  the  uplifted 
margin."  (Wilson.) 

The  Adirondacks  were  in  part  reduced  to  base  level  at  the  same 
time,  but  in  the  eastern  portion  either  the  surface  was  not  base- 
leveled,  or  subsequent  movements  have  raised  it  and  given  it  vary- 
ing altitudes. 

Rate  of  the  Denudation  of  Continents.  —  The  land  surface  of  the 
United  States  is  being  lowered  at  an  average  rate  of  about  one  inch 
in  760  years,  or  of  one  foot  in  a  little  more  than  9000  years.  The 
total  amount  carried  to  the  sea  each  year  from  the  United  States  is 
approximately  270,000,000  tons  of  dissolved  matter  and  513,000,000 
tons  of  suspended  matter.  "  If  this  erosive  action  had  been  con- 
centrated upon  the  Isthmus  of  Panama  at  the  time  of  the  American 
occupation,  it  would  have  excavated  the  prism  for  an  eighty-five 
foot  level  canal  in  about  seventy-three  days."  l 

How  the  Load  of  Streams  is  Measured.  —  Estimates  such  as  the 
above  are  obtained  by  measuring  the  amount  of  water  discharged 
by  rivers,  together  with  the  minerals  in  solution  and  the  insoluble 
silt,  and  pebbles  which  are  either  carried  in  suspension  or  rolled  along 
the  bottom.  The  volume  of  water  discharged  by  a  river  is  found  by 
multiplying  the  number  of  square  feet  in  its  cross  section  by  the  ve- 
locity a  second  to  obtain  the  discharge  a  second.  The  quantity  of 
silt  is  found  by  filtering  samples  of  water  from  various  portions  of 
the  section  at  different  times  of  the  year,  and  the  quantity  of  soluble 
material  is  determined  by  evaporating  samples  of  the  water  after 
filtering.  The  difficulty  in  obtaining  accurate  results  is  due  to  the 
1  U.  S.  Water-Supply  Paper  No.  234,  p.  83. 


THE  WORK  OF   STREAMS 


119 


fact  that  the  velocity  and  volume  of  rivers  fluctuate  often  from  day 
to  day,  and  the  quantity  of  silt  varies  with  the  velocity.  Moreover, 
the  material  in  solution  in  a  cubic  foot  is  greater  at  low  than  at  high 
water,  since  the  proportion  of  spring  water  is  then  greater.  It  is 
also  difficult  to  measure  the  quantity  of  material  roiled  along  the 
bottom.  It  is  believed,  however,  that  notwithstanding  these  diffi- 
culties, the  estimate  of  the  rate  of  denudation  of  the  United  States 
of  one  foot  in  about  900x5  years  is  accurate  within  20  per  cent. 

DEPOSITION 

Causes  of  Deposition.  —  Streams  bearing  a  full  load  will  deposit 
their  sediment  when  their  velocities  are  diminished,  (i)  A  stream 
flowing  from  a  steep  to  a  gentle  gradient  will  deposit  its  coarser  sedi- 
ment. (2)  When  a  stream  emerging  from  a  straight,  narrow  channel 
flows  into  a  wide,  winding  one,  its  current  is  diminished  by  friction 
with  its  bottom  and  sides,  and  deposition  may  take  place.  (3)  When 
tributary  streams  with  steep  gradients  flow  into  slow-moving  main 
streams,  they  may  deposit  a  part  of  their  load.  (4)  Since  the  velocity 
of  a  stream  increases  with  its  volume,  it  is  evident  that,  if  the  volume 
is  diminished  in  any  way,  as  by  seepage  or  evaporation,  its  ability 
to  carry  sediment  will  be  correspondingly  decreased.  Consequently, 
rivers  in  arid  regions  are  often  depositing  streams  (p.  81),  even  when 
they  have  steep  gradients.  (5)  When  slow-moving  streams,  carrying 
much  fine  sediment,  meet  any  obstruction,  such  as  a  stranded  log  or 
a  tree  which  has  fallen  from  the  bank,  the  slight  check  to  the  current 
produced  in  this  way  may  cause  the  formation  of  a  sand  bar  or 
island.  Many  of  the  islands  of  the  lower  Mississippi  River  began 
as  "  snags."  (6)  When  a  stream  reaches  a  body  of  still  water,  either 
a  large  lake  or  the  ocean,  all  of  the  sediment  soon  finds  a  resting 
place.'  The  goal  of  all  sediment  is  the  sea,  but  in  its  journey  ocean- 
ward  it  makes  many  halts,  forming  the  alluvium  of  the  river  valley. 

Flood  Plains.  —  Flood  plains  are  formed  by  graded  streams,  as 
a  result  of  both  lateral  erosion  and  of  deposition  during  overflow. 
A  broad,  flat  valley  may  be  formed  in  this  way.  Since  rivers  nor- 
mally first  reach  base  level  where  they  enter  the  sea,  their  flood  plains 
are  usually  widest  there.  The  lower  Mississippi  flood  plain  (which 
is,  perhaps,  more  correctly  described  as  a  delta)  is  five  to  eight  miles 
wide  and  is  bounded  on  the  east  by  clay  bluffs  100  to  300  feet  high, 
and  on  the  west  side,  as  far  as  the  Red  River,  by  less  prominent 


120 


PHYSICAL  GEOtOGY 


banks.  The  downstream  slope  of  a  flood  plain  varies  with  the  vol- 
ume of  the  water  in  the  stream  and  its  load.  The  slope  of  the  flood 

plain  of  the  lower 
Mississippi,  for  ex- 
ample, is  only  two  to 
three  inches  a  mile, 
while  streams  carry- 
ing coarse  material 
may  build  up  flood 
plains  with  slopes  of 
50  to  75  feet  to  the 

mile.  Flood  plains  are  highest  near  the  river  and  slope  gradually 
away  from  it  (Fig.  107).  This  is  due  to  the  fact  that,  at  flood, 
the  coarser  and  more  abundant  material  is  deposited  where  the 
silt-laden  main  current  is  checked  by  contact  with  the  slow-moving 
waters  of  the  sides. 


FIG.  107.  —  The  flood  plain  of  a  river.  The  natural 
levees  on  each  side  are  shown,  as  is  also  the  structure  of 
flood-plain  deposits. 


FIG.  108.  —  Meanders,  Owens  valley,  California.     (U.  S.  Geol.  Surv.) 


THE  WORK  OF  STREAMS 


121 


The  fertility  of  the  Nile  valley  in  Egypt  is  due  to  the  thin  layer 
of  silt  which  is  spread  over  the  flood  plain  each  year.  If  the  sedi- 
ment deposited  on  a  flood  plain  is  coarse,  the  plain  will  be  infertile. 

Meanders.  —  After  a  river  has  become  more  sluggish  and  is  con- 
sequently unable  to  cut  downward,  it  may  undercut  its  banks  on  the 
outside  of  its  curves  and  thus  widen  its  valley  floor.  As  the  outside 
of  a  curve  is  cut  away,  the  inside  is  filled  with  sediment  to  flood  level, 
and  a  strip  of  land  is  thus  formed.  In  this  way,  as  well  as  by  deposi- 
tion during  overflow,  a  broad,  flat  valley  is  developed  which,  as  has 
been  said,  is  called  a  flood  plain  because  covered  by  water  during  floods. 
On  such  a  flood  plain  a  river 
will  take  a  still  more  winding 
or  meandering  course  (Fig.  108). 
The  origin  of  these  meanders  is 
easily  conjectured.  Imagine  a 
perfectly  straight  stream  flow- 
ing through  a  level  alluvial  plain. 
If,  under  such  conditions,  a  tree 
is  blown  over  into  the  stream, 
a  rock  falls  from  the  bank,  or 
a  tributary  stream  forces  the 
current  against  the  opposite  bank, 
or  brings  in  gravel  and  builds 
a  natural  jetty,  the  current  will 
be  deflected  a  little,  the  bank 
will  be  undercut,  and  the  chan- 
nel changed  at  this  point  (Fig. 

109).  The  stream  will  then  strike  the  opposite  bank  obliquely 
a  little  further  down  its  course,  wearing  it  away  at  this  point,  and 
thus,  one  after  another,  new  meanders  will  be  formed.  A  single 
obstruction  may,  therefore,  affect  the  oscillations  of  the  current  for 
an  indefinite  distance  down  its  course.  The  length  of  the  Mississippi 
River  (Fig.  no),  from  the  mouth  of  the  Ohio  to  the  Gulf  of  Mexico, 
is  1000  miles,  so  meandering  is  its  course,  although  the  direct  dis- 
tance is  only  600  miles.  One  of  the  plans  for  improving  the  Missis- 
sippi is  to  straighten  the  channel  by  cutting  off  the  curves. 

Oxbow  Lakes.  —  Once  initiated  (Fig.  109),  meanders  tend  to  be- 
come more  pronounced  in  form,  changing  from  an  open  loop  to  one 
which  is  horseshoe-shaped.  The  neck  of  land  separating  one  bend 
from  the  next  may  become  more  and  more  narrow  (Fig.  in)  until, 


•1- 


FIG.  109. —  Diagram  showing  the  initiation 
of  meanders.     (Modified  after  Salisbury.) 


122 


PHYSICAL  GEOLOGY 


in  time  of  flood,  the  river  may 
straighten  its  course  by  cutting 
a  channel  across  this  narrow 
strip,  leaving  horseshoe  or  oxbow 
lakes,  or  bayous,  which  are  soon 
separated  from  the  new  and 
shorter  channel  by  deposits  of 
silt.  Brooks  tend  to  develop  a 
greater  number  of  bends  than 
larger  rivers,  since  they  are 
easily  deflected  by  accidental 
disturbances,  such  as  a  fallen 
tree  or  a  landslide,  while  a  larger 
river  tends  to  obliterate  its 
smaller  irregularities  and  to 
develop  the  larger  ones.  As  a 
result,  we  find  many  close-set 
meanders  in  small  brooks,  while 
in  large  riverjs  there  are  a  small 
number  of  well-spaced  meanders 
which  grow  to  large  size  before 
FIG.  i io.  — Meanders  of  the  Mississippi  they  are  cut  off.  Many  ex- 
River.  The  successive  positions  of  the  amples  might  be  cited  of  cities 
river  in  1883,  1895,  and  later  are  shown.  d  m  j  d  h  b  fc 

Ihe    movement    of   the    meanders  down-  &    . 

stream  and  their  tendency  to  increase  are     of  meandering  rivers,  which  have 
shown.    (After  Salisbury.)  been  left  far  inland  by  the  cut- 

ting   off    of    the    meanders    on 

which  they  were   situated.      Because   of  their   changing   channels 
rivers  make  very  poor  political  boundaries. 

In  the  course  of  time  the  "  oxbow  "  lakes  formed  by  the  "  cut- 
offs "  are  destroyed, 
as  they  are  apt  to 
be  filled  with  sedi- 
ment when  the 
stream  is  at  flood, 
and  at  other  times 
sand  is  blown  in  by 
the  wind,  and  vege- 
tation takes  root  FlG>  Iir>  _  An  oxbow  lake  formed  by  the  cutting 
there.  through  of  the  neck  of  a  meander. 


THE  WORK  OF  STREAMS 


123 


Natural  Levees.  —  A  study  of  a  topographic  map  of  the  lower 
Mississippi  River  shows  that  it  flows  between  banks  which  rise  ten  or 
more  feet  above  the  surrounding  swamps,  and  occasionally  constitute 
the  only  dry  land  for  long  distances.  Such  embankments  are  called 
natural  levees  (Fig.  107).  They  are  gradually  built  up  in  time  of  flood 
when  the  water  is  swift  and  contains  much  sediment.  The  current 
in  the  channel  is  sufficient  to  carry  the  sediment  onward,  but  its 


FIG.  112.  —  Map  showing  the  changes  in  the  course  of  the  Hoang  Ho  on  its  delta 
(shaded).  The  river  is  useless  for  navigation  because  it  is  so  changeable,  and  its 
waters  are  restrained  only  by  an  elaborate  system  of  dikes  and  canals.  (Richtofen.) 

velocity  is  checked  when  it  comes  in  contact  with  the  slow-moving 
flood  water  on  the  sides,  sediment  is  deposited,  and  an  embank- 
ment is  thus  erected  above  the  swamp.  Natural  levees  are  often 
strengthened  and  heightened  artificially  to  prevent  floods,  but  it  is 
readily  seen  that  during  a  flood  a  river  may  break  through  its 
levees,  spread  over  its  swamps,  and  perhaps  change  its  course.  The 
Mississippi  River  broke  through  its  levees  in  1912,  causing  great 
destruction  of  life  and  property.  The  levees  of  the  Hoang  Ho  in 


I24 


PHYSICAL  GEOLOGY 


China  have  been  increased  artificially  so  that,  in  places,  the  surface 
of  the  river  is  30  feet  above  the  surrounding  plain,  but  in  spite  of 
man's  efforts  it  has  often  changed  its  course  and  is  called  "  China's 
Sorrow  "  because  of  its  great  destructiveness  (Fig.  112).  In  1904  the 
mouth  was  250  miles  north  of  its  position  40  years  before  (p.  133). 
Natural  levees  are  sometimes  high  enough  to  turn  the  courses  of 
the  tributary  streams  for  long  distances;  thus  the  Yazoo  travels 
for  200  miles  parallel  to  the  Mississippi  before  entering  it,  and  the 


FIG.  113.  —  Shallow  basins  formed  as  a  result  of  the  building  of  natural  levees  along 
the  stream  are  filled  at  flood  time  with  water  from  the  stream  and  from  the  sides  of  the 
valley.  (Minneapolis  topographic  sheet,  U.  S.  Geol.  Surv.) 

St.  Francis  for  100  miles.  Lakes  are  sometimes  formed  when  a 
stream  in  winding  through  a  valley  builds  up  its  levees  and  thus 
incloses  basins  between  them  and  the  banks  of  the  valley  (Fig.  113). 
Alluvial  Cones  and  Fans,  (i)  In  Arid  Regions.  —  In  desert  re- 
gions streams  are  fed  chiefly  by  tributaries  whose  sources  are  in  the 
mountains  where  the  rainfall  is  greater  than  on  the  arid  plains.  At 
rare  intervals  heavy  downpours  (cloud-bursts)  may  occur  on  the 
lower  courses  which,  though  often  of  only  a  few  minutes'  duration, 
may  fill  the  valleys,  producing  torrents  of  great  erosive  power.  But 
ordinarily  such  streams  rapidly  lose  volume  as  they  flow  out  on  the 
thirsty  land,  as  their  lower  courses  are  seldom  fed  by  springs.  Dur- 
ing certain  seasons,  when  the  rainfall  in  the  mountains  is  heavy, 
some  desert  rivers  are  a  hundred  miles  longer  than  at  other  times. 
Streams  flowing  from  high  lands  into  deserts  quickly  drop  their  sedi- 
ment at  the  mouths  of  their  gorges,  both  because  their  gradients  are 
diminished  and  because  their  velocity  is  decreased  as  water  is  lost 


THE  WORK  OF  STREAMS 


125 


FIG.  114.  —  An  alluvial  fan  near  Salt  Lake  City.     (U.  S.  Geol.  Surv.) 

by  evaporation  and  by  absorption  into  the  porous  soil.  In  this  way 
a  pile  of  waste  is  accumulated,  half  cone-shaped,  with  a  base  varying 
in  diameter  from  a 
foot  to  forty  or  more 
miles.  Accumula- 
tions such  as  this  are 
called  alluvial  cones 
when  steep,  or  fans 
(Fig.  114)  when  the 
slope  is  not  great. 
In  general  they  are 
composed  of  coarser 
materials  at  the  apex 
and  progressively 
finer  ones  toward  the 
base,  since  a  stream 
first  drops  the  larger 
debris  with  which  it 
is  burdened  when  its 

velocity    is    checked. 

TV™*   ci-r^Qmc   flrtwino-          FlG-  H5-  — The  San  Joaquin  valley  and  Tulare  Lake, 
Ihe  streams  Mowing    CaUfornia;»  The  basin  of  Tulare  Lake  is  due  chiefly  to 

Over  alluvial  cones  or     tne  building  up  of  alluvial  fans  across  the  San  Joaquin 
fans       seldom       have     valley  by  Kings  River  and  Los  Gatos  Creek. 


126 


PHYSICAL  GEOLOGY 


single  channels  throughout  their  courses,  because,  as  they  lose 
volume,  they  are  unable  to  carry  all  of  their  load  and  therefore 
deposit  it  along  the  sides  of  their  channels,  so  narrowing  them 
that  the  water  breaks  through  the  banks  and  forms  other  channels. 
This  process  may  be  repeated  again  and  again  until  at  the 
base  of  the  cone  a  stream  has  been  divided  into  a  number  of 
distributaries.  These  distributaries  tend  to  keep  the  fan  or  cone 
symmetrical.  The  angle  of  the  slope  of  these  accumulations  varies 
(i)  with  the  rapidity  with  which  the  velocity  of  the  stream  is 


UNCONSOLIDATEO    SEDIMENTS 


Porous  sand         Quartzite          Limestone 
ay  gravel  above  ground-    and  gravel  below 

water  table  ground-water  table 


Impervious    Porous  sand  and 
d 


Crystalline 
rock 


FIG.  116.  —  Cross  section  of  a  typical  valley  in  an  arid  region.  Beneath  the  alluvial 
slope  gravel  predominates,  but  towards  the  central  flats  it  gives  way  to  alternate 
layers  of  sand  and  clay.  Water  is  obtained  when  wells  reach  the  porous  sediments,  as 
at  c,  d,  and  e.  The  dotted  line  shows  the  base  of  the  alluvial  slope.  (U.  S.  Geol. 
Surv.) 

diminished;  (2)  with  the  kind  and  amount  of  the  sediment;  and 
(3)  with  the  size  of  the  stream.  The  slope  of  cones  and  fans  of  large 
streams  usually  is  less  than  that  of  small  torrents,  which  may  be  as 
steep  as  from  5  to  15  degrees. 

An  alluvial  fan  sometimes  causes  the  formation  of  a  lake  by  build- 
ing a  dam  across  a  river.  Where  Kings  River  enters  the  San  Joaquin 
River  of  California  it  has  deposited  a  fan  which  has  dammed  the  San 
Joaquin,  forming  the  shallow  Tulare  Lake  (Fig.  115). 

Piedmont  or  Alluvial  Plains  are  formed  by  the  coalescing  of 
adjoining  fans.  The  slope  of  such  plains  may  be  so  uniform  that 
the  angle  is  not  easily  detected  by  the  naked  eye  by  one  traveling 


THE  WORK  OF  STREAMS 


127 


over  the  region  (Fig.  116).  Almost  any  topographic  map  of  a  desert 
basin,  however,  shows  that  the  slope  of  Piedmont  plains  is  usually 
considerable. 

In  desert  regions  oases  are  often  found  on  alluvial  fans,  since  water 
can  be  obtained  here  from  wells  or  from  the  mountain  streams.  The 
principal  settlements  of  Utah  are  on  the  alluvial  slopes  at  the  foot  of 
the  Wasatch  Mountains,  and  many  of  the  cities  of  Persia  and  Turke- 
stan are  situated  on  alluvial  fans. 

(2)  In  Humid  Regions.  —  Alluvial  cones  and  fans  are  also  de- 
posited in  moist  regions  where  a  main  stream  is  unable  to  remove  the 
rock  and  silt  carried  into  it  by  its  tributaries.  In  such  cases,  the 


FIG.  117.  —  Lake  Brienz  and  Lake  Thun  were  formerly  one  lake,  but  have  been 
separated  by  an  alluvial  fan  upon  which  Interlaken  is  situated. 

cone  or  fan  forces  the  main  stream  over  to  the  opposite  side  of  the 
valley,  compelling  it  to  undercut  its  bank.  This  may  cause  the  for- 
mation of  rapids,  with  a  shallow  lake  above.  The  Liitschine  River,  in 
the  Lauterbrunnen  valley  in  Switzerland,  has  built  an  alluvial  fan 
which  has  divided  the  lake  into  which  the  river  flows  into  two  parts, 
Lake  Brienz  and  Lake  Thun  (Fig.  117).  Fans  in  humid  regions  may 
be  of  considerable  extent,  and  are  well-developed  in  portions  of  the 
Rhone  valley  in  Switzerland  and  in  the  larger  valleys  of  the  French 
Alps.  Since  they  are  well-drained  and  usually  fertile,  they  are  often 
the  sites  of  villages. 

Alluvial  Terraces.  —  Terraces  are  not  uncommon  in  river  valleys 
and  are  composed  either  of  rock,  when  they  are  called  rock  terraces 
(p.  128),  or  of  stratified  clay,  sands,  and  gravels,  when  they  are  known 

CLELAND    GEOL. — 9 


128  PHYSICAL  GEOLOGY 

as  alluvial  terraces.  The  latter  are  fragments  of  sediments  which 
once  filled  the  valleys  to  their  level,  and  may  be  accounted  for  by 
meandering  and  swinging  streams,  slowly  degrading  valleys  which 
had  previously  been  aggraded ;  in  other  words,  by  streams  slowly 
eroding  their  flood  plains.  Such  a  change  from  deposition  to  erosion 
may  be  the  result  of  one  or  more  of  several  causes,  (i)  If  a  region 
is  elevated  so  as  to  increase  the  velocity  of  the  streams,  deposition  is 
succeeded  by  erosion.  (2)  This  is  also  true  if  the  volume  of  water 
in  a  stream  increases  without  a  corresponding  increase  of  sediment. 
Such  a  condition  may  result  when  a  moist  climate  follows  a  dry 
one,  or  when  a  stream  captures  the  headwaters  of  another  stream. 
Alluvial  terraces  in  many  dry  regions  appear  to  indicate  oscillations 
between  dry  conditions,  when  soil  and  rock  waste  were  washed  down 
from  the  mountain  sides  into  the  valleys,  and  moist  conditions,  when 
the  deposits  formed  in  the  valley  bottoms  were  dissected  because  the 
load  of  the  streams  had  been  diminished.  This  resulted  from  the 
fact  that  during  wet  years  the  soil  was  held  in  place  by  the  flourish- 
ing vegetation ;  while  during  the  dry  years,  although  the  rainfall  was 
less,  the  amount  of  waste  removed  was  great  because  of  the  disap- 
pearance of  the  vegetation  which  formerly  bound  the  weathered 
rock. 

(3)  If  the  quantity  of  sediment  is  decreased,  as  occurs  when  a 
stream  ceases  to  erode  at  its  head,  deposition  gives  place  to  erosion. 

(4)  As  a  valley  lengthens,  so 
much  of  its  load  may  be 
dropped  in  the  upper  and 
newer  portions  of  its  flood 
plain  that  it  is  enabled  to 
degrade  its  older  flood  plain. 

FIG.  1 18. -Rock  terraces  due  to  uplift.  <5)  When  a  region  suffers 

successive  uplifts,  so  that  a 

stream  is  unable  to  cut  away  its  former  flood  plain  before  its  grade 
is  again  increased,  terraces  will  be  formed  which  correspond  on  the 
two  sides  of  the  valley  (Fig.  118).  (6)  If  a  degrading  stream 
decreases  in  volume,  it  will  not  be  able  to  occupy  the  full  width  of 
its  valley  and  will  cut  a  narrower  valley  in  the  older  one.  The  last- 
mentioned  cause,  although  perhaps  the  one  which  first  suggests  itself, 
appears  to  have  been  rarely  effective  in  the  formation  of  terraces. 

The  close  of  glacial  times  (p.  663)  seems  to  have  been  especially 
favorable  for  valley  filling,  because  of  the  overloading  of  the  streams 


THE  WORK  OF   STREAMS 


I29 


which  derived  their  material  more  or  less  directly  from  the  glaciers 
as  well  as  from  the  rapid  erosion  of  new  gorges.  The  depression  of 
the  land  in  many  places,  as  in  the  Connecticut  valley,  reduced  the 
velocity  of  the  streams,  and  occasionally  ice  jams  of  long  duration 
also  caused  deposition.  The  deposits  thus  built  in  valleys  have 
since  been  partly  removed,  thus  causing  the  formation  of  terraces. 
Discontinuity  of  Terraces.  —  The  terraces  on  the  two  sides  of  a 
valley  do  not  necessarily  agree  in  height.  This  is  due  to  the  fact 
that,  in  swinging  to 
and  fro  across  its 
valley,  a  stream  not 
only  cuts  laterally 
but  also  at  the  same 
time  degrades  its  bed 
(Fig.  119  A,  £), 
the  flood  plain  often 
being  higher  on  one 
side  than  on  the 
other.  In  the  Brat- 
tleboro,  Vermont, 
region,  for  example, 
the  stream  appears 
to  deepen  its  valley 
about  12  feet  in 
each  swing.  (E.  H. 
Fisher.)  If  a  stream 
meanders  entirely 
across  its  valley,  it 
will  destroy  its  flood 
plain,  but  if  it  fails 
to  make  a  complete  swing,  a  fragment  will  remain  as  a  terrace. 
When  in  its  meanderings  a  stream  encounters  a  rock  ledge  (Fig. 
119)  in  its  valley  floor,  the  lateral  cutting  may  be  retarded  to 
such  a  degree  that  it  will  begin  to  swing  to  the  opposite  side  of  its 
valley  before  completing  its  usual  lateral  movement.  In  this  way  a 
portion  of  the  flood  plain  will  be  preserved  as  a  terrace.  When  other 
rock  ledges  are  encountered  in  its  further  swings  across  the  valley 
more  terraces  will  be  left,  and  the  "  meander  belt "  will  be  narrowed. 
The  theory  of  defending  rock  ledges  affords  a  better  explanation 
than  any  other  for  many  of  the  terraces  of  the  New  England  valleys. 


Fig.  119.  —  Block  diagrams  showing  the  origin  of  stream 
terraces  defended  by  rock  ledges.  The  terraces  H,  K,  A, 
B,  C,  D,  E,  F,  G,  owe  their  preservation  to  the  presence  of 
rock  ledges  which  prevented  the  stream  from  cutting  them 
away  as  the  valley  was  deepened.  The  relation  of  rock 
to  alluvium  on  the  right  of  the  block  diagram  is  also 
shown  in  figure  B.  (Modified  after  E.  H.  Fisher.) 


130 


PHYSICAL  GEOLOGY 


Characteristics  of  River  Deposits.  —  A  cross  section  through  a 
river  deposit  does  not  show  a  homogeneous  deposit  of  stratified  sedi- 
ment, but  rather  lens-shaped  masses  of  coarse  sands  and  gravels  at 
different  levels,  buried  in  stratified  sands  and  clays  (Fig.  107).  When 
traced  up  or  down  the  valley,  these  deposits  are  found  to  lie  in  long 
and  comparatively  narrow  belts.  They  represent  the  former  channels 
of  the  aggrading  river,  where  the  current  was  strong  enough  to  remove 
all  but  the  coarser  material  of  its  load.  The  finer  deposits  —  the 
mud  and  fine  sand  —  were  laid  down  in  the  more  sluggish  water  on 
either  side  of  the  channel  and  on  the  flood  plain.  Beds  of  muck, 
marking  the  sites  of  shallow  lakes  and  swamps,  are  also  common. 


Scale  of  Miles.      ^^ 
0  25  50  75i8r\ 


DELTAS 

Deltas  are  formed  where  streams  enter  either  lakes  or  seas.  If 
the  body  of  water  into  which  the  river  flows  is  large,  all  of  the  sedi- 
ment carried  in  by  the  stream  is  dropped,  and  the  bottom  is  gradually 

built  up  at  the  river's  mouth. 
Since  sediment  settles  much 
more  quickly  in  salt  than  in 
fresh  water,  it  is  dropped 
more  quickly  in  the  ocean. 
Because  of  the  low  gradient, 
a  river  often  splits  into  sev- 
eral channels  (Fig.  120)  as  it 
enters  its  delta,  the  branches 
being  known  as  distributaries. 
The  shape  of  a  delta,  as 
the  name  implies,  is  usually 
that  of  the  Greek  letter  of 
that  name,  with  one  angle  of  the  triangle  pointing  upstream. 

Growth  of  Deltas.  —  The  rate  of  growth  of  a  delta  depends  upon 
(i)  the  amount  of  sediment  carried  by  the  river,  (2)  the  depth  of  the 
sea  or  lake,  (3)  the  strength  of  the  waves  or  currents,  and  (4)  the 
stability  of  the  bottom  of  the  sea  or  lake  where  the  deposition  is  tak- 
ing place.  Deltas  are  apt  to  be  largest  in  seas  in  which  the  tide  is 
weak,  since  under  such  conditions  practically  all  of  the  sediment  is 
dropped  soon  after  it  reaches  still  water.  When  the  ocean  bottom  at 
the  mouths  of  rivers  is  subsiding,  the  upbuilding  of  the  bottom  may 
be  insufficient  to  compensate  for  the  subsidence.  The  Mississippi 


FIG.  120.  —  The  delta  of  the  Mississippi  River. 


THE  WORK  OF  STREAMS  13 1 

delta  has  been  built  upward  and  outward  in  spite  of  subsidence; 
the  sinking  which  produced  the  Chesapeake  and  Delaware  bays, 
however,  was  so  rapid  that  estuaries  were  formed  (p.  114).  The  Mis- 
sissippi River  is  extending  its  delta  at  the  remarkable  rate  of  one 
mile  in  sixteen  years;  the  Rhone  has  added  a  mile  to  its  delta  in 
Lake  Geneva  since  Roman  times.  It  leaves  the  lake  as  a  clear 
stream,  but  gathers  sediment  from  its  tributaries  in  France,  with 
which  it  builds  another  delta  at  its  mouth  at  the  rate  of  about  a  mile 
a  century.  In  220  B.C.  the  town  of  Pu-tai,  China,  stood  one  third  of  a 
mile  from  the  sea,  but  in  1730  it  was  47  miles  inland,  and  to-day  it 
is  48  miles  from  the  shore.  (King.)  Many  of  the  "  points  "  in  lakes 
are  deltas  which  have  been  built  out  by  streams. 

Structure  of  Deltas.  —  A  section  through  a  delta  shows  approxi- 
mately horizontal  beds  of  fine  material  at  the  bottom,  which  do  not 
differ  greatly  from  other 
deposits  at  a  similar 
depth  where  no  delta 
occurs.  These  are  termed 
the  bottom-set  beds. 
Above  these  are  the  ^IGp  I2I-  —  Ideal  section  of  a  delta  built  into 
,  ,.  j  f  quiet  waters  of  constant  level.  The  lower  horizontal 

steeply    inclined   fore-set  beds  are  caUed  bottom.set>  the  inclined>  fore_set)  and 

beds,  composed  of  coarser   the  upper,  top-set.     (After  Barrell.) 

sediments     which     have 

been  swept  outward  by  the  currents  and  waves  and  may  have  a 

slope  approaching  the  angle  of  repose  (Figs.  121,  122).     The  top-set 

beds  are  nearly  horizontal  and  are  laid  down  upon  the  fore-set  beds. 

These  are  usually  the  last  deposits  of  the  river  in  the  upbuilding  of 

the  delta. 

The  surface  of  a  delta  is  comparatively  level,  but  gradually  rises 
upstream.  In  it  large  and  small  lakes  may  occur;  the  depressions 
in  which  they  lie  being  those  portions  of  the  delta  which,  because  of 
the  accidental  position  of  the  distributaries,  were  not  filled  to  the 
general  level  with  sediment.  Their  life  is  necessarily  short,  since 
they  are  gradually  being  filled  by  accumulations  of  silt  during  floods, 
and  by  swamp  vegetation. 

Deltas  may  be  very  extensive.  That  of  the  Ganges  and  Brahma- 
putra has  an  area  of  50,000  to  60,000  square  miles,  with  its  head 
200  miles  from  the  sea.  The  length  of  the  Mississippi  delta  is  more 
than  200  miles,  and  its  area  is  more  than  120,000  square  miles. 
The  Orinoco  delta  has  an  area  larger  than  that  of  New  Jersey.  The 


132 


PHYSICAL  GEOLOGY 


head  of  the  delta  of  the  Hoang  Ho  is  350  miles  from  the  coast. 
The  Imperial  valley  in  California  is  the  result  of  delta  building.  The 
Gulf  of  California  formerly  extended  150  miles  further  northwest 
than  now,  and  across  it  a  delta  was  built  by  the  Colorado  River,  so 
high  as  to  shut  off  the  upper  part  of  the  gulf  and  inclose  a  lake  of 
salt  water.  This  lake  has  almost  entirely  disappeared  and  its  bed 


FIG.   122.  —  Longitudinal  section  of  a  delta,  showing  the  dipping,  fore-set  beds. 
(Photo.  R.  S.  Tarr.) 

has  become  the  Salton  sink.  Thanks  to  irrigation  this  basin  is  ex- 
tremely fertile. 

The  depth  of  delta  deposits  is  often  great.  A  boring  at  New  Or- 
leans encountered  driftwood  at  1042  feet,  and  depths  of  500  feet 
are  not  uncommon  in  other  deltas.  It  has  been  shown  that  in  many 
cases  the  subsiding  of  deltas  progresses  at  a  pace  about  equal  to  the 
deposition. 

Deltas  are  usually  noted  for  their  fertility.  The  three  most 
densely  populated  regions  of  the  world,  outside  of  cities,  are  the 
deltas  of  eastern  China,  India,  and  the  Po  River  in  Italy.  This  is 
true  in  spite  of  the  fact  that,  because  of  their  level  surfaces,  deltas 
are  especially  subject  to  floods.  The  great  flood  in  the  Mississippi 
River  delta  in  1912  destroyed  many  lives  and  millions  of  dollars'  worth 
of  property,  and  this  was  also  the  case  with  earlier  floods.  One  of 
the  notable  examples  of  such  easily  flooded  districts  is  the  delta  of 


THE  WORK  OF  STREAMS 


133 


the  Hoang  Ho  in  China.  This  river  is  restrained  by  great  dikes 
(p.  124),  some  of  which  are  30  feet  above  the  level  of  the  region;  but 
notwithstanding  these  precautions  many  disastrous  floods  have 
occurred.  For  several  hundreds  of  years  previous  to  1852  this  river 
emptied  into  the  Yellow  Sea.  In  that  year,  when  in  unusual  flood, 
it  broke  through  its  north  levees  and  emptied  into  the  Gulf  of  Chihli, 
some  300  miles  farther  north.  This  is  only  one  of  the  many  shiftings 
which  this  river  has  made  during  its  history  (Fig.  112).  During  a 
flood  in  1887  many  villages  were  destroyed,  and  the  loss  of  life  through 
drowning  and  famine  exceeded  1,200,000  people,  more  than  the  entire 
population  of  Nebraska. 


DEPOSITION  IN  LAKES  BY  STREAMS  AND  BY  OTHER  AGENTS 

Mechanical  Deposits.  —  Streams  deposit  their  loads  when  they 
flow  into  lakes,  forming  deltas  (p.  130)  at  their  mouths  and  covering 
the  bottom  of  the  lake  with  the  finer  silt,  which  is  carried  farther  out 
since  it  remains  in  suspension 
longer.  Lakes  may  in  time  be 
entirely  filled  by  the  growth  of 
their  deltas,  first  becoming 
swamps  and  then  level  meadows 
through  which  the  streams  may 
flow  in  meandering  courses  (Fig. 
123  A,  B).  Meadows  of  this 
history  are  abundant  in  regions 
which  have  been  glaciated,  such 
as  Michigan,  New  York,  and 
Minnesota.  Lakes  are  shallowed 
by  the  waves  cutting  back  the 
cliffs  along  their  shores  and  carry- 
ing out  into  them  the  material 
thus  derived. 

It  is  thus  seen  that  as  soon  as 
a  lake  comes  into  existence, 
agencies  arise  which  tend  to 
obliterate  it;  sediment  begins 
to  fill  it,  and  the  outgoing  stream 
commences  to  deepen  the  outlet 
and  thus  in  time  to  drain  it. 


FIG.  123.  —  Map  A  shows  a  lake  being 
filled  in  with  sediment  carried  by  streams. 

Map  B  shows  the  same  lake  converted 
into  a  marsh,  with  the  streams  flowing  in 
meandering  courses. 


134  PHYSICAL  GEOLOGY 

Lakes  equalize  the  flow  of  streams,  preventing  floods,  and  also  act 
as  filters. 

Chemical  Deposits.  —  In  addition  to  such  mechanical  deposits  as 
those  described,  chemical  deposits  are  also  found  in  lakes.  Lime  is 
sometimes  deposited,  and  iron  in  the  form  of  limonite  (p.  686)  is 
precipitated.  In  some  of  the  lakes  of  Sweden  and  Canada  iron  of 
this  origin  is  so  abundant  as  to  be  of  economic  importance. 

Organic  Deposits,  (a)  Diatoms.  —  Dredgings  in  lakes  show  that 
the  bottoms  are  sometimes  covered  with  thick  deposits  of  diatoms 
(microscopic  plants  which  secrete  siliceous  tests,  p.  581).  Since  these 
organisms  multiply  with  great  rapidity,  they  may  form  extensive 
deposits,  called  diatomaceous  earth. 

(b)  Marl.  —  Calcareous  deposits  in  the  form  of  marl  may  accu- 
mulate to  great  depths  in  lakes.     This  is  a  white,  or  gray,  clay-like 
deposit  which  is  composed  largely  of  calcium  carbonate.     It  is  formed 
either  by  the  accumulation  of  shells,  or  through  the  agency  of  certain 
plants  (algae)  which  extract  carbon  dioxide  from  the  water  and  thus 
cause  the  deposition  of  the  lime  dissolved  in  the  water.     Marl  is 
formed  only  where  small  quantities  of  clay  are  washed  into  the  lake, 
since,  if  large  quantities  are  carried  in,  the  deposit  would  be  termed 
mud.     Deposits  of  marl  may  be  a  score  or  more  feet  in  depth  and  are 
often  overlain  by  peat.     In  regions  where  limestone  is  not  accessible, 
marl  is  sometimes  used  in  the  manufacture  of  Portland  cement. 

(c)  Peat.  —  A  brown  deposit,  called  peat,  composed  of  the  partially 
decayed    remains   of   plants,    sometimes     accumulates    in   swamps, 
marshes,  and  shallow  lakes.     Peat  forms  most  rapidly  in  cool,  moist 
climates  where,  although  the  vegetation  may  not  grow  rapidly,  the 
low  temperature  retards  decay.     Under  favorable  conditions  it  also 
accumulates  in  warm  countries.     In  Florida,  for  example,  there  are 
considerable  areas  of  peat.     Extensive  areas  of  peat  occur  in  the 
United  States,  such  as  that  of  the  Dismal  Swamp  of  Virginia  and 
North  Carolina.     In  Massachusetts,  it  is  estimated  that  there  are 
15,000,000  cubic  feet  of  peat.     One  tenth  of  the  surface  of  Ireland  is 
underlain  by  peat,  and  large  areas  in  Europe  and  elsewhere  are  pro- 
vided with  it.     Peat  is  dried  and  used  for  fuel  in  some  regions  where 
it  occurs  in  great  abundance,  and  where  its  extraction  is  easy. 

Playas.  —  In  desert  regions,  where  no  permanent  lakes  occur, 
streams  sometimes  reach  depressions  when  their  volumes  are  increased 
during  the  wet  season  or  by  cloud-bursts,  and  form  temporary, 
shallow  lakes  which  may  cover  large  areas.  The  largest  in  Nevada 


THE  WORK  OF   STREAMS 


135 


is  in  the  Black  Rock  desert  and  is  450  to  500  square  miles  in  area, 
although  seldom  more  than  a  few  inches  deep.  Such  temporary 
desert  lakes  are  called  playas.  Their  beds,  when  dry,  are  covered  with 
fine  clay  and  sand,  and  sometimes  with  gypsum  and  salt.  On  the 
mud  of  ancient  playa  beds  the  footprints  of  extinct  animals  have 
been  preserved  (p.  379). 

Salt  Lakes. — A  salt  lake  may  be  formed  (i)  by  the  cutting  off 
of  an  arm  of  the  sea  by  a  delta,  as  in  the  case  of  the  Salton  Sea, 
California  (p.  132),  or  by  an  elevation  of  the  sea  bottom,  which 
isolates  a  body  of  water.  Under  such  conditions,  the  water  will, 
at  first,  have  the  same  composition  as  sea  water.  If,  however,  the 
water  flowing  into  the  lake  exceeds  the  evaporation  of  its  surface, 
it  will  gradually  be  freshened.  Such  was  the  history  of  Lake  Cham- 
plain.  If,  on  the  other  hand,  such  a  lake  has  been  formed  in  a 
desert  region  where  evaporation  is  excessive,  the  water  will  become 
more  salty  as  time  goes  on.  The  Caspian  Sea  was  formerly  con- 
nected with  the  Black  Sea,  but  is  now  isolated  and  is  growing  more 
salty. 

(2)  Salt  lakes  are  also  formed  by  the  concentration  of  fresh  water. 
Basins  in  arid  regions  which  do  not  receive  enough  water  to  cause 
them  to  overflow  may,  in  time,  become  saturated  with  salts  of  various 
kinds.  The  streams  bring  in  common  salt  (NaCl),  gypsum 
(CaSO4-2fI2O),  Epsom  salt  (MgSO4-7H2O),  and  calcium  carbonate 
(CaCO3),  which  they  obtain  from  the  rocks  over  which  they  flow. 
These  salts  may  accumulate  in  the  lake  as  evaporation  proceeds,  until 
the  water  becomes  so  concentrated  that  they  are  precipitated.  Iron 
oxide  and  calcium  carbonate  will  be  deposited  first;  upon  further 
concentration,  gypsum,  which  is  insoluble  in  strong  brine,  will  be 
precipitated ;  then  common  salt  and  Glauber  salts  (Na2SO4),  in 
the  order  of  their  solubility.  This  order  is  often  interfered  with 
under  certain  conditions.  Cold  weather,  for  example,  will  cause  the 
precipitation  of  Glauber  salts  (Na2SO4)  before  the  common  salt  has 
all  been  precipitated.  If  the  evaporation  of  the  surface  of  the  salt 
lake  does  not  equal  the  amount  of  water  received  during  a  wet  season, 
the  deposition  of  gypsum  and  salt  will  cease,  and  the  beds  of  salt 
may  be  covered  by  the  sediment  brought  in  by  the  streams.  With 
the  recurrence  of  the  dry  season  the  deposit  of  gypsum  and  salt  will 
commence  again.  Many  alternations  of  mud  and  salt  are  encoun- 
tered in  wells  sunk  on  the  margins  of  salt  lakes.  In  some  salt  lakes 
most  of  the  salt  has  been  deposited,  and  the  liquid  remaining,  called 


136  PHYSICAL  GEOLOGY 

"  bittern,"  contains  chiefly  Epsom  and  Glauber  salts.  The  Dead 
Sea  is  such  a  lake. 

(3)  The  salt  of  some  salt  lakes  has  been  attributed  to  an  accumu- 
lation of  wind-blown  salt.  Perhaps  the  best  example  of  a  salt  lake 
in  which  this  origin  is  evident  is  furnished  by  a  lake  in  northern 
India  (Sambhar  Lake).  This  lake  is  situated  in  an  inclosed  basin 
more  than  400  miles  inland  and  appears  to  receive  the  greater  part,  if 
not  all,  of  its  salt  from  dust-laden  winds  which  sweep  over  the  plains 
between  it  and  an  arm  of  the  sea  during  the  dry  months.  Analysis 
of  the  air  during  the  dry  season  shows  that  at  least  3000  metric  tons 
of  salt  are  carried  over  the  lake  annually,  an  amount  sufficient  to 
account  for  the  accumulations  of  salt  in  the  lake. 

Alkaline  Lakes.  —  Alkaline  and  borax  lakes  differ  from  salt  lakes 
in  that  they  contain  a  predominance  of  sodium  carbonate  or  borax. 
The  source  of  this  carbonate  and  borax,  as  in  the  case  of  common 
salt,  is  the  rocks  over  which  the  streams  which  feed  such  lakes  flow. 

Origin  of  Rock  Salt.  —  Deposits  of  salt  underlie  many  hundreds 
of  square  miles  of  sedimentary  rocks  in  New  York  and  other  states. 
The  thickness  of  the  salt  beds  varies  greatly,  the  thickest  reported 
in  New  York  consisting  of  325  feet  of  solid  salt.  The  greatest  salt 
deposit  known  is  that  at  Stassfurt,  Germany,  which  is  4794  feet  deep. 
Since  salt  and  gypsum  occur  together,  it  is  believed  that  such  deposits 
have  been  formed  as  a  result  of  the  evaporation  of  salt  lakes.  One 
objection  to  this  theory  is  the  great  thickness  of  some  beds  and  their 
purity.  In  the  case  of  such  deposits  it  is  believed  that  an  estuary  or 
lagoon  was  separated  from  the  sea  by  a  bar  over  which  water  was 
carried  during  storms  or  perhaps  at  high  tide.  If  the  region  in  which 
this  occurred  was  hot  and  arid,  it  is  conceivable  that  salt  might  be 
deposited  to  the  depth  of  the  lagoon  or  estuary.  If  such  a  basin 
should  slowly  subside,  a  bed  of  salt  of  great  thickness  could  result. 
Such  remarkably  thick  deposits  as  those  in  Louisiana,  where  the 
bottom  has  not  been  reached  at  a  depth  of  2000  feet,  requires  a  still 
further  modification  of  the  theory. 

Extinct  Lakes.  —  Upon  their  disappearance  lakes  leave  behind 
them  proofs  of  their  former  existence.  If  they  were  of  comparatively 
short  duration,  as  would  be  the  case  if  they  had  been  formed  by  ice 
jams  (p.  186),  their  former  presence  might  be  attested  by  (i)  the 
deltas  deposited  by  the  streams  which  flowed  into  them,  as  well  as 
by  (2)  the  stratified  sand  and  clay  which  were  spread  over  their  beds. 
When  their  life  was  long,  (3)  wave-cut  terraces,  (4)  sand  bars  and 


THE  WORK  OF   STREAMS 


137 


FIG.  124.  —  Deltas  formed  in  Lake  Bonneville  by  the  Logan  River,  Utah. 
(U.  S.  Geol.  Surv.) 

spits,  deltas,  and  other  thick  deposits  are  left  (Fig.  124).  One  of  the 
most  remarkable  lakes  of  this  sort  was  Lake  Bonneville  (Fig.  125), 
of  which  the  Great  Salt  Lake  is  a  withered  remnant.  This  lake  at 


FIG.  125.  —  Contour  map  of  the  shore  terraces  of  Lake  Bonneville,  Utah. 
The  terraces  were  cut  and  built  at  different  lake  levels.     (U.  S.  Geol.  Surv.) 


138  PHYSICAL  GEOLOGY 

its  greatest  extent  covered  19,750  square  miles  and  was  1000  feet 
deep.  At  this  time  it  had  an  outlet  to  the  north  which  carried  the 
excess  waters  to  the  Pacific.  During  this  period,  too,  great  terraces 
were  cut  and  immense  deltas  were  built.  From  Salt  Lake  City  one 
can  see  these  terraces  on  the  lower  slopes  of  the  mountains  and  from 
them  can  learn  the  former  levels  of  the  lake.  A  change  in  climate 
finally  reduced  this  extensive  lake  to  the  present  relatively  small 
Great  Salt  Lake,  which  has  an  area  of  2000  square  miles  and  an 
average  depth  of  15  feet.  Since  the  water  now  contains  18  per  cent, 
of  salt,  it  is  so  dense  that  the  bather  is  required  to  exert  no  effort  to 
keep  his  head  above  water,  as  it  is  impossible  to  sink. 

REFERENCES  FOR  THE  WORK  OF  STREAMS 

GENERAL 
CLELAND,  H.  F.,  —  North    American    Natural   Bridges,    with  a  Discussion  of   their 

Origins:   Bull.  Geol.  Soc.  America,  Vol.  21,  1910,  pp.  313-338. 
DE  MARTONNE,  E.,  —  Geographic  Physique,  pp.  413-442. 
GILBERT,  G.  K.,  —  Re-port  on  the  Geology  of  the  Henry  Mountains:  U.  S.  Geog.  and 

Geol.  Surv.  of  the  Rocky  Mountain  Region,  1877,  pp.  99-150. 
HAUG,  E.,  —  Traite  de  Geologic,  pp.  406-436. 
RUSSELL,  I.  C.,  —  Rivers  of  North  America. 
SALISBURY,  R.  D., —  Physiography  (Advanced),  pp.  114-203. 
SALISBURY  AND  ATWOOD, —  The  Interpretation  of  Topographic  Maps:    Professional 

Paper,  U.  S.  Geol.  Surv.  No.  60,  1908. 
SHALER,  N.  S.,  —  Aspects  of  the  Earth,  pp.  143-196. 
TAYLOR,  F.  B.,  — Niagara  Falls  Folio :  U.  S.  Geol.  Surv.  No.  190, 1913. 

FLOOD  PLAINS 
DAVIS,  W.  M.,  —  The  Development  of  River  Meanders :  Geol.  Mag.,  Vol.  10,  1903, 

pp.  145-148. 
JEFFERSON,  M.  S.  W.,  —  Limiting  Widths  of  Meander  Belts :  Nat.  Geog.  Mag.,  Vol.  13, 

1902,  pp.  373-384- 

CYCLE  OF  EROSION 

DAVIS,  W.  M.,  —  Geographical  Cycle,  Geographical  Essays,  1909. 
DAVIS,  W.  M.,  —  Base    Level,    Grade,    and    Peneplain :   Jour.    Geol.,  Vol.   10,  1902, 

pp.  77-109. 
DAVIS,  W.  M., —  The  Peneplain:   Am.  Geologist,  Vol.  23,  1899,  pp.  207-239. 

STREAM  PIRACY 

BOWMAN,  I.,  —  A  Typical  Case  of  Stream  Capture  in  Michigan :  Jour.  Geol., 
Vol.  12,  1904,  pp.  326-334. 

DARTON,  N.  H.,  —  Examples  of  Stream  Robbing  in  the  Catskill  Mountains :  Bull.  Geol. 
Soc.  America,  Vol.  7,  1896,  pp.  505-507. 

DAVIS,  W.  M.,  —  Stream  Contest  along  the  Blue  Ridge:  Bull.  Geog.  Soc.  Philadel- 
phia, Vol.  3,  1905,  pp.  213-244. 


THE  WORK  OF   STREAMS 


TERRACES 


139 


DAVIS,  W.  M.,  —  The  Terraces  of  the  Westfield  River,  Massachusetts:  Am.  Jour.  Sci., 
Vol.  14,  1902,  pp.  77-94. 

DAVIS,  W.  M.,  —  River  Terraces  in  New  England:  Bull.  Harvard  Collection, 
Museum  of  Comparative  Zoology,  Vol.  38,  1902,  pp.  281-346. 

DODGE,  R.  E.,  —  The  Geographical  Development  of  Alluvial  River  Terraces :  Pro- 
ceedings Boston  Soc.  Nat.  Hist.,  Vol.  26,  1894,  pp.  257-273. 

FISHER,  E.  F., —  Terraces  of  the  West  River,  Brattleboro,  Vermont:  Proceedings 
Boston  Soc.  Nat.  Hist.,  Vol.  33,  1906,  pp.  9-42. 

INTRENCHED  MEANDERS 

DAVIS,  W.  M.,—  The  Seine,  the  Meuse,  and  the  Moselle :  Nat.  Geog.  Mag.,  Vol.  7,  1896, 
pp.  189-202;  228-238. 

DEPOSITION 

BARRELL,  J.,  —  The  Geological  Importance  of  Continental,  Littoral,  and  Marine  Sedi- 
mentation: Jour.  Geol.,  Vol.  14,  1906,  pp.  316-356;  430-457;  524-568. 

SALT  LAK^S 

HARRIS,  G.  D.,  —  Rock  Salt,  Its  Origin  and  Importance:  Bull.  La.  Geol.  Surv.  No.  7, 
1907. 

TOPOGRAPHIC  MAP  SHEETS,  U.  S.  GEOLOGICAL  SURVEY,  ILLUSTRATING  THE  WORK 

OF  RUNNING  WATER 

Regions  in  Topographic  Youth  Regions  in  Topographic  Maturity 

Casselton,  North  Dakota.  Charleston,  West  Virginia. 

Fargo,  North  Dakota.  Briceville,  Tennessee. 

Bright  Angel,  Arizona.  Lancaster,  Wisconsin-Iowa-Illinois. 

Kaibab,  Arizona.  Becket,  Massachusetts. 

Bisuka,  Idaho.  Arnoldsburg,  West  Virginia. 

Milan,  Illinois.  Monterey,  Virginia- West  Virginia. 
Niagara  Falls,  New  York. 
Great  Falls,  Montana. 

Regions  in  Topographic  Old  Age  Stream  Piracy 

Caldwell,  Kansas.  Kaaterskill,  New  York. 

Butler,  Missouri.  Lake,  Yellowstone  National  Park, 

Morrilton,  Arkansas.  Wyoming. 

Gloversville,  New  York. 

Lykens,  Pennsylvania. 

Rejuvenated  Streams  and  Entrenched  Meanders 

Lockport,  Kentucky.  Huntingdon,  Pennsylvania. 

Harrisburg,  Pennsylvania.  Ravenswood,  West  Virginia-Ohio. 


140  PHYSICAL  GEOLOGY 

TOPOGRAPHIC  MAP  SHEETS,  ILLUSTRATING  STREAM  DEPOSITS 
Alluvial  Fans  Braided  Streams 

Cucamonga,  California.  North  Platte,  Nebraska. 

Sierraville,  California.  Kearney,  Nebraska. 

Desert  Well,  Arizona.  David  City,  Nebraska. 

Parker,  California-Arizona.  Gothenburg,  Nebraska. 

Disaster,  Nevada. 

Natural  Levees 

Donaldsonville,  Louisiana. 
Baton  Rouge,  Louisiana. 
Hahnville,  Louisiana. 

Flood  Plains  and  Meanders  Terraces 

St.  Louis,  Missouri.  Cohoes,  New  York. 

Butler,  Missouri.  Lacon,  Illinois. 

Lake  Providence,  Louisiana.  Hartford,  Connecticut. 

Jefferson  City,  Missouri.  Mountain  Home,  Idaho. 


CHAPTER  V 
THE  WORK    OF  GLACIERS 

WHEN  viewed  from  an  eminence,  a  mountain  glacier  has  the  appear- 
ance of  a  river  of  ice  flowing  down  a  valley  to  a  point  where  it  ends 
abruptly  and  a  stream  emerges  from  beneath  it  and  courses  toward  the 
sea.  If  the  climate  is  cold,  as  in  Greenland,  glaciers  may  even  reach 
the  sea,  where  their  shattered  fronts  are  carried  away  as  icebergs  by 
the  ocean  currents. 

GENERAL  CONSIDERATIONS 

Distribution  and  Size  of  Glaciers.  —  Glaciers  exist  on  high  moun- 
tains, even  in  the  tropics.  In  temperate  regions  they  abound  on  high 
ranges,  especially  on  those  against  which  moisture-laden  winds  blow; 
as,  for  example,  the  Sierra  Nevada,  the  Cascade  ranges,  the  Alps, 
the  Caucasus,  the  Andes,  and  the  Himalayas. 

Mountain  glaciers  vary  in  size  from  those  which  barely  extend 
beyond  their  cirques  (hanging,  clifF,  or  corrie  glaciers)  to  the  great 
Seward  Glacier  of  Alaska,  more  than  50  miles  long  and  3  miles  wide 
where  narrowest.  In  the  Alps  there  are  2000  glaciers,  the  largest  of 
which,  the  Aletsch  Glacier  (p.  187),  is  more  than  10  miles  long, 
although  the  majority  are  less  than  a  mile.  These  Alpine  glaciers 
vary  in  width  from  a  few  hundred  feet  to  about  one  mile.  u  The 
thickness  of  ice  in  the  Alpine  glaciers  must  often  be  as  much  as  800 
to  1 200  feet,"  the  depth  usually  being  least  at  the  lower  end.  Great 
glaciers  are  confined  to  polar  regions  (continental  glaciers,  p.  168) 
and  to  high  mountains  of  the  temperate  zones. 

Position  of  the  Snow  Line.  —  The  level  on  the  earth's  surface 
above  which  some  of  the  snow  of  one  winter  lasts  through  the  fol- 
lowing summer,  thus  forming  areas  of  "  perpetual  snow  "  or  snow 
fields,  is  called  the  snow  line.  Its  position  depends  upon  the  normal 
temperature  of  the  region  as  well  as  upon  other  factors.  In  general, 
the  snow  line  varies  little  from  the  line  on  which  the  average  tempera- 
ture is  32°  F.  Near  the  equator  it  is  15,000  to  19,000  feet  above  the 
sea,  while  in  polar  regions  it  is  almost,  or  quite,  at  sea  level.  In 

141 


142  PHYSICAL  GEOLOGY 

intermediate  regions  the  height  increases  toward  the  equator.  In  the 
Alps  the  snow  line  is  8500  feet  above  the  sea ;  in  the  western  United 
States  and  in  British  Columbia  the  higher  mountains  are  covered  with 
perpetual  snow;  in  Massachusetts  it  has  been  shown  by  kites  that 
glaciers  would  exist  at  an  altitude  of  11,470  feet;  and  it  is  estimated 
that  glaciers  would  develop  in  the  Scottish  Highlands  if  the  average 
temperature  were  lowered  three  degrees. 

The  position  of  the  sun  with  reference  to  a  mountain  range  in- 
fluences the  height  of  the  snow  line.  In  the  northern  hemisphere,  for 
example,  other  things  being  equal,  the  snow  line  will  be  lower  on  the 
north  side  of  a  mountain  than  on  the  south  side,  since  the  former 
receives  heat  from  the  sun  fewer  hours  each  day.  Certain  forms  of 
topography  also  favor  the  retention  of  snow.  For  instance,  snow 
gathers  to  greater  depths  in  deep  ravines  than  on  a  level  surface,  as  it 
is  blown  in  by  the  wind  and  protected  from  the  sun's  heat  so  that  it 
may  remain  from  one  winter  to  the  next.  A  moist  climate  also  favors 
a  low  snow  line  on  account  of  the  greater  snowfall,  since  more  time 
is  required  to  melt,  or  evaporate,  a  thick  layer  of  snow  than  a  thin  one. 
On  the  Himalayas  the  snow  line  is  3000  to  40x30  feet  lower  on  the  south 
than  on  the  shaded  north  side,  because  of  the  greater  amount  of  snow 
precipitated  there  by  the  moist,  south  winds  from  the  Indian  Ocean. 
The  few  inches  of  snow  which  fall  on  the  north  slope  may  be  melted 
in  a  few  warm  summer  days,  while  the  several  feet  of  snow  on  the 
south  side  may  not  disappear,  even  when  subjected  to  a  longer  period 
of  warmth.  In  dry  climates  the  snow  may  disappear  entirely  by 
direct  evaporation.  As  far  as  temperature  is  concerned  portions  of 
Siberia  are  under  glacial  conditions,  but  the  climate  is  so  arid  and  the 
snowfall  so  scanty  that  the  snow  which  falls  is  soon  evaporated. 

Formation  of  Ice  in  Snow  Fields.  —  Snow  differs  from  ice  in  being 
composed  of  fine  crystals,  loosely  consolidated  and  separated  from 
one  another  by  air,  whereas  ice  consists  of  crystals  in  contact.  In  a 
snow  field  there  is  every  gradation  from  fluffy  snow  to  granular  snow 
or  neve  and  finally  to  solid  ice.  The  change  from  one  state  to  the 
other  is  well  shown  in  snowdrifts  of  the  temperate  zone,  which  become 
granular  if  they  exist  for  a  few  months,  the  granules  being  about  the 
size  of  small  hailstones.  If  they  exist  still  longer,  the  drifts  are  rep- 
resented by  small  mounds  or  ridges  of  solid  ice.  The  transforma- 
tion from  snow  to  neve  and  then  to  ice  is  very  gradual  and  is  accom- 
plished (i)  by  the  pressure  of  the  overlying  snow  which  forces  the  air 
from  between  the  snow  crystals  and  thus  tends  to  compact  them; 


THE  WORK  OF  GLACIERS 


143 


(2)  by  rain  and  the  water  from  the  upper  layers  of  the  melting  snow, 
which  soaks  down  into  the  snow,  freezes,   and  expels  the  air;    and 

(3)  by  the  growth  of  the  snow  crystals.     It  is  in  this  way  that  the 
coarsely  granular  snow  seen  in  drifts  in  the  early  spring  and  in  the 
neve  of  snow  fields  is    produced.      The  growth  of  the   crystals  is 
accomplished  partly  at  the  expense  of  the  smaller   crystals  which 
lose  bulk  by  evaporation,  while  their  larger  neighbors  take  the  mois- 
ture given  off  to  increase  their  own  size,  and  partly  from  the  thaw 
water  which  bathes  them.     Neve  passes  insensibly  into  snow,  on  the 
one  hand,  and  into  ice  on  the  other.     A  crystallographic  study  shows 
that  ice  is  made  up  of  crystals,  the  external  form  of  which  has  been 
obliterated  by  pressure  and  as  a  result  of  their  growth.      Ice  is,  there- 
fore, a  crystalline  rock,  like  marble,  and  is  classed  as  a  rock. 

Snow  does  not  accumulate  indefinitely  above  the  snow  line;  a 
part  melts  and  runs  off,  a  part  is  evaporated,  and  a  part  is  carried 
away  by  glaciers.  It  has  been  estimated  that  if  glaciers  had  ceased 
to  drain  the  snow  fields  at  the  beginning  of  the  Christian  era,  the 
Alps  would  now  be  buried  under  a  mantle  of  snow  about  5000  feet 
thick. 

MOUNTAIN  GLACIERS 

Formation.  —  When  ice  has  accumulated  to  a  considerable  depth 
it  tends  to  spread,  much  (so  far  as  external  appearance  is  concerned) 
as  does  a  mass  of  stiff  molasses  candy ;  and  if  it  rests  on  an  inclined 
surface,  it  tends  to  move  down  the  slope.  When  the  ice  in  an  ice 
field  begins  to  move  it  is  called  a  glacier. 

If  we  study  typical  glaciers,  such  as  those  in  the  Alps,  in  Glacier 
National  Park,  in  British  Columbia,  or  in  Alaska,  we  find  that  in 
general  they  are  similar  but  show  individual  differences.  We  find, 
upon  following  a  glacier  to  its  head,  that  it  begins  in  a  broad  amphi- 
theater (Fig.  126),  called  a  cirque  (French  for  amphitheater),  above 
the  snow-covered  floor  of  which  rocky  walls  rise  precipitously,  often 
to  a  height  of  several  hundred  feet.  In  this  amphitheater  snow 
gathers  to  great  depths,  often  to  hundreds  of  feet.  The  snow  comes 
from  the  frequent  storms  which  rage  there  and  from  the  accumula- 
tions on  the  walls  of  the  cirque,  from  which  it  is  swept  in  by  winds  or 
carried  by  avalanches.  Cirques  are  therefore  the  feeding  grounds  of 
mountain  glaciers.  In  them  one  finds  every  gradation,  from  snow 
which  is  freshly  fallen,  through  granular  neve  or  half-formed  ice, 
to  compact  ice.  From  the  cirque  the  solid  ice  of  the  glacier  moves 

CLELAND   GEOL.  —  IO 


144 


PHYSICAL  GEOLOGY 


slowly  down  the  mountain  valley  until  it  reaches  a  point  where  the 
melting  equals  the  forward  movement  (p.  159),  the  size  of  the  glacier 

depending  (i)  upon 
the  area  of  the  neve 
field  drained  by  it, 
(2)  upon  the  amount 
of  the  precipitation, 
and  (3)  upon  the  rate 
of  melting.  Some- 
times glaciers  flow 
between  forests  and 
even  cultivated  fields, 
as,  for  example,  in  the 
valleys  of  Grindel- 
wald  and  Chamonix, 
f  where  glaciers  lie 

FIG.  126.  —  Cirques  or  reeding  ground,  and  medial  moraine         ...         ° 

of  the  Breithorn  Glacier.  (Photo.  L.  E.  Westgate.)          within  a  few  hundred 

feet  of  the  homes  of 

the  inhabitants.     In  New  Zealand  a  glacier  from  the  Mt.  Cook  range 
discharges  its  debris  in  the  midst  of  subtropical  vegetation. 

Cirques.  —  One  of  the  most  striking  and  beautiful  features  of  the 
Alps  in  Switzerland, 
of  the  Selkirks  in 
Canada,  of  the  Rocky 
Mountains  of  the 
United  States,  and  of 
other  high  mountains 
of  the  temperate  re- 
gions are  the  ragged 
crests  (Fig.  127)  sepa- 
rating the  gigantic 
semicircular  cirques, 
which  hang  high  up 
on  the  mountain  sides. 
These  cirques  domi- 
nate the  high  moun- 


FIG.  127.  —  Cirques  and  small  glacier,  bt.  Christophe, 
France. 


tains  and  correspond 
to   the  limit  of  per- 
petual snow  of  the  Glacial  Period  (p.  141).     Their  walls  are  rough 
and   precipitous,  while  their  floors  are  comparatively  smooth  and 


THE  WORK  OF  GLACIERS 


level,  the  former  having  been  roughened  by  the  attacks  of  the 
frost  and  other  weathering  agents  and  the  latter  having  been 
scoured  by  glaciers  into  rounded  surfaces.  Most  of  the  lakes  of 
the  high  mountains,  which  give  such  scenery  much  of  its  charm,  rest 
in  cirques.  They  lie  in  basins,  formed  either  by  dams  of  glacial 
debris  (moraines)  left  by  glaciers,  or  in  depressions  cut  into  the  solid 
rock  of  the  cirque  by  the  ice  (rock  basins).  An  understanding  of  the 
origin  of  cirques  is,  therefore,  necessary  for  an  appreciation  of  the 
scenery  of  high  mountains. 

Origin  of  Cirques.  —  If  the  average  temperature  of  a  mountain  region  is  being 
lowered  as  a  result  of  a  change  in  climate,  the  drifts  of  snow  which  accumulate  in  ra- 
vines and  spots  sheltered  from  the  full  heat  of  the  sun  may  last  from  one  season  to 
the  next.  On  account  of 
the  weight  of  the  snow  and 
for  other  causes  (p.  142), 
the  lower  layer  will  be 
compressed  into  ice  and 
will  slowly  move  down  the 
slope.  This  movement  will 
separate  the  moving  mass 
of  snow  and  ice  from  the 
snow  which  rests  upon  the 
upper  slope,  near  the  valley 
wall.  The  rock  wall  will 
thus  be  partially  exposed, 
and  a  crevasse,  called  the 
Bergschrund  (German  for 
mountain  gap  or  fissure), 
will  be  formed  (Fig.  128). 
The  Bergschrund,  in  fact, 
marks  the  line  where  the 

real  downward  motion  of  the  ne"ve  begins.  Crevasses  of  this  sort  vary  in  width  from 
two  or  three  feet  to  more  than  80  feet,  and  play  an  important  part  in  the  formation  and 
enlargement  of  cirques.  One  such  Bergschrund,  150  feet  deep,  which  extended  down 
to  the  rock  bottom  of  the  cirque,  was  explored,  and  its  floor  was  found  to  be  composed 
of  rock  masses,  partly  or  completely  dislodged  from  the  wall  of  the  cirque.  During  the 
days  of  summer  the  water  from  the  melting  snow  drips  down  into  the  crevasse,  wetting 
the  rocks  and  filling  the  cracks.  As  soon  as  the  sun  sets  the  temperature  of  such 
regions  is  rapidly  lowered  and  the  water  filling  the  cracks  and  joints  freezes,  forcing 
the  blocks  from  the  sides.  Since  the  cracks  at  the  base  of  the  rock  wall  are  more  com- 
pletely filled  with  water  than  are  those  in  the  upper  portion,  the  greatest  disruptive 
effect  is  at  the  bottom  of  the  crevasse,  thus  tending  to  produce  and  maintain  vertical 
walls.  As  the  cirque  is  enlarged  by  the  wedge  work  of  the  ice  on  the  rock  in  the 
Bergschrund,  the  crevasse  also  moves  back.  The  circular  form  of  the  cirque  results 
from  the  movement  of  the  snow  and  ice  away  from  the  surrounding  walls  toward  the 
center  of  the  depression. 


FIG.  128.  —  The  Bergschrund  of  a  glacier.     Swiss  Peak, 
British  Columbia.     (Photo.  L.  E.  Westgate.) 


146 


PHYSICAL  GEOLOGY 


The  development  of  cirques  is  apparently  not  necessarily  limited  to  the  heads  of 
former  stream  valleys,  although  this  is  generally  the  case,  but  they  may  have  their  origin 
in  a  somewhat  different  way.  If  the  drifts  on  a  mountain  slope  last  year  after  year  until 

late  in  the  spring,  it  will  be 
found  that  their  edges  are 
usually  bordered  by  fine 
soil  which  is  slowly  being 
removed  by  water  and 
deposited  in  deltas  at  the 
lower  margins  of  the  drifts. 
This  fine  soil  is  the  result 
of  the  alternate  freezings 
and  thawings  of  the  water 
in  the  cracks  and  pores 
of  the  rock,  which  is  thus 
finally  broken  up  into 
small  fragments.  In  this 
way,  by  nivation,  a  niche, 
the  beginning  of  a  cirque, 
may  be  formed  on  a  moun- 
tain slope. 

Development  of 
Cirques.  — A  high  re- 
gion which  has  been 
partly  cut  into  cirques 
"  resembles  nothing 
so  much  as  a  layer 
of  dough  from  which 
biscuit  have  been 
cut."  As  the  amphi- 
theaters or  cirques  on 
the  two  sides  of  a 
mountain  ridge  en- 
large, they  finally  en- 
croach on  each  other, 
first  forming  a  nar- 
row, ragged,  comb- 
like  ridge  (Fig.  129 
A,  5),  and  somewhat 
later,  as  the  separat- 


FIG.  129.  —  A  shows  mountain  valleys  formed  by 
stream  erosion.  B  shows  the  same  valleys  after  they 
have  been  occupied  and  strongly  eroded  by  glaciers.  The 
main  valley  has  become  U-shaped,  and  the  side  valleys 
have  become  hanging  valleys  with  strongly  developed 
cirques.  An  attempt  has  been  made  to  show  the  prob- 
able approximate  deepening,  in  feet,  by  glacial  erosion. 


ing  walls  of  the  cirques  are  partially  quarried  away,  producing  tooth- 
like  peaks. 

Fate  of  Cirques.  —  If  changes  in  climate  cause  a  glacier  gradually 


THE  WORK  OF  GLACIERS 


to  wither  back  into  its  cirque  and  finally  to  disappear,  the  character- 
istic features  of  the  abandoned  cirque  are  slowly  obliterated ;  land- 
slides and  talus  descending  from  the  cliffs  are  heaped  upon  the  bottom, 
filling  the  lakes  and  covering  the  bottom;  the  morainic  (p.  159)  or 
rock  ridge  at  its  entrance  is  breached  by  a  gorge  cut  by  the  out- 
flowing stream ;  side  valleys  are  developed  ;  and  the  resulting  topog- 
raphy presents  few  features  to  indicate  that  it  was  developed  from 
a  cirque. 

Ablation.  — The  surfaces  of  glaciers  are  constantly  being  lowered  by 
direct  evaporation  and  by  melting ;  those  of  the  Alpine  glaciers  are 
lowered  from  18  to  25  feet  during  the  summer  months,  that  of  the 
Mer  de  Glace  having  been  lowered  twenty-four  and  a  half  feet  in  1842. 
Since  the  advance  of  a  glacier  depends  upon  the  thickness  of  its  mass, 
it  follows  that  when  ablation  is  excessive  the  front  will  retreat.  A 
retreating  glacier  is,  consequently,  thinner  and,  unless  its  valley  walls 
are  vertical,  narrower  than  when  it  was  advancing. 


SURFACE  OF  MOUNTAIN  GLACIERS 

The  surface  of  a  glacier  is  usually  rough  (Fig.  130)  as  a  result  of 
a  number  of  causes. 

(i)  Irregularities  Due  to  Tension.  —  Because  of  the  brittleness 
of  the  ice  mass,  glaciers  are  broken  by  cracks  called  crevasses.  Some 
of  these  are  the  result  of  the  more  rapid  motion  of  the  center  than  the 
retarded  sides,  which 
produces  strains  un- 
der which  the  ice 
fractures.  The  cre- 
vasses formed  in  this 
way  are  diagonal  and 
extend  up  the  valley 
(Fig.  136  C,  p.  151). 
When  a  glacier 
emerges  from  a  nar- 
row portion  of  its 
valley  longitudinal 
cracks  are  developed, 
and  the  tension  on  a 

Curve  produces  trans-          pIG    I30  _  Surface  of  a  glacier  showing  seracs  and 
verse  crevasses  which  crevasses. 


148 


PHYSICAL  GEOLOGY 


rise  obliquely  from  the  bottom,  since  the  latter  portion  of  the  ice  is 
retarded  by  friction  with  the  bed.     Crevasses  when  first  formed  are 

usually  separated  from 
one  another  by  rela- 
tively level  surfaces, 
but  since  their  upper 
portions  are  soon  wid- 
ened by  melting,  the 
intervening  ice  often 
becomes  blade-like  in 
its  sharpness,  so  that 
the  surface  of  the  gla- 
cier presents  a  maze  of 
sharp  ridges.  Such  a 
ridge  of  ice  is  called  a 
serac  (Fig.  131).  In 


FIG.  131.— The  Aletsch  Glacier,  Switzerland. 


crossing  a  glacier,  such 
as  the  Mer  de  Glace, 
these  sharp,  steep  ridges 


the    chief   difficulties    encountered    are 
of  ice. 

The  most  conspicuous  roughness  of  a  glacier's  surface  develops 
where  there  is  a  sudden  change  in  the  slope  of  the  bed  (Figs.  132,  133). 
In  a  river  this  would  produce  a  waterfall,  and  in  a  glacier  it  produces 
an  ice/all.  Such  icefalls  make  travel  on  a  glacier  extremely  difficult 
and  dangerous.  The  ice 
passes  over  the  fall  slice 
by  slice,  the  fall  (as  in  a 
river)  remaining  station- 
ary. Below  the  fall  the 
blocks  heal  together,  but 
the  resulting  surface  is 
extremely  rough,  although 
it  gradually^  becomes  FIG.  132.  —  Longitudinal  sections  of  a  glacier 
Smoother.  showing  icefalls  formed  where  the  slope  of  the  bed 

ki  j          of  a  glacier  increases  suddenly.     (After  Heim.) 

is  not  to   be   under- 
stood that  a  glacier  is  much  fractured  in  all  parts.     The  absence  of 
cracks  on  portions  of  the  Aar  Glacier  is  shown  by  the  fact  that  a 
pond  20  feet  deep  and  covering  10  acres  existed  for  24  years  and 
was  carried  a  distance  of  600  feet. 

(2)  Irregularities  Due  to  Streams  and  Ice  Tables.    The  surfaces 


THE  WORK  OF  GLACIERS 


149 


FIG.  133.  — Denver  Glacier,  Alaska,  showing  an  ice- 
^>  feeding  grounds,  and  lateral  and  medial  moraines. 
(Photo.  F.  B.  Sayre.) 


of  glaciers  become  irregular  in  other  ways  besides  fracturing.  Water 
from  the  melting  ice  forms  rivulets,  which  erode  and  melt  channels  in 
the  ice.  When  such  a  . 
stream  reaches  a  cre- 
vasse it  plunges  down, 
forming  a  circular 
shaft  called  a  moulin 
(French  for  mill).  As 
the  ice  moves  on,  this 
opening  is  closed  and 
a  new  one  formed  in 
its  place.  Thus  a 
series  of  inactive 
moulins,  in  various 
stages  of  preserva- 
tion, are  left  extend- 
ing down  the  glacier 
from  the  active  one. 
The  active  moulin,  however,  may  be  said  to  remain  stationary  or 
confined  to  narrow  limits,  and  may,  in  time,  excavate  potholes 

(p.   93)    many   feet   in   depth   in   the   rock 
beneath  the  glacier  (Fig.  134). 

Since  the  ice  of  a  glacier  varies  in  com- 
pactness it  melts  unevenly,  and  this  also 
tends  to  produce  a  rough  surface. 

The  surface  of  a  glacier  is  also  roughened 
by  the  irregular  melting  of  the  ice,  due  to 
the  accumulation  of  debris.  If  a  fragment 
of  rock  which  has  fallen  on  the  ice  is  too 
thick  to  be  heated  through  by  the  sun  it 
will  protect  the  ice  beneath  from  melting. 
Because  of  this  it  may  in  time  stand  on  an 
ice  pillar  several  feet  in  height,  forming  an 
ice  table  (Fig.  135).  After  a  time  the  pillar 
may  become  so  high  that  the  sun  will  be  able 

F  to  melt  it.     The  protecting  cap  of  rock  will 
FIG.   134. —  A  giant  pot-  f  b       r 

hole  formed  in  the  bed  of  a  then  be  undermined  and  will   slide  ott,  on 

glacier  by  the  water,  sand,  the  south  side  in  the  northern  hemisphere, 

and  gravel  carried  through  a  an(J  wjn  then  be    rea(j     tQ   cauge  the  forma. 
crevasse.     Near  Chnstiama. 

(After  A.  Geikie.)  tion  of  another  column. 


150 


PHYSICAL  GEOLOGY 


FIG.  135.  —  Ice  pillars  protected  by  slabs 
of  rock.  Parker  Creek  Glacier,  California. 
(After  Russell.) 


The  portions  of  a  glacier  over  which  dust  or  thin  layers  of  earth  are 
spread  will  be  melted  more  rapidly  than  those  not  so  covered,  since 
the  dark  dust  absorbs  heat  more  rapidly  than  does  ice.  In  this  way 
dust  wells  and  other  irregular  hollows  several  inches  in  depth  are 
formed,  the  depth  depending  upon  the  diameter  of  the  hollow  and  the 
angle  at  which  the  sun's  rays  strike  it.  The  great  drifts  of  snow  which 

had  to  be  removed  each  spring 
during  the  construction  of  a 
railroad  in  Norway  were  scat- 
tered over  with  fine  dust  in 
order  that  they  might  be  more 
quickly  melted  by  the  sun.  If, 
however,  dust  is  more  than  an 
inch  thick  it  prevents  the  under- 
lying ice  from  melting  and 
forms  dirt  cones. 

Often  the  greatest  irregulari- 
ties on  mountain  glaciers  are  the  long  lines  of  rock  debris  (surface 
moraines)  which  may  be  of  considerable  thickness  and  which  usually 
rest  upon  high  ice  ridges  formed  by  the  protecting  cover  of  the 
former. 

The  water  from  the  moulins  and  that  which  reaches  the  bottom 
of  the  glacier  in  other  ways,  as  for  example  that  melted  from  the 
lower  surface  of  the  glacier  by  friction,  that  which  comes  from  the 
springs  in  the  valley  through  which  the  glacier  is  moving,  and  that 
which  seeps  through  the  cracks  of  the  ice,  all  emerges  from  a  tunnel 
in  the  end  of  the  glacier  as  a  single  stream,  often  of  considerable  size. 
These  streams  flow  even  throughout  the  extreme  winters  of  glaciated 
regions. 

MOVEMENT  OF  GLACIERS 

The  Swiss  early  had  reason 'to  believe  that  glaciers  move,  as  was 
shown  when  two  glaciers  advanced  over  fields  and  meadows,  up- 
setting barns  and  filling  the  quarries  from  which  the  citizens  of  Bern 
obtained  their  marble.  A  recent  example  of  this  sort  occurred  in 
1909-1910,  when  the  advancing  Child  Glacier  in  Alaska  threatened 
to  destroy  a  $1,400,000  steel  bridge. 

Rate  of  Movement.  —  It  was  not,  however,  until  1827  that  any 
serious  attempts  were  made  to  determine  the  rate  at  which  glaciers 
move.  In  that  year  Hugi  built  a  hut  upon  the  Aar  Glacier  in  Switzer- 


THE  WORK  OF  GLACIERS 


land  and  noted  its  position  from  year  to  year.  In  fifteen  years  it  had 
moved  1428  meters,  or  about  100  meters  a  year.  Forty- four  years 
later  the  remains  of  the  hut  were  found  2408  meters  lower  down  the 
valley.  Since  these  first  measurements  careful  surveys  have  been 
made  from  time  to  time,  and  it  has  been  found  that  the  motion  of 
Alpine  glaciers  seldom  exceeds  one  third  to  two  thirds  meter  (one  to 
two  feet)  a  day.  In  1861  the  heads  of  three  guides  with  some  hands 
and  fragments  of  clothing  appeared  at  the  foot  of  the  Bossons  Glacier 
on  whose  neve  they  had  been  buried  beneath  an  avalanche  forty-one 
years  before.  So  perfect  was  the  preservation  that  they  were  easily 
recognized  by  a  guide  who  had  known  them  in  life.  The  rate  of 
movement  had  been  eight  inches  a  day. 

In  large  glaciers,  however,  the  rate  is  much  more  rapid.  It  is 
estimated  that  the  Child  Glacier  in  Alaska  moves  about  30  feet  a  day 
during  the  summer,  and  a  large  glacier  which  drains  the  snow  fields 
of  north  Greenland  is  said  to  have  moved  more  than  60  feet  in  a  single 
day.  These  latter  figures  are  exceptional  and  apply  only  to  very  large 
and  thick  glaciers.  Large  glaciers,  however,  do  not  always  move 
faster  than  small  ones,  since  other  conditions  may  counterbalance 
the  greater  thickness. 

Differential  Movement  of  Glaciers.  —  By  placing  stakes  in  a 
straight  line  across  the  surface  of  a  glacier  and  a  vertical  row  on  a 


I 

0  < 

1  / 
;       / 

0  O 

1  I 


ooe 


ABC 

FIG.  136.  —  Diagrams  showing  the  movement  of  glaciers.  A,  a  line  of  stakes 
placed  in  a  straight  row  across  a  glacier  becomes  more  and  more  curved  each  day. 
By  a  line  of  stakes  placed  in  a  vertical  row  on  the  exposed  side  of  a  glacier  becomes 
more  and  more  inclined.  C  shows  the  formation  of  marginal  fissures  produced  by 
the  pulling  of  the  more  rapid  central  portions  upon  the  slower  marginal  portions. 

side  exposure,  it  was  found  that  the  middle  of  a  glacier  moves  faster  than 
the  sides  (Fig.  136  A}  and  the  top  faster  than  the  bottom  (136  A,  B). 
In  one  glacier,  while  the  top  moved  6  inches,  the  middle  moved  only  4.5 
inches,  and  the  bottom  2.5  inches.  The  reason  for  the  slower  motion 
of  the  sides  and  bottom  is  evidently  to  be  found  in  the  friction  with 


152  PHYSICAL  GEOLOGY 

the  walls  and  bed  of  the  valley  through  which  the  glacier  flows.  It 
follows  from  the  above  that  the  rate  of  movement  will  be  reduced 
if  the  bed  of  the  glacier  is  rough,  and  that  a  smooth  bed  will  favor 
rapid  motion. 

It  has  been  found  that  the  line  of  swiftest  motion  is  not  always  in 
the  middle  of  glaciers,  but  that  as  in  the  case  of  rivers,  although  to  a 
lesser  degree,  it  is  deflected  from  side  to  side,  being  nearer  the  outside 
of  a  curve. 

Factors  Influencing  the  Rate  of  Movement.  —  The  rate  of  move- 
ment of  a  glacier  increases  with  (i)  the  slope  of  the  bed  upon  which  it 
rests,  but  depends  even  more  upon  (2)  the  slope  of  the  upper  surface 
of  the  ice  and  upon  (3)  its  thickness.  A  general  inclination  of  the 
upper  surface  of  a  glacier  is  necessary  for  glacial  movement,  although 
for  short  stretches  the  surface  of  the  ice  may  even  have  a  backward 
slope.  The  beds  of  valley  glaciers  slope  in  the  general  direction  of  the 
movement  of  the  ice,  but  there  are  many  local  exceptions,  as  is  shown 
by  the  deep  basins  in  valleys  formerly  occupied  by  glaciers.  The  great 
ice  sheets  of  North  America  moved  to  the  south  over  a  land  surface 
which  for  many  miles  sloped  towards  the  north,  i.^.,  in  the  direction 
opposite  to  that  of  the  movement  of  the  ice.  In  all  such  cases  the 
upper  surface  of  the  ice  must  have  sloped  in  the  direction  of  the 
glacial  movement. 

The  velocity  of  a  glacier  is  greater  in  summer  than  in  winter,  and 
at  midday  than  at  night ;  that  is,  when  the  glacier  is  melting  more 
rapidly  and  is  most  thoroughly  saturated  with  water.  The  Mer  de 
Glace,  France,  moves  in  summer  at  an  average  rate  of  27  inches  a 
day  in  the  middle  and  13  to  19  inches  a  day  near  the  sides;  in  winter 
the  rate  is  about  half  as  much.  Other  factors  influencing  the  rate 
of  motion  of  a  glacier,  besides  the  slope  of  the  ground,  the  slope  of  its 
upper  surface,  and  the  quantity  of  water  with  which  the  ice  is  sat- 
urated, are  the  amount  of  load  in  its  basal  portions,  which  tends  to 
retard  the  rate,  and  the  straightness  of  its  course  and  the  smoothness 
of  its  bed,  which  tend  to  increase  it. 

Lower  Limit  of  Glaciers.  —  Glaciers  move  down  their  valleys  until 
they  reach  a  point  where  the  melting  (ablation)  equals  the  forward 
movement  (Fig.  137).  When  the  melting  exceeds  the  forward  move- 
ment, the  glacier  is  said  to  retreat;  when  it  is  less  the  glacier  advances. 
It  is  evident  that  the  lower  limit  of  a  glacier  will  not  be  fixed  (except 
when  it  reaches  the  sea)  unless  the  conditions  of  temperature  and 
snowfall  remain  constant.  Since  both  temperature  and  snowfall 


THE  WORK  OF  GLACIERS 


153 


usually  vary  from  year  to  year  and,  more  widely,  in  cycles,1  the  ends 
of  glaciers  are  seldom  stationary  for  long  periods.  If  the  depth  of 
the  snow  in  the  cirque  increases  during  a  single  year  or  a  number  of 
years,  the  glacier  will  advance ;  while  if  the  snowfall  is  slight  or  the 
average  temperature  high  so 
that  little  snow  can  accumu- 
late, the  glacier  will  retreat. 
For  example,  because  of  the 
hot  summer  of  1911  practi- 
cally all  of  the  glaciers  of  the 
Alps  were  in  retreat  in  1912, 
one  (the  Brenva  Glacier  on 
Mt.  Blanc)  receding  50  me- 
ters. The  Muir  Glacier  in 
Alaska  has  retreated  seven 
miles  in  the  past  twenty 
years,  and  the  glaciers  of 
the  Chamonix  valley  in  the, 
Alps,  one  quarter  to  one  half 
of  a  mile  since  1812.  In 
1858  there  was  a  harbor  in 
Bell  Sound,  Spitsbergen,  at 
the  head  of  which  was  a 
strip  of  lowland  and  beyond 
this  a  low,  but  broad  glacier. 
In  1860-1861  the  glacier  advanced  over  the  lowland,  filled  up  the 
harbor,  and  extended  far  into  the  sea.  It  is  now  one  of  the  largest 
glaciers  in  Spitsbergen. 

A  large  glacier  responds  to  excessive  or  deficient  snowfall  more 
slowly  than  a  small  one,  and  several  years  may  elapse  before  it  shows 
the  effect  of  such  changes. 

An  unusual  cause  of  rapid  glacial  advance  is  recorded  from  Alaska, 
where  the  ice  fronts  of  a  number  of  glaciers  have  moved  forward  as  a 
result  of  earthquake  shocks.  During  an  earthquake  in  1899  the 
mountains  from  which  the  snow  supply  of  these  glaciers  is  derived 
were  so  vigorously  shaken  that  great  avalanches  of  snow  and  rock 
were  thrown  down  on  the  neves.  This  increased  supply  caused  all 
of  the  glaciers  in  the  region  affected  to  advance.  They  did  not  all, 

1  There  appears  to  be  a  climatic  cycle  of  35  years  during  which  a  series  of  cold  or  rainy  years 
is  followed  by  years  which  are  warmer  or  drier. 


FIG.  137.  —  The  Rhone  Glacier,  showing 
crevasses  and  front. 


154 


PHYSICAL  GEOLOGY 


however,  show  a  simultaneous  forward  movement.  The  reason  for 
this  is  to  be  found  in  the  size  of  the  glaciers.  It  was  discovered  that 
the  smaller  glaciers  advanced  more  quickly  and  had,  indeed,  in  1909 
already  passed  completely  through  the  period  of  advance,  while  the 
long  glaciers  were  at  that  date  (1909)  either  still  advancing  or  merely 
beginning  to  advance.1 


TRANSPORTATION  OF  MOUNTAIN  GLACIERS 

Surface  Moraines.  —  We  have  seen  (p.  29)  that  in  regions  where 
frost  is  an  active  agent  of  the  weather,  talus,  composed  of  angular  rock 

5,  fragments  of  various  sizes, 
rests  at  the  bases  of  cliffs. 
If  the  bottom  of  a  valley 
with  high  and  steep  sides  is 
occupied  by  a  glacier,  these 
fragments  will  fall  upon  its 
surface ;  and  as  the  ice  moves 
on  all  parts  of  the  glacier's 
side,  will  pass  under  the  cliffs 
which  supply  the  debris.  In 
the  process  of  time,  a  regular 
ridge  of  angular  rocks  and 
soil  will  rest  upon  the  edge  of 
the  ice.  Such  a  deposit  is 
called  a  lateral  moraine  (Figs. 
*38>  J39>  I4°)-  The  ice  of 
glaciers  does  not  commingle 
at  their  confluence,  but  the 
masses  move  on  side  by  side, 
the  two  adjacent  lateral 
moraines  uniting  to  form  a 
medial  moraine  (Figs.  138, 
139,  140).  In  this  way,  by 
the  union  of  several  branches,  a  glacier  may  be  covered  with  several 
medial  moraines.  A  medial  moraine  may  also  be  formed  when  a 
glacier  passes  over  an  elevation  in  its  bed  from  which  it  scrapes  off 
rock  fragments.  After  the  glacier  has  passed  this  point  the  debris 

1  Physiography  and  Glacial  Geology  of  the  Yakutat  Bay  Region,  Professional   Paper 
No.  64,  U.  S.  Geol.  Surv.,  1909. 


FIG.  138.  —  Map  showing  the  formation  of 
the  Mer  de  Glace  by  the  union  of  several  gla- 
ciers. The  development  of  lateral  and  medial 
moraines  is  illustrated  also. 


THE  WORK  OF  GLACIERS 


155 


f. 


may  be  exposed  by  the  melting  of  the  surface  of  the  ice  and 
continue  to  the  end  of  the  glacier  as  a  medial  moraine.  It  will 
readily  be  seen  that  the  material  of  the  various  surface  moraines 
of  a  glacier  may  differ  widely  in  composition,  since  they  were 
derived  from  the  rocks  of  many  parts  of  the  valley.  The  Baltoro 
Glacier  of  Hindu  Kush  has  fifteen  moraines  of  different  colors. 
(Bonnersheim.) 

Since  the  surface  moraines  are  usually  sufficiently  thick  to  protect 
from  the  sun's  rays  the  ice  upon  which  they  rest,  they  are  generally 
situated  on  ridges  of 
ice,  sometimes  50  to 
80  feet  in  height. 
After  a  time  the  ridges 
become  so  high  that 
the  morainic  material 
slips  off,  thus  widen- 
ing the  morainic  belt. 
After  several  repeti- 
tions of  this  process 
the  medial  and  lateral 
moraines  may  cover 
completely  the  lower 
end  of  a  glacier. 

The  size  of  some 
of  the  rock  frag- 
ments carried  on  the 
surfaces  of  glaciers 
is  very  great.  One 

such    bowlder    con-    fc^  ^lVV^l>~/^"\*i      f^f^l)  v~/^V' 
tained  244,000  cubic 


Kfn 


£„„      /IT   ..        ^  FIG.  139.  —  Diagram  and  cross  section  of  a  mountain 

feet  (Forbes),  which   glader     Lateral  morajnes  are  seen  to  produce  medial 

IS     equivalent     to     a  moraines.     The  movement  of  the  superglacial  material  to 

squared  Stone  122  feet  f°rm  englacial  and  finally   subglacial   material   is  shown. 

In         CO     f              *H  Icefalls  occur  near  the  confluence  of  the  glaciers  on  the 

K'     3                             '  right, 
and  3 6  feet  high.  The 

weight  of  material  which  a  glacier  can  carry  on  its  surface  is  limited 
only  by  what  it  may  receive,  and  the  very  weight  of  the  surface  load 
will  hasten  the  movement  of  the  glacier.  Upon  the  disappearance 
of  a  glacier,  these  great  rock  masses  are  often  left  in  unstable  posi- 
tions and  are  then  known  as  balanced  bowlders,  or  rocking  stones. 


156 


PHYSICAL  GEOLOGY 


All   bowlders  transported   and  deposited  by  glaciers   are  given  the 
general  name  erratics.     Figure  141  shows  a  balanced  bowlder. 

_         Subglacial  Material. 

—  The  bottom  por- 
tions of  glaciers  con- 
tain stones  andground- 
up  rock.  This  ma- 
terial is  derived,  either 
directly  from  the  rock 
bed  or  from  the  super- 
glacial  material  which 
reaches  the  bottom 
through  the  crevasses. 
The  subglacial  mate- 
rial is  usually  much 


FIG.   140.  —  Aar  Glacier  showing  lateral  and  medial 
moraines,  and  cirques.     (Photo.  L.  E.  Westgate.) 


worn. 


En  glacial  Material. 
—  Between  the  sur- 
face and  the  bottom  of  a  glacier  some  debris  may  be  carried.  This 
is  derived  in  part  from  the  superglacial  material  which  has  not 

reached    the    bottom  , , 

through  the  crevasses, 
in  part  from  that 
which  gathered  on  the 
surface  of  the  snow  or 
neve  and  was  subse- 
quently covered,  and 
in  part  from  that 
which  was  scraped  off 
an  elevation  in  the 
bed.  The  englacial 
material  may  become 
superglacial  by  abla- 
tion, and  subglacial 
by  gradually  settling 
or  by  the  melting  of 
the  lower  ice.  All  will 
be  deposited  when  the 


FIG.    141.  —  Balanced    bowlder,    Hoosac    Mountain, 


i        r  Massachusetts.     The  bowlder  is  so  nicely  balanced  that 

tne  glacier  is  although  of  great  weight  it  can  be  made  to  vibrate  with 
reached.  little  effort. 


THE  WORK  OF  GLACIERS 


157 


EROSION  BY  MOUNTAIN  GLACIERS 

Plucking  and  Abrasion.  —  Glaciers  accomplish  their  work  of  erosion 
in  two  ways,  (i)  The  ice  secures  a  hold  on  the  material  of  its  bed 
either  by  freezing  about  projecting  points  of  rock  or  by  being  pressed 
into  the  joints  and  other  cracks  by  its  great  weight.  As  the  glacier 
moves  on  rock  fragments  are  pulled  out  and  carried  along.  This 
process  is  called  plucking,  and  by  it  a  glacier  may  remove  a  great 
quantity  of  material  in  much-jointed  rock.  The  process,  however, 
is  of  little  effect  on 
rocks  which  have  few 
joints ;  and  conse- 
quently one  some- 
times finds  that  a 
glacier  has  been  able 
to  deepen  its  valley 
more  easily  in  hard 
granite  and  gneiss 
than  in  the  softer 
limestone,  because 
the  former  were 
much  fractured,  per- 
mitting the  plucking 
out  of  blocks,  while 
the  latter  being  less 
broken  was  little 
affected.  (2)  Glaciers 
also  deepen  and  widen  their  valleys  by  abrasion.  The  tools  which 
accomplish  the  work  of  abrasion  are  the  rocks  which  have  been 
torn  from  the  bed  by  plucking  and  those  which  have  reached 
the  base  of  the  ice  from  the  surface  through  crevasses.  Hold- 
ing these  rock  fragments  in  a  firm  grasp  and  pressing  with  great 
force,  estimated  to  be  48,600  pounds  to  the  square  yard  in  portions 
of  the  Aar  Glacier,  a  glacier  acts  as  a  gigantic  file,  cutting  down  pro- 
jecting points,  and  deepening  and  smoothing  its  bed.  The  hard 
pebbles  scratch  and  the  bowlders  groove  the  bedrock ;  while  the  clay 
and  rock,  ground  fine  by  the  process,  polish  the  surface,  producing  the 
smoothed  and  striated  appearance  so  characteristic  of  glaciated  rocks. 

It  will  readily  be  seen  that  a  thick  glacier,  because  of  its  great 
weight,  will  be  able  to  erode  its  bed  more  rapidly  than  a  thin  one, 


FIG.  142.  —  Roche  moutonne'e,  Central  Park,  City  of 
New  York.     (U.  S.  Geol.  Surv.) 


158 


PHYSICAL  GEOLOGY 


and  that  a  very  thin  glacier  or  one  with  clear  ice  at  its  bottom  may, 
indeed,  not  only  be  unable  to  wear  down  its  rock  bed,  but  may  even 
override  loose  sand  or  glacial  drift. 

In  moving  over  a  projection  in  its  valley  a  glacier  smooths  off  the 
side  upon  which  it  impinges  (the  stoss  side)  and  plucks  angular  frag- 

^ ments  from  the  lee  side,  often 

leaving  it  rough  and  jagged. 
It  also  smooths  rough  sur- 
faces into  forms  (Figs.  142, 
143)  which,  because  of  their 
rounded  shapes,  have  been 
given  the  name  roches  mouton- 
n'ees  (French  for  rock  sheep). 
If  one  looks  down  a  glaciated 
valley,  the  smooth  stoss  slopes  of  the  roches  moutonnees  are  very 
striking.  If,  however,  one  glances  up  the  valley,  the  rough  lee  sides 
of  the  roches  moutonnees  are  often  so  conspicuous  as  to  seem  to  con- 
tradict the  statement  that  glaciers  deepen  their  valleys  by  abrasion. 
Effect  on  the  Material  Carried.  —  Not  only  are  the  rock  beds  of 
glaciers  grooved  by  bowlders,  scratched  by  pebbles,  and  polished  by 


FIG.  143.  —  Diagram  showing  a  projecting 
rock  smoothed  and  rounded  on  the  side  upon 
which  the  glacier  impinged  (stoss),  and  rough- 
ened on  the  lee  side  by  plucking, —  a  roche 
moutonnee. 


FIG.  144.  —  Glaciated  pebbles.     (After  Blackwelder  and  Barrows.) 


clay,  but  these  tools  are,  in  turn,  scratched  and  polished  (Fig.  144). 
If  one  axis  of  a  glaciated  pebble  is  much  longer  than  the  other,  it  is  usu- 
ally found  that  the  striations  are  parallel  to  the  longer  axis.  This  is 
due  to  the  fact  that  the  pebble  was  held  in  this  position  in  the  ice,  as  it 
offered  less  resistance  in  this  way  than  in  any  other.  If  the  axes  of  a 
pebble  are  approximately  equal,  it  may  have  scratches  running  in 
many  directions,  as  this  shape  would  enable  it  to  turn  more  easily 
in  the  ice  and  therefore  to  be  carried  onward  in  various  positions. 


THE  WORK  OF  GLACIERS 


159 


Since  much  of  the  rock  of  the  valley  floor  over  which  a  glacier  moves 
as  well  as  the  pebbles  which  it  holds  in  its  grasp  are  ground  to  powder, 
it  is  not  surprising  to  find  the  water  of  glacial  streams  so  turbid  with 
sediment  as  to  be  spoken  of  as  glacier  milk.  The  light  color  of  such 
streams  differs  from  the  yellow  color  of  ordinary  streams  because  the 
former  carry  freshly  ground,  unoxidized  rock,  and  the  latter  the  prod- 
ucts of  weathering.  The  Aar  Glacier,  for  example,  a  comparatively 
small  glacier  of  the  Alps,  is  estimated  to  discharge  280  tons  of  rock 
flour  a  day  during  a  summer  month. 

Factors  Influencing  the  Rate  of  Erosion.  —  As  has  been  said, 
glaciers  accomplish  little  erosion  in  passing  over  smooth  surfaces. 
The  amount  of  morainic  material  carried  by  Greenland  glaciers,  for 
example,  is  surprisingly  small.  This  is  due  to  the  fact  that  they  have 
moved  over  their  beds  so  long  as  to  render  them  comparatively 
smooth.  If,  however,  a  glacier  moves  over  a  bed  whose  surface  is 
sufficiently  rough  to  permit  the  ice  to  tear  away  fragments  by  pluck- 
ing (p.  157),  it  is  likely  to  deepen  its  bed  rapidly,  since  under  these 
conditions  a  large  surface  is  exposed  to  the  wear  of  the  debris  held  in 
the  base  of  the  ice.  Erosion  is  also  favored  by  the  incoherence  of  the 
material  over  which  the  ice  moves,  by  the  weight  of  the  ice,  and  by  a 
rapid  rate  of  movement;  but  is  unfavorably  affected  by  an  over- 
loading of  the  basal  portion  of  the  glacier  with  debris  and  by  the 
resistance  of  the  rock. 


DEPOSITS  OF  MOUNTAIN  GLACIERS 

Terminal  Moraines.  —  At  the  lower  end  of  a  glacier,  where  the 
melting  equals,  or  nearly  equals,  the  advance,  all  of  the  debris  (the 
superglacial,  englacial,  and  subglacial)  is  deposited  as  a  terminal 
moraine.  Terminal  moraines  (Fig.  145)  are  usually  crescent  or 
horseshoe-shaped,  concave  towards  the  glacier,  and  often  form  con- 
spicuous hills  in  valleys  once  occupied  by  glaciers.  The  heights  of 
terminal  moraines  vary  greatly,  since  the  quantity  of  material  de- 
posited in  them  depends  upon  a  number  of  factors,  (i)  The  length 
of  time  during  which  the  front  of  a  glacier  remained  stationary  is 
important.  If  a  glacier  advances  600  feet  a  year  and  for  a  number  of 
years  melts  back  at  the  same  rate,  it  is  evident  that  each  year  all  of 
the  debris  carried  on,  in,  and  under  it  will  be  left  at  the  same  place, 
with  the  exception  of  that  which  is  carried  away  by  the  stream  which 
flows  from  it.  If,  on  the  other  hand,  the  ice  melts  back  600  feet  a  year, 

CLELAND  GEOL.  —  II 


160  PHYSICAL  GEOLOGY 

while  it  advances  500  feet,  it  is  evident  that  comparatively  little  debris 
will  be  left  at  any  one  spot,  and  no  conspicuous  hills  or  ridges  will  be 
formed.  (2)  The  velocity  of  the  glacier,  (3)  the  quantity  of  material 
transported  by  it  (p.  154),  and  (4)  the  amount  carried  away  by  the 
stream  which  flowed  from  its  end  are  also  determining  factors  in  the 
size  of  terminal  moraines.  They  sometimes  reach  a  height  of  several 
hundred  feet,  but  heights  of  100  or  200  feet  are  more  common.  If 
the  front  of  a  waning  glacier  halts  for  considerable  periods  at  different 
points,  a  series  of  terminal  moraines  (also  called  recessional  moraines) 
will  be  left. 

The  material  of  terminal  moraines  usually  consists  of  a  heterogene- 
ous mixture  of  large  and  small  pebbles  and  bowlders  of  different  kinds, 


FIG.  145.  —  Moraine  near  Dansville,  New  York.     (Photo.  H.  L.  Fairchild.) 

embedded  in  clay  and  sand.  Occasional  patches  of  stratified  sand  and 
gravel  from  the  water  of  the  melting  ice  also  occur.  All  the  glacial 
debris  is  called  drift,  the  unstratified  is  called  till  or  bowlder  clay,  and 
the  stratified  (sorted  and  laid  down  in  water)  is  called  stratified  drift. 
On  glaciers  which  move  between  precipitous  walls  supplying  great 
quantities  of  talus,  the  lateral  moraines  will  be  large ;  and  upon  the 
disappearance  of  the  ice,  especially  if  the  retreat  be  slow,  a  high 
ridge  of  unstratified  drift  will  be  left  on  each  side  of  the  valley. 
Some  of  these  are  a  thousand  feet  or  more  in  height.  The  terminal 
moraine  of  such  a  glacier  may  be  comparatively  insignificant.  Ter- 
minal moraines  are  breached  by  streams  and  are  sometimes  entirely 
removed  by  them.  Sometimes,  however,  the  moraine  constitutes  an 
effective  dam  for  many  years,  behind  which  a  picturesque  lake  lies. 


THE  WORK  OF  GLACIERS  161 

Thousands  of  mountain  lakes  owe  their  existence  to  such  morainic 
dams. 

Submarginal  Moraines.  —  Another  kind  of  moraine  is  formed 
under  the  sides  of  a  glacier  by  the  movement  of  the  ice  from  the 
center  to  the  sides.  This  should  not  be  confused  with  the  lateral 
moraines  of  the  surface  (p.  154).  In  valley  glaciers  which  receive  little 
superglacial  debris  these  submarginal  moraines  maybe  thicker  than  the 
surface  moraines.  The  presence  of  polished  and  striated  pebbles  and 
bowlders  in  such  moraines  is  abundant  proof  that  the  drift  composing 
them  had  been  carried  between  the  ice  and  its  bed. 

Ground  Moraine.  —  A  glacier  may  be  so  full  of  debris  in  its  basal 
portion  that  it  is  unable  to  carry  all  of  it.  Under  such  conditions 
some  of  the  load  is  deposited  and  is  overridden.  Such  deposition  takes 
place  (i)  where  the  ice  is  thinning  near  the  end,  as  this  makes  its 
movement  less  rapid,  so  that  it  is  unable  to  carry  all  of  the  load  which 
it  has  acquired  in  its  progress  through  a  rough  valley.  Such  deposi- 
tion also  occurs  (2)  after  a  glacier  has  passed  over  a  projection  in  its 
bed,  as  the  bottom  of  the  ice  is  then  heavily  loaded  with  the  debris 
which  it  has  plucked  or  abraded  from  the  obstacle. 

In  a  valley  formerly  occupied  by  a  glacier  there  is  usually  a  layer 
of  compact  till  composed  of  clay  and  much-worn  pebbles.  This 
deposit  is  known  as  the  ground  moraine  and  was  derived  either  from 
the  bottom  of  the  advancing  ice,  as  described  above,  or  from  the  base 
of  the  ice  upon  its  disappearance.  It  is  usually  thickest  near  the 
terminal  moraine  and  thinnest  near  the  head  of  the  glacier,  while  over 
portions  of  the  valley  it  may  be  entirely  lacking.  Since  conditions 
in  valley  glaciers  favor  erosion  rather  than  deposition,  their  ground 
moraines  are  seldom  important,  being  in  contrast  in  this  respect  to 
continental  glaciers  (p.  171),  whose  ground  moraines  are  of  con- 
siderable thickness,  although  seldom  attaining  the  depth  of  terminal 
moraines. 

The  Work  of  Glacial  Streams.  —  The  streams  which  flow  from 
beneath  glaciers  or  from  their  sides  are  supplied  with  pebbles  from 
the  moraine  and  an  abundance  of  rock  particles  derived  from  the 
rock  ground  to  fine  flour  between  the  ice  and  its  bed.  With  such  tools 
they  are  able  to  deepen  their  channels  as  long  as  they  have  sufficient 
velocity.  The  streams  from  certain  glaciers  emerge  from  their  fronts 
in  deep  gorges  which  they  have  cut  in  the  rock.  The  Lammer  Glacier 
of  Switzerland  and  the  Mer  de  Glace  are  examples.  It  is  doubtful, 
however,  if  the  deepening  which  is  such  a  marked  feature  of  valleys 


162 


PHYSICAL  GEOLOGY 


long  occupied  by  glaciers,  has  been  accomplished  to  any  great  extent 

by  subglacial  stream  erosion. 

If  the  streams  which  issue  from  glaciers  are  ponded  by  terminal 

moraines,  lakes  are  formed  in  which  they  deposit  their  loads.     If 

they  have  a  free 
course,  however,  they 
will  carry  their  loads 
of  rock  flour  and  peb- 
bles farther  down  the 
valleys.  The  coarse 
gravel  will  soon  be 
dropped,  but  the  finer 
material  may  be  car- 
ried some  distance. 
When,  however,  a 
stream  thus  loaded 
reaches  a  more  gentle 


FIG.  146.  —  The  union  of  the  Rhone  and  Arve  rivers 
near  Geneva,  Switzerland.  The  water  of  the  Rhone,  hav- 
ing been  filtered  by  Lake  Geneva,  is  clear  and  blue,  while 
that  of  the  Arve  is  grey  with  the  rock  flour  carried  into 
it  by  glacial  streams.  To  the  right  is  seen  the  cement 
works  for  recovering  the  Arve  sediments.  (Hobbs,  Earth 
Features.) 


grade,  it  may  lose  so 
much  velocity  that 
it  becomes  overloaded 
and  compelled  to  take 
a  braided  course  (p.  86).  The  stratified  deposits  laid  down  in 
valleys  by  glacial  streams  are  called  valley  trains  (p.  178). 

As  has  been  stated 
(p.  1 59), streams  flow- 
ing from  glaciers  are 
milky  with  rock  flour, 
while  those  which 
gather  their  water 
from  the  land  sur- 
face may  be  yellow 
with  the  clay  of  the 
weathered  rock  which 
they  bear  along,  but 
streams  filtered  by 


FIG.  147.  —  Block  diagram  showing  a  valley  blocked 
by  a  moraine;  the  stream  having  been  diverted  from  its 
old  course  has  cut  a  steep-sided,  postglacial  gorge. 


lakes  are   clear.     At 

the  confluence  of  the 

Rhone  and  the  Arve  (Fig.  146)  a  striking  contrast  is  seen  between 

the  clear  water  flowing  from  Lake  Geneva  and  the  turbid  water  of 

the  Arve  which  has  its  source  in  the  glacier  of  that  name.     For  a 


THE  WORK  OF  GLACIERS  163 

short  distance  after  their  union  the  waters  of  the  two  streams  flow 
side  by  side,  but  gradually  they  merge. 

Since  streams  are  no  longer  overloaded  after  the  retreat  of  their 
glaciers,  they  begin  to  erode  the  alluvial  deposits  of  their  former 
flood  plains  and  in  this  way  form  the  terraces  which  so  often  border 
stream  valleys  in  glaciated  regions.  As  the  streams  deepen  their  beds 
in  their  partially  filled  valleys,  they  occasionally  fail  to  find  their 
former  channels,  and  after  excavating  broad  valleys  in  the  recently 
deposited  gravels  may  cut  narrow  gorges  into  solid  rock.  This  is 
shown  in  the  diagram  (Fig.  147),  in  which  a  stream  flows  from  its 
alluvium-filled  valley  (in  the  background)  into  a  deep,  postglacial 
gorge  in  the  foreground. 

LANDSCAPE  MODIFIED  BY  GLACIAL  ACTION 

Characteristics  of  Glaciated  Valleys.  —  A  striking  feature  of  moun- 
tain valleys  which  have  been  subjected  to  the  long-continued  erosion 
of  thick  glaciers  is  the  flatness  of  the  floors  and  the  steepness  of  the 
valley  sides,  as  contrasted  with  the  V-shaped  valleys  cut  by  streams. 
A  cross  section  of  a  valley  which  has  been  shaped  by  glaciers  is 
typically  a  gigantic  U,  sometimes  more  than  3000  feet  deep  and  three 
miles  wide.  The  tributary  streams  of  such  valleys  usually  enter 
them  over  falls.  The  high,  tributary  valleys  are  called  hanging  val- 
leys (Fig.  148),  and  their  occurrence  is  proof  that  the  main  valley  has 
been  deepened  by  glacial  action. 

This  peculiar  relation  between  the  main  valley  and  its  tributaries 
can  best  be  understood  by  following  the  history  of  a  valley  from  the 
time  it  was  first  occupied  by  a  glacier  until  it  again  became  free  from 
ice.  When  a  main  valley  is  occupied  by  a  thick  glacier,  it  will  in 
time  be  deepened  and  broadened,  especially  near  the  bottom,  and  the 
valley  sides  will  at  the  same  time  be  oversteepened.  This  excavation 
is  termed  overdeepening.  Since  the  main  valley  is  well  filled  with 
ice,  it  is  evident  that  the  glaciers  of  the  tributary  valleys  will  not 
be  able  to  lower  their  beds  far  below  the  surface  of  the  main  glacier. 
Consequently,  when  the  glaciers  disappear  from  the  valleys,  the 
side  valleys  will  no  longer  enter  the  main  valley  at  grade,  but  by 
falls.  In  other  words  they  have  become  hanging  valleys.  In  this 
way  those  steep-sided,  picturesque  valleys  were  formed  for  which 
Switzerland  and  British  Columbia  are  famous.  The  many  falls  of 
the  valleys  of  the  Yosemite,  California  (Fig.  148),  and  Lauterbrunnen, 


164 


PHYSICAL  GEOLOGY 


THE  WORK  OF  GLACIERS  165 

Switzerland,  and  many  other  of  the  high  falls  of  the  world  are  of 
this  origin.  Hanging  valleys  of  a  different  origin  have  been  dis- 
cussed elsewhere. 

The  courses  of  valleys  are  straighter  after  glaciation  than  before. 
This  is  due  to  the  fact  that  the  glaciers,  because  of  their  rigidity,  cut 
off  the  "  spurs  "  on  the  inside  of  the  curves. 

The  line  on  the  side  of  a  U-shaped  valley  above  which  glacial  erosion 
was  not  effective  is  marked  by  a  change  in  slope,  forming  a  sort  of 
"  shoulder."  Such  "  shoulders  "  are  of  some  economic  importance  in 
Switzerland,  since  they  usually  afford  good  pasturage  a.nd  are  favorite 
spots  for  hamlets,  as  they  are  not  subject  to  the  severe  cold  of  the  deep 
valleys.  The  "  shoulders  "  are  usually  about  1000  feet  above  the 
bottom  of  the  valleys  in  the  Alps,  but  are  sometimes  as  much  as  3000 
feet  above. 

Mature  Glaciated  Valleys.  —  It  is  evident  that  valleys  which  were 
formerly  occupied  by  glaciers  will  not  be  U-shaped  unless  the  glaciers 
were  at  work  for  a  long  time,  and  every  gradation  can  be  seen  between 
them  and  V-shaped  valleys,  in  which  the  inside  of  the  curves  (spurs) 
have  been  little  cut  away  and  the  beds  are  still  broken  by  falls.  In 
valleys  long  subjected  to  glacial  action  the  spurs  are  cut  away,  the 
bottoms  widened,  and  the  sides  smoothed.  In  the  upper  and  middle 
portions  where  the  weight  of  the  ice  and  its  movement  were 
greatest,  basins  may  have  been  formed  in  which  lakes  now  rest. 
Lake  Chelan,  Washington,  is  probably  such  a  lake,  as  are  also 
the  beautiful  lakes  of  the  Scottish  Highlands.  Even  maturely 
glaciated  valleys  may  not  have  graded  beds,  since,  under  certain 
conditions  (p.  157),  a  glacier  erodes  one  portion  of  its  bed  more 
deeply  than  another. 

Destruction  of  Features  of  Glaciated  Valleys.  —  The  characteristic 
features  of  glacial  valleys  are  destroyed  in  process  of  time  by  the  work 
of  erosion  and  weathering,  very  much  as  are  those  of  cirques.  Talus 
slopes  accumulate  at  the  bases  of  the  cliffs ;  landslides  sometimes  cover 
considerable  areas  of  the  bottoms  with  debris ;  the  streams  from  the 
mountains  build  out  alluvial  fans  and  cones;  and  in  the  course  of 
time  the  "  shoulders  "  are  worn  away  by  weathering  and  the  action 
of  the  rills  which  tumble  over  them.  The  streams  from  the  hanging 
valleys  cut  down  their  beds  so  that  they  enter  the  main  valleys 
through  deep  canyons.  It  is  possible  by  these  criteria  roughly  to 
determine  the  length  of  time  since  the  disappearance  of  the  glacier 
from  the  valley. 


1 66 


PHYSICAL  GEOLOGY 


Fiords.1  —  The  coast  of  Norway  is  noted  for  the  long,  narrow  bays, 
called  fiords  (Fig.   149),  which  may  be  navigated  for  many  miles. 


FIG.  149.  —  Fiord,  Grenville  Channel,  British  Columbia.     (U.  S.  Geol.  Surv.) 

Into  these  fiords  streams  enter  from  hanging  valleys  over  falls. 
Soundings  show  that  while  the  end  towards  the  sea  is  very  deep,  it 
is  not  so  deep  as  at  some  distance  inland.  The  maximum  depth  of 
the  Sogne  fiord  in  Norway  (Fig.  150)  is  4000  feet,  and  that  of  three 

...._  others  is  2550,  2298,  and 
1800  feet.  The  greatest 
depth  occurs  where  the 
fiord  is  bounded  by  moun- 
tain masses  of  great  extent 
and  elevation. 

There  seems  little  doubt 
that  fiords  are  valleys 
which  were  greatly  deep- 
ened by  glacial  erosion. 
Their  increased  depth  from  the  outlets  inward  is  due  either  to  the 
greater  erosion  of  the  glaciers  some  distance  inland,  where  they  were 
presumably  thicker  and  their  erosive  power  consequently  greater;  or 
to  the  piling  up  of  morainic  matter  where  they  entered  the  ocean ;  or 
probably  both  cooperated  to  produce  the  result.  Whether  or  not  the 
glaciers  actually  cut  the  valleys  below  sea  level  has  not  been  proved. 

1  It  has  been  maintained  that  fiords  owe  their  characteristics  to  earth  movements  and  not 
to  glacial  action,  and,  in  fact,  that  fiords  occur  in  non-glaciated  regions.  According  to  this 
theory  areas  were  fractured  along  certain  belts  as  they  were  being  raised  to  form  plateaus. 
These  belts  of  more  or  less  shattered  and  fissured  rocks  are  supposed  to  have  subsided,  with  the 
formation  of  steep-sided  troughs.  In  support  of  this  theory  it  is  pointed  out  that  fiords  are 
arranged  along  a  kind  of  angular  network  believed  to  be  caused  by  intersecting  lines  of  frac- 
tures. (Gregory,  J.  W.,  —  The  Nature  and  Origin  of  Fiords,  1913.) 


FIG.  150.  —  Map  of  Sogne  fiord,  Norway. 


THE  WORK  OF  GLACIERS 


167 


During  the  process  of  the  glacial  deepening  of  the  Scandinavian 
fiords  the  land  was  higher  than  now.  This  was  followed  by  a  period 
of  great  submergence  and  later  by  a  reelevation  of  a  few  hundred 
feet.  Fiords  are  common  in  Greenland,  Alaska,  British  Columbia, 
southern  Chile,  and  Patagonia. 

PIEDMONT  GLACIERS 

When  mountain  glaciers  reach  the  plain  at  the  foot  of  the  mountains 
from  which  they  flow,  they  spread  out,  as  they  are  no  longer  confined 
by  valley  walls,  and  coalesce  to  form  piedmont  (foot  of  mountain) 


* 


FIG.  151.  — Model  of  the  Malaspina  Glacier.  The  dark  margin  is  the  moraine- 
covered  area  upon  which  a  forest  of  spruce,  cottonwood,  and  alder  grows.  (Model  by 
Lawrence  Martin.  Copyright,  University  of  Wisconsin.) 


1 68  PHYSICAL  GEOLOGY 

glaciers.  A  typical  example  of  such  a  glacier  is  the  Malaspina  in 
Alaska  (Fig.  151).  It  is  formed  by  the  union  of  several  glaciers  which 
move  down  the  valleys  of  the  St.  Elias  range  upon  a  nearly  flat  plain. 
The  area  of  the  united  glacier  is  nearly  1500  square  miles,  about  the 
size  of  Rhode  Island.  The  lateral  margin  where  the  ice  is  probably 
1000  feet  thick  is  covered  with  a  belt  of  morainic  matter  a  few  feet 
thick  and  several  miles  wide,  on  which  grows  a  luxuriant  vegetation. 
Extensive  areas  of  bushes  are  found  and,  near  the  outer  edge,  trees 
some  of  which  reach  a  diameter  of  three  feet.  On  the  surface  of 
the  nearly  stagnant  glacier  are  numerous  ponds  in  which  stratified 
deposits  are  laid  down.  The  central  portion  of  the  glacier  is  com- 
paratively free  from  debris  and  is  much  broken  by  crevasses  into  which 
streams  from  the  melting  ice  flow.  Piedmont  glaciers  are  rare  at  the 
present  time,  but  were  much  more  numerous  during  glacial  times, 
when  they  existed  at  the  foot  of  the  Alps,  the  foot  of  the  mountains 
of  western  North  America,  the  southern  Andes,  and  elsewhere. 

CONTINENTAL  ICE  SHEETS 

Up  to  this  point  mountain  glaciers  have  been  discussed  because,  on 
account  of  their  small  size  and  accessibility,  they  are  more  easily 
studied  and  their  phenomena  are  better  known  than  are  those  of  the 
great  continental  glaciers  such  as  now  cover  Greenland  and  the  Ant- 
arctic Continent.  At  one  time  ice  sheets  covered  the  northern 
portions  of  North  America  and  Europe.  These  were  of  great  extent, 
those  of  North  America  covering  an  area  estimated  at  4,000,000 
square  miles;  of  long  duration,  and  probably  of  great  thickness. 
The  stratified  and  unstratified  drift  so  conspicuous  in  many  of  these 
once  glaciated  regions  was  formerly  believed  to  have  been  transported 
to  its  present  position  and  the  underlying  rock  scratched  and  polished, 
by  a  great  flood  (Mosaic  flood)  which  swept  down  from  the  north, 
carrying  with  it  pebbles  and  bowlders  which  striated  and  grooved  the 
rocks  over  which  they  were  borne.  The  term  drift  is  a  relic  of  this 
ancient  theory.  One  cannot  obtain  a  clear  conception  of  the  con- 
ditions which  existed  in  North  America  and  Europe  during  the 
Glacial  Period  (p.  645)  without  a  study  of  the  existing  continental 
glaciers  of  the  polar  regions. 

Greenland.  —  Greenland  is  a  continent  1400  miles  long  and  900 
miles  wide.  Of  this  area,  fully  three  quarters  are  covered  at  all  times 
with  ice,  the  only  inhabited  portion  of  the  continent  being  a  narrow 


THE  WORK  OF  GLACIERS 


169 


70      60    50     40     30      20 


strip  of  land  along  the  coast,  generally  from  5  to  25  miles  wide  (Fig. 
152).  The  great  snow  desert  of  the  interior  is  devoid  of  life,  with  the 
exception  of  lowly  forms, 
such  as  the  microscopic  red 
plants  (Sphcerilla  nivalis) 
which  sometimes  exist  in 
such  abundance  as  to  give 
a  red  color  to  the  snow  and 
produce  the  "  red  snow  "  of 
both  mountain  and  conti- 
nental glaciers. 

All  Greenland  explorers 
give  nearly  identical  de- 
scriptions of  the  interior. 
Near  the  coast  the  ice  sheet 
rises  with  comparative  ab- 
ruptness, being  steeper  on 
the  east  than  on  the  west 
coast,  while  the  central  por- 
tion is  nearly  flat  (Fig.  153). 
The  gradient  of  the  surface, 
as  a  whole,  gradually  de- 
creases as  the  interior  is 
approached,  and  "the  mass 
thus  presents  the  form  of 
a  shield  with  a  surface  cor- 
rugated by  gentle,  almost 
imperceptible  undulations, 
lying  more  or  less  north 
and  south."  (Nansen.)  The  highest  recorded  point  in  the  interior  is 
about  9000  feet  above  the  sea,  although  it  is  possible  that  unexplored 
portions  may  reach  an  altitude  of  10,000  feet.  A  hundred  miles  from 


FIG.  152.  —  Map  of  Greenland,  showing  a  con- 
tinental ice  sheet. 


FIG.  153.  —  A  section  across  Greenland,  showing  the  profile  of  the  ice  and  the  prob- 
able configuration  of  the  land. 

the  coast  no  depressions  or  elevations  in  the  ice  mark  the  presence  of 
valleys  or  mountain  ranges  beneath ;   but  within  50  or  70  miles  of 


170  PHYSICAL  GEOLOGY 

the  coast  broad,  shallow,  basinlike  depressions  appear  which,  when 
followed  seaward,  grade  into  great  glaciers  which  flow  from  the  central 
mass  into  the  sea  through  valleys.  The  scenery  of  these  broad  de- 
pressions resembles,  on  a  grand  scale,  the  gathering  grounds  of 
Alpine  glaciers.  The  ice  of  the  depressions  which  give  rise  to  these 
separate  tongues  of  ice  is  probably  a  mile  in  thickness,1  but  on  the 
mountain  ridges  it  is  much  thinner. 

The  smooth  almost  flat  central  portion  gives  place  near  the 
margins  to  a  surface  of  a  decidedly  different  character.  Here,  where 
the  motion  of  the  ice  is  more  rapid,  being  greater  in  one  portion  than 
in  another,  the  surface  is  much  broken  by  crevasses  which  make 
travel  well-nigh  impossible.  Near  the  coast,  where  the  mountains 
protrude  through  the  ice  as  islands  (nunataks),  the  whiteness  of  the 
surface  is  broken  by  patches  and  lines  of  rock  waste  derived  from  these 
projections.  Nunataks  are  often  surrounded  by  deep  ditches,  due 
to  the  absorption  of  the  sun's  heat  by  the  dark  rock  walls  and  its 
radiation  from  them,  with  the  consequent  melting  of  the  adjacent 
ice. 

As  has  been  said,  the  great  interior  plateau  is  drained  by  glaciers 
which  descend  through  valleys.  Many  of  these  reach  the  sea,  where 
their  fronts  are  broken  off  and  carried  away  as  icebergs.  Some  of 
these  glaciers  are  among  the  largest  known.  One  of  the  most  remark- 
able is  the  Humboldt  Glacier,  which  has  a  breadth  of  more  than  50 
miles  where  it  enters  the  ocean.  Its  front  rises  precipitously  from 
the  level  of  the  water  to  a  height  of  300  feet,  and  the  total  thickness 
above  and  below  the  water  level  at  this  point  is  probably  2700  feet. 
Some  of  these  glaciers  fail  to  reach  the  sea  but  spread  out  on  flat 
plains.  In  such  glaciers  it  is  seen  that  the  ice  is  stratified  and  that 
the  white  upper  layers  are  in  marked  contrast  to  those  near  the  base, 
which  are  often  so  filled  with  debris  that  it  is  difficult  to  tell  where  the 
ice  ends  and  the  ground  moraine  begins.  This  loading  of  the  basal 
portions  of  the  ice  and  the  almost  total  freedom  of  the  surface  from 
debris  should  be  borne  in  mind  when  the  work  of  the  ancient  ice 
sheets  is  considered. 

The  rate  of  movement  of  the  Greenland  glaciers  near  their  ends  is 
sometimes  more  than  50  feet  a  day,  but  in  the  interior  of  the  ice  sheet 
the  rate  may  be  as  slow  as  an  inch  a  day  or  practically  zero;  in  other 
words  the  motion  is  from  the  center  of  the  ice  sheet  outward,  the 
movement  being  caused  by  the  weight  of  the  ice.  Moreover,  the 
1  Geikie  A., —  Textbook  of  Geology. 


THE  WORK  OF  GLACIERS  171 

movement  is  locally  concentrated  and  therefore  increased  where  the 
ice  finds  a  relatively  narrow  outlet  between  inclosing  ridges. 

The  contour  of  this  buried  continent  can  be  conjectured  from  the 
fact  that  the  greatly  indented  coast  with  its  numerous  islands  re- 
sembles that  of  Norway,  so  that  it  is  probable  that  a  rough,  moun- 
tainous surface  like  that  of  Norway  underlies  the  ice. 

The  Antarctic  Continent.  —  The  Antarctic  Continent  is  larger  than 
the  whole  of  Europe  and  differs  from  Greenland  in  its  greater  height, 
in  the  greater  severity  of  its  climate,  and  in  the  absence  of  a  strip 
of  ice-free  land  bordering  the  ice.  The  interior,  as  in  Greenland,  is 
dome  or  shield-shaped  and  was  found  by  Amundsen  to  be  10,500  feet 
above  the  sea  at  the  pole.  Above  the  general  level  of  the  shield, 
mountains  rise  to  heights  of  15,000  feet.  The  excess  ice  is  drained 
by  valley  glaciers,  as  well  as  by  the  Great  Ice  Barrier,  a  floating  ice 
shelf  which  in  Victoria  Land  forms  an  ice  cliff  many  miles  long  and 
varying  in  height  from  280  feet  to  places  so  low  that  it  can  be  used 
as  a  wharf. 

ANCIENT  GLACIATION 

The  proofs  that  glaciers  at  one  time  covered  a  large  area  in  North 
America  are  so  conclusive  and  the  belief  is  now  so  universal  that  it 
seems  remarkable  that  the  theory  was  not  advanced  until  1846  (by 
Agassiz),  and  that  nearly  thirty  years  elapsed  before  its  general  accept- 
ance by  geologists. 

DEPOSITION 

(i)  Bowlders.  —  One  of  the  most  striking  proofs  that  a  region  has 
been  covered  with  glaciers  is  to  be  found  in  the  occurrence  of  bowlders 
in  the  soil  and  on  the  surface,  which  differ  in  composition  from  the  un- 
derlying rock  and  consequently  were  not  derived  from  it.  When 
traced  to  their  parent  ledges,  some  of  these  bowlders  are  found 
to  have  been  carried  several  hundreds  of  miles  over  hill  and  dale. 
In  regions  of  rough  topography,  glacial  bowlders  are  often  found  at  a 
much  higher  level  than  the  ledges  from  which  they  came.  For 
example,  bowlders  of  quartzite  have  been  found  on  Mt.  Greylock, 
Massachusetts,  which  must  have  been  carried  into  a  valley  and  then 
to  the  top  of  the  mountain,  a  vertical  distance  of  almost  3000  feet. 
Bowlders  of  jasper  conglomerate,  composed  of  bright  red  pebbles  of 
jasper  embedded  in  white  quartz,  have  been  found  from  the  northern 


172 


PHYSICAL  GEOLOGY 


to  the  southern  confines  of  Ohio,  and  when  traced  northward  are  seen 
to  have  been  derived  from  a  deposit  on  the  north  shore  of  Georgian 
Bay  in  Canada.  Native  copper  from  the  copper  deposits  of  Lake 
Superior  has  been  found  in  the  drift  many  miles  to  the  south.  Pieces 
of  copper  transported  in  this  way  were  often  picked  from  the  drift 

and  made  into  ornaments  by  the  ancient 
mound  builders.  In  New  England  trains 
of  bowlders  have  often  been  traced  to 
outcrops  which  have  some  distinctive 
character  (Fig.  154).  In  any  one  deposit 
the  number  of  kinds  of  rock  is  usually 
not  very  great,  although  in  some'cases 
one  may  find  as  many  as  twenty  different 
varieties  in  a  single  bed  of  till.  The 
bowlders  are  often  angular  and  scratched, 
and  in  both  of  these  features  differ  from 
stones  which  have  been  rolled  about  by 
streams.  It  should  not  be  inferred  from 
the  above  that  no  rounded  and  un- 
scratched  bowlders  occur  in  glacial  debris, 
for  sometimes  the  angular  and  scratched 
bowlders  are  in  the  minority. 

(2)  Unstratified  Drift.  —  As  has  been 
said  in  connection  with  mountain  glaciers, 
all  of  the  debris  transported  by  the  ice  is 
included  under  the  general  term  drift;  till 
or  bowlder  clay  being  the  unstratified 
tjie  drift,  and  that  carried  and  later  deposited 
by  streams  being  called  stratified  drift 
(p.  178).  Till  (Fig.  155)  is  composed  of 
a  heterogeneous  mixture  of  bowlders  of 
many  sizes  embedded  in  clay.  The  mix- 
ture of  coarse  and  fine  material  and  the  lack  of  stratification  proves 
without  question  that  such  deposits  were  not  made  by  streams. 
The  stratified  drift  (Fig.  156)  was  deposited  by  melting  waters 
under  various  conditions.  Drift  is  not  confined  to  the  valleys  of 
glaciated  regions  but  is  spread  over  hills  as  well,  being  in  a  measure 
independent  of  topography. 

The  deposition  of  drift  may  render  a  region  either  rougher  (Fig  157) 
or  smoother  (Fig.  1 58).     If,  for  example,  in  passing  over  a  region  some- 


FIG.  154.  —  Map  of 
"bowlder  train"  from  Iron  Hill, 
Rhode  Island.  The  direction 
of  the  movement  of  the  ice  is 
generalized.  (After  Hobbs.) 


THE  WORK  OF  GLACIERS 


173 


what  cut  up  into  valleys,  the  ice  sheets  filled  them  with  drift,  compel- 
ling the  streams  upon  the  retreat  of  the  ice  to  make  new  valleys,  the 


FIG.  155.  — Till.     Note  the  heterogeneous  mixture  of  large  and  small  bowlders  and 
fine  clay.     (Pennsylvania  Geol.  Surv.) 

result  may  be  a  smoother  surface.  Many  portions  of  the  northern 
United  States  have  been  modified  in  this  way.  On  the  other  hand,  the 
irregular  dumping  and  piling  up  of  drift  in  moraines  may  roughen  the 
landscape.  The  northern  half  of  the  peninsula  of  Michigan,  which  in 
certain  places  now 
rises  from  1000  to 
noo  feet  above  the 
surface  of  the  Great 
Lakes,  seems  to  have 
no  rock  standing 
more  than  250  to 
300  feet  above  the 
lakes,  there  being 
from  700  to  800  feet 
of  drift  on  its  higher 
parts.  The  average 
thickness  of  the  drift 
in  southern  Illinois  is 
somewhat  less  than 
30  feet;  in  north- 
western Ohio,  north- 
ern Indiana,  and  FIG.  156.  —  Stratified  lake  clay  resting  on  till. 


PHYSICAL  GEOLOGY 


northeastern  Illinois  the  average  is  almost  200  feet,  and  in  the  south- 
ern peninsula  of  Michigan  about  300  feet.  Borings  near  Cleveland, 
Ohio,  show  that  the  drift  extends  470  feet  below  lake  level. 


FIG.  157.  —  Diagram  showing  a  region  made  rougher  by  glaciation. 


The  drift  deposited  under  the  ice  was  often  compacted  by  the  great 
weight  of  the  ice  mass  into  a  dense  bowlder  clay  which  is  excavated 
with  difficulty. 

Mention  should  be  made  of  an  area  in  Wisconsin  and  adjacent  por- 
tions of  Iowa,  Illinois,  and  Minnesota,  which  was  not  covered  by  the  ice 


FIG.  158.  —  A  region  made  smoother  by  glacial  debris.     The  uncertainty  in  coal 
mining  (black  bands)  in  such  a  region  is  shown. 

sheets,  the  driftless  area  (Fig.  159  A).  In  this  area  the  rock  is  deeply 
weathered,  and  rock  pillars  are  not  uncommon ;  the  drainage  is  per- 
fect, the  streams  being  without  swamps,  lakes,  or  waterfalls.  It 
differs  markedly  in  these  respects  from  the  adjoining  regions  (159  B). 
This  freedom  from  ice  was  due  to  a  combination  of  causes  :  its  position 
with  reference  to  the  centers  from  which  the  ice  moved,  the  higher 
ground  to  the  north,  and  the  presence  of  the  deep  Michigan  and 
Superior  basins  which  diverted  the  flow  of  the  ice. 

Moraines.  —  The  drift  was  laid  down  either  as  terminal,  ground,  or 
lateral  moraines,  or  as  stratified  deposits. 

(A)  Terminal  moraines  (see  p.  159)  were  formed  where  the  ice 
front  remained  stationary  or  nearly  so,  for  a  long  time,  so  that  its 
forward  movement  was  almost  or  quite  equal  to  the  melting  at  its 
margin,  sometimes  being  slightly  in  excess  and  sometimes  slightly 
less.  Under  such  conditions  it  will  readily  be  seen  that  the  glacial 
debris  would  be  left  in  extremely  irregular  deposits,  unless  the  drift 
had  been  uniformly  distributed  throughout  the  ice,  which  apparently 
was  seldom  the  case.  The  term  "  recessional  moraine  "  is  often  used 
to  indicate  those  moraines  which  were  formed  during  the  various  halts 
of  the  ice  as  it  retreated  to  the  north,  "  outer  terminal  moraine  " 


THE  WORK  OF  GLACIERS 


I7S 


being  reserved  for  the  moraine  formed  at  the  time  of  the  greatest  ex- 
tension of  the  ice.  In  this  discussion  the  term  "  terminal  "  will  be  used 
to  include  both.  "  The  surface  of  these  moraines  (Fig.  145,  p.  160) 
is  a  jumble  of  elevations  and  depressions,  which  vary  from  low,  gentle 
swells  and  shallow  sags  to  sharp  hills  a  hundred  feet  or  so  in  height, 
and  deep,  steep-sided  hollows.  Such  tumultuous  hills  and  hum- 
mocks, set  with  depressions  of  all  shapes,  which  are  usually  without 
outlet  and  are  often  occupied  by  marshes,  ponds,  and  lakes,  surely 
cannot  be  the  work  of  running  water.  The  hills  are  heaps  of  drift, 
lodged  beneath  the  ice  edge  or  piled  along  its  front.  The  basins  were 
left  among  the  tangle  of  morainic  knolls  and  ridges  as  the  margin 
of  the  ice  moved  back  and  forth.  Some  bowl-shaped  basins  were 
made  by  the  melting  of  a  mass  of  ice  left  behind  by  the  retreating 
glacier  and  buried  in  its  debris."  (Norton.) 

Moraines  of  the  Last  Great  Ice  Sheet  in  North  America.  —  These 
moraines  usually  occur  in  belts  three  to  ten  or  fifteen  miles  in  width, 
their  position  being  marked  in  Minnesota,  Wisconsin,  and  other  states 
by  thousands  of  lakes.  In  regions  of  little  topographic  relief  moraines 
may  be  the  most  conspicuous  features  of  the  landscape.  Some  of 
the  moraines  have  been  traced  several  hundreds  of  miles  and  if  the 
correlations  are  correct  have  been  identified  over  a  distance  of  a 
thousand  miles  or  more  (Fig.  160). 


FIG.  159.  —  A  shows  the  drainage  of  a  portion  of  the  "driftless  area"  indicating  that  it 
had  a  normal  development.    It  is  in  decided  contrast  to  the  adjoining  glaciated  area  B. 


CLELAND    GEOL.  —  12 


176 


PHYSICAL  GEOLOGY 


At  its  greatest  extent  (Fig.  565,  p.  646)  the  ice  may  have  stretched 
on  the  east  as  a  great  wall  from  Massachusetts  to  northern  Labrador, 
discharging  icebergs  throughout  its  length.  The  terminal  moraine 
Forms  the  backbone  of  Long  Island  and  stretches  across  northern 


Moraines  of  the  Wisconsin 
Ice  Sheet  and  the  limit  of 
maximum  glaclation 


FIG.  1 60. —  Map  showing  moraines  and  direction  of  ice  movement  (indicated  by 
arrows)  of  the  last  continental  ice  sheet.  The  lobate  character  of  the  moraines  is  pro- 
nounced. The  "driftless  area"  was  not  covered  by  ice.  (After  U.  S.  Geol.  Surv.) 

New  Jersey.  From  there  its  direction  is  northwest  to  the  state  of 
New  York  and  from  there  to  the  southwest  as  far  as  northern  Ken- 
tucky and  almost  to  the  southern  tip  of  Illinois.  From  Kansas  it 
extends  a  little  west  of  north  into  North  Dakota,  from  which  point  it 
has  a  general  east  and  west  direction  except  in  the  mountainous  regions. 


THE  WORK  OF  GLACIERS 


177 


(B)  Ground  Moraine.  —  The  till  which  covers  regions  between 
moraines  and  which  constitutes  by  far  the  largest  area  of  the  gla- 
ciated surface  is  called  the 
ground  moraine.  Its  topog- 
raphy varies  widely,  being 
usually  rolling  and  inter- 
spersed with  swamps,  but 
sometimes  nearly  flat  over 
large  areas.  The  ground  mo- 
raine is  of  variable  thickness, 
being  thinner  in  Canada 
(Fig.  161)  than  in  the  United 
States,  since  the  ice  in  its 
movement  to  the  south  car- 
ried away  much  of  the  ma- 
terial derived  from  the  under- 
lying rock,  leaving  little  to 
be  deposited  on  the  melting 
of  the  ice. 

Drumlins,  where  they 
occur,  are  conspicuous  fea- 
tures of  the  ground  moraine. 
These  are  smooth,  elliptical  hills  composed  of  till  and  have  their 
longest  axes  parallel  to  the  movement  of  the  ice  (Fig.  162).  They 
are  not  by  any  means  found  everywhere  in  the  ground  moraine,  but 


FIG.  161.  —  Map  showing  the  effect  of  glacia- 
tion  in  different  parts  of  North  America.  The 
area  of  maximum  deposition  lies  chiefly  south 
of  the  Great  Lakes,  where  the  ice  was  relatively 
thin  and  its  erosive  power  was  generally  feeble. 
(Modified  after  Dryer.) 


FIG.  162.  —  Drumlins.     Wayne  County,  New  York.     (Photo.  H.  L.  Fairchild.) 

are  abundant  in  portions  of  central  and  western  New  York,  Wis- 
consin, eastern  Massachusetts,  and  elsewhere.  The  islands  in  Boston 
Harbor  are  drumlins.  There  is  still  much  doubt  as  to  the  precise 


I78 


PHYSICAL  GEOLOGY 


mode  of  their  formation,  but  there  is  abundant  evidence  that  they 
were  developed  beneath  the  margin  of  the  ice  and  were  built  up  by 
the  addition  of  successive  layers  of  till. 

Stratified  Drift. — An  estimate1  has  been  made,  based  upon  a 
study  of  the  Malaspina  Glacier  of  Alaska  (p.  168),  that  at  certain 
stages  of  the  withdrawal  of  the  great  ice  sheets  the  Mississippi  River 
had  a  volume  sixty  times  greater  than  at  present.  When  one  con- 
siders the  number  of  streams  flowing  on,  under,  and  in  front  of  the 
ice,  whose  combined  volumes  made  the  greater  rivers  of  the  time, 
we  can  understand  the  abundance  of  stratified  deposits,  such  as 
kames,  eskers,  deltas  (laid  down  in  temporary  lakes),  outwash  plains, 
and  like  deposits,  so  common  in  the  glaciated  portions  of  North 
America. 

Outwash  Plains.  —  The  streams  which  flowed  from  the  fronts  of 
the  continental  ice  sheets  were  usually  heavily  charged  with  silt, 


FIG.  163.  — Outwash  plain.     New  York.     (Photo.  H.  L.  Fairchild.) 

sand,  and  gravel,  which  they  obtained  from  the  ground  and  terminal 
moraines  (p.  175),  so  that  they  were  able  to  aggrade  their  beds,  often 
leaving  thick  deposits.  If  they  flowed  through  well-defined  valleys, 
the  loads  were  deposited  in  the  valley  bottoms  and  fond  valley 
trains.  If  a  number  of  streams  issued  from  the  ice 
plain,  or  in  a  shallow  valley,  they  gradually  raised  its 
deposited  their  surplus  loads.  In  this  way  the  streams  quickly  built 
their  beds  above  the  level  of  the  surrounding  regions  and  were  in 
consequence  forced  to  shift  their  positions  to  lower  places,  forming 
braided  streams  (p.  86).  When  valley  trains  grew  to  such  an  extent 
that  they  overlapped,  an  outwash  plain  resulted  (Fig.  163),  much  as 

1  O.  D.  von  Engeln. 


THE  WORK  OF  GLACIERS  179 


FIG.  164.  —  Kettle  holes.     (U.  S.  Geol.  Surv.) 

alluvial  slopes  are  formed  from  the  growth  of  adjacent  alluvial  fans 
(p.  124).  Outwash  plains  differ  from  valley  trains  in  being  shorter 
and  wider. 

Outwash  plains  are  closely  associated  with  terminal  moraines.  The 
longer  the  front  of  a  glacier  remained  stationary,  the  more  favorable 
were  the  conditions  for  the  accumulation  of  gravel,  since  the  streams 
then  had  an  abundant  supply  of  debris  which  they  continued  to  de- 
posit. The  material  of  outwash  plains  and  valley  trains  is  sand  and 
gravel,  the  coarser  material  being  nearest  the  moraine  and  the  finer 
further  away.  Outwash  plains  may  be  very  extensive,  and  since 
they  are  composed  of  sand  and  gravel  are  usually  infertile,  sometimes 
to  such  a  degree  as  to  form  miniature  deserts.  The  sandy,  desert- 
like  plain  south  of  the  terminal  moraine  on  Long  Island  is  an  outwash 
plain. 

Outwash  plains  and  morainic  plains  may  be  "  pitted  " ;  that  is, 
the  general  level  may  be  broken  by  more  or  less  rounded  depressions. 
These  pits  are  called  kettle  holes  (Fig.  164)  and  were  usually  devel- 
oped from  the  melting  of  blocks  of  ice  which  had  been  buried  in  the 
drift  as  the  ice  retreated.  In  the  outwash  plain  of  the  Hidden 
Glacier  in  Alaska  kettle  holes  are  seen  to  be  forming;  and  "their 
development  is  due  to  the  melting  out  of  ice  from  beneath  the  plain, 
although  in  no  case  was  the  ice  actually  seen."  (Tarr.) 

Terraces.  —  The  valleys  of  the  rivers  of  Ohio,  Indiana,  and  Illi- 
nois which  carried  off  the  water  of  the  melting  ice  are  now  bordered 
by  terraces  of  stratified  drift,  and  the  conspicuous  terraces  of  the 
Connecticut  and  Merrimac  rivers  (p.  127)  and  their  tributaries  are 
remnants  of  deposits  of  stratified  drift  laid  down  either  by  over- 


i8o 


PHYSICAL  GEOLOGY 


loaded  glacial  streams  or  by  streams  which  were  overloaded  as  a 
result  of  the  erosion  of  till  soon  after  it  was  uncovered  by  the  ice. 

Deltas.  —  When  glacial  streams  entered  bodies  of  water  they 
rapidly  built  out  deltas  (Fig.  122).  Ancient  deltas  of  this  origin 
often  afford  the  best  evidence  of  the  former  existence  of  a  glacial 
lake.  In  western  Massachusetts,  for  example,  a  glacial  lake  of  large 
extent  was  formed  by  the  damming  of  the  Hoosic  River  by  the  ice 
sheet.  Although  it  did  not  exist  a  sufficient  length  of  time  to  permit 
the  waves  to  cut  back  the  shores  to  form  cliffs,  yet  the  heavily  loaded 
streams  which  flowed  into  it  built  conspicuous  deltas. 

Eskers.  —  Eskers  (Fig.  165)  are  narrow,  usually  winding  ridges  of 
gravel  and  sand,  ten  or  more  feet  wide  at  the  summit  and  from  a  few 
feet  to  fifty  or  more  feet  high,  and  resembling  abandoned  railroad 
grades.  They  usually  follow  valleys  but  sometimes  extend  across 
the  country  with  little  regard  to  the  topography,  even  when  the  hills 
stand  200  or  more  feet  above  the  valleys.  Single  esker  ridges  in 
Ireland,  Scandinavia,  Finland,  Maine,  and  elsewhere  have  been 
traced  many  miles.  Usually,  however,  they  are  less  than  a  mile  in 
length. 

Eskers  are  believed  to  have  been  formed  beneath  glaciers  by  sub- 
glacial  rivers  which  flowed  in  tunnels  beneath  the  ice,  and  are  most 


FIG.  165.  —  The  long,  narrow,  winding  ridge  is  an  esker.     It  is  composed  of  strati- 
fied sand  and  gravel.     (Photo.  F.  B.  Taylor.) 


THE  WORK  OF  GLACIERS 


181 


abundant  where  subglacial  streams  emptied  into  bodies  of  water. 
Under  such  conditions  the  outlet  of  the  stream  from  the  glacier  would 
be  more  readily  closed  by  the  delta  which  formed  rapidly  where  the 
sediment  was  dropped  as  the  stream  emerged  from  the  ice  into  the 
lake,  and  the  tunnel  under  the  ice  would  thus  be  gradually  filled  with 
sand  and  gravel.  Most  eskers  were  probably  formed,  for  the  most 
•part,  in  connection  with  the  melting  of  stagnant  ice,  since  it  is  evi- 
dent that  had  the  ice  been  even  in  slight  movement  the  winding 
ridges  would  have  been  destroyed. 

Kames.  —  In  glaciated  regions  groups  of  sand  and  gravel  hills 
and  ridges,  as  well  as  isolated,  conical  hills  with  high  and  steep  sides, 
are  not  uncommon. 
These  hills  of  strati- 
fied drift  are  called 
kames  (Fig.  166). 
They  are  often  con- 
fused with  eskers  and 
indeed  the  two  are, 
in  individual  cases,  so 
closely  associated  and 
shade  into  each  other 
so  perfectly  that  it 
is  difficult  to  state 
whether  the  deposit  is 
a  kame  or  an  esker. 
Kames  are  composed 
of  stratified  sand  and 

gravel,  while drumlins    ^ame.s  are  c°mpose«  or  sana/.na  «raveyna  « 
°  quently  are  always  characterized  by  rounded  slopes, 

(p.  177)  are  composed 

of  till.  They  are  often  excavated  for  building  and  road  material, 
and  are  favored  sites  for  cemeteries.  The  same  origin  cannot  be 
assigned  to  all  the  deposits  that  are  classed  as  kames.  '"'Some  were 
formed  at  the  margin  of  the  ice,  where  the  streams  issuing  from 
beneath  under  pressure  heaped  up  their  loads  against  the  ice  front. 
Upon  the  melting  of  the  ice  these  deposits  assumed  a  more  or  less 
irregular  surface,  depending  upon  the  character  of  the  ice  front. 
Kames  of  this  origin  are  especially  common  near  terminal  moraines, 
and  some  of  the  conspicuous  knolls  and  hills  of  moraines  are  often, 
individually,  kames.  Isolated  kames  may  have  been  formed  from 
the  deposits  of  small  lakes  resting  in  depressions  on  the  surface  of 


FIG.   166.  —  Kame.     North    Adams,    Massachusetts. 
Kames  are    composed    of   sand    and   gravel,   and   conse- 


1 82  PHYSICAL  GEOLOGY 

the  glacier.  As  the  ice  melted  these  would  be  inverted  to  form 
mounds.  Sand  and  gravel  carried  into  moulins  (p.  149)  whose  sub- 
glacial  passage  had  been  closed  would  produce  such  hills.  When 
stagnant  ice  occupies  deep  valleys,  drainage  along  the  sides  may  give 
rise  to  large  deposits  of  sand  and  gravel,  which  may  be  left  in  some- 
what the  form  of  a  terrace  with  a  kame  topography  when  the  ice  has 
disappeared.  Outwash  plains  and  valley  trains  sometimes  begin  in 
kame  areas. 

Relation  between  Stratified  and  Unstratified  Drift.  —  It  should 
not  be  understood  that  stratified  and  unstratified  drift  always  have 
topographic  forms  which  distinguish  them,  or  that  they  can  always 
be  clearly  separated.  The  mingling  of  the  unstratified  and  stratified 


FIG.  167.  —  Diagram  showing  the  relation  between  stratified  and  unstratified  drift. 
In  this  figure  the  rough  moraine  leads  to  a  sandy  outwash  plain  on  the  left.  On  the 
right  is  stratified  drift  which  was  laid  down  in  a  temporary  pond  between  the  terminal 
moraine  and  the  retreating  ice  front. 

deposits  (Fig.  167)  is  readily  comprehended  when  it  is  remembered 
that  the  edge  of  the  ice  sheet  probably  seldom  remained  in  the  same 
position  long,  but  oscillated  back  and  forth  during  its  advance  and 
retreat.  In  this  way  till  has  been  covered  with  sand  and  gravel 
which  in  turn  has  been  overridden  by  the  ice  and  covered  with  till. 
Moreover,  when  temporary  lakes  existed  between  the  ice  front  and 
its  moraine,  stratified  deposits  were  laid  down  in  the  midst  of  the 
unstratified. 

EROSION  BY  CONTINENTAL  GLACIERS 

The  amount  of  erosion  formerly  ascribed  to  continental  glaciers 
was  probably  excessive.  There  is  no  doubt  that  the  ice  sheets 
modified  the  topography  over  which  they  passed, — in  some  cases 
profoundly,  —  but  in  general  the  more  pronounced  features  of  the 
landscape  were  little  changed  by  erosion,  although  large  areas  were 
altered  to  a  greater  or  less  degree  by  the  irregular  deposition  of  drift. 

Effect  on  the  Underlying  Rock.  —  Previous  to  the  appearance  of 
the  continental  ice  sheets  the  surface  of  the  rock  of  North  America 
was  deeply  weathered  (p.  651),  much  as  it  is  now  in  the  southern 
states.  Consequently,  when  the  ice  covered  and  moved  over  this 
"  rotten  "  rock  (p.  27)  and  soil,  it  found  an  abundance  of  material 


THE  WORK  OF  GLACIERS 


183 


which  it  carried  along  for  a  time  and  later  dropped,  either  as  hetero- 
geneous, unstratified  till,  or  as  stratified  sands,  clays,  and  gravels. 
The  rock  under- 
lying the  drift  is 
often  smoothed  and 
striated  (p.  157) 
(Fig.  1 68),  differ- 
ing from  that  of 
the  non-glaciated 
regions  in  this  par- 
ticular, as  well  as 
in  the  fact  that  the 
surface  rock  is  usu- 
ally fresh  and  does 
not  pass  gradually 
into  soil,  as  the  rot- 
ten rock  has  been  FIG-  168.  —  A  rock  surface  polished  and  striated  by 
removed  by  the  glacial  action.  (Photo.  L.  .E.  Westgate.) 

glaciers.  The  scratches  and  grooves  (Fig.  169)  on  the  surfaces  of 
glaciated  rocks  usually  have  a  common  direction  (with  some  variation) 
and  show,  as  do  the  glacial  bowlders  or  erratics  (p.  156),  the  direction 

of  the  movement  of 
the  ice.  Harder  por- 
tions of  the  rock  being 
less  easily  smoothed 
by  the  ice,  project 
slightly  above  the  gen- 
eral surface  and  also 
show  by  the  greater 
abrasion  on  one  side 
(stoss)  the  direction 
from  which  the  ice 
The  effect  of 
different 


on 


came, 
erosion 

kinds  of  rock  is  not 
always  in  proportion 
to  their  softness,  al- 
though the  softer  the 
rock  the  more  easily  it  will  be  worn  away.  More  material  may 
actually  be  removed  from  a  hard  but  much-jointed  granite  by  the 


FIG.  169.  —  Rock  ground  and  polished  by  glaciers. 
The  excavation  on  the  right  is  artificial. 


i84 


PHYSICAL  GEOLOGY 


"  plucking  "  of  the  blocks  of  the  rock  than  is  removed  from  a  soft 
limestone  in  which  joints  are  poorly  developed  (p.  157).  Under 

certain  conditions  glaciers  may  have 
little  effect  on  the  underlying  forma- 
tions, as  is  shown  by  till  and  even 
sand  and  clay  deposits  (Fig.  170)  laid 
down  by  an  earlier  ice  sheet,  which 
were  but  slightly  affected  when  over- 
ridden by  a  later  ice  sheet.  In 
Switzerland  glaciers  have  overridden 
Alpine  landslides  without  carrying 
away  many  blocks.  It  is  possible, 
however,  that  such  unconsolidated 
deposits  as  those  just  described  were 
frozen  when  the  ice  moved  over  them. 
Modification  in  the  Shape  of  the 
Hills.  —  The  shape  of  the  hills  in 
glaciated  regions  sometimes  shows 
the  direction  from  which  the  ice 
came,  the  side  upon  which  the  ice 
impinged,  called  the  stoss  side,  having 
TIG.  170.  —  Till  overlying  lake  a  more  gentle  slope  than  the  other, 
clays,  showing  that  a  lake  first  ex-  tne  lee  side  (Fig.  171).  Hummocks 
isted  and  that  the  ice  sheet  advanced  /*  i  111  i  11 

over  the  clays  without  being  able  °f  rock  eroded  b^  g^Ciers  and  known 
to  remove  them.  Williamstown,  as  roches  moutonnees  (p.  158)  are 
Massachusetts.  well-developed  in  many  places,  but 

may  be  especially  well  studied  in  portions  of  Canada. 

Effect  of   Glaciation  on  Drainage.  —  In  general   it  can   be  said 
that  glaciation  tended  to  disturb  the  preexisting  drainage,  with  the 


FIG.  171.  —  Diagram  showing  the  effect  of  glacial  erosion.  The  stoss  side  suffers 
more  than  the  lee  side,  and  the  slopes  are  more  gentle.  The  direction  of  ice  movement 
is  shown  by  the  arrow. 

result  that  land  which  in  preglacial  times  was  as  thoroughly  drained, 
for  example,  as  portions  of  West  Virginia  and  Kentucky  to-day, 
became  swampy,  with  abundant  lakes  and  ponds. 


THE  WORK  OF  GLACIERS 


185 


/.  Lakes  and  Ponds.  —  A  glance  at  any  good  map  of  the  United 
States  shows  that  south  of  the  limit  of  glaciation  lakes  are  almost 
absent  except  (i)  those  formed  by  rivers  in  their  meanderings  (p. 
120);  (2)  those  in  limestone  regions  (e.g.,  Florida)  (p.  69);  and 


FIG.   172.  —  Map  showing  the  great  abundance  of  lakes  in  a  portion  of  a 
glaciated  region.     Ashby  Quadrangle,  Minnesota. 

(3)  those  formed  by  wave  and  current  action  along  the  coast  (p.  220). 
This  condition  is  in  decided  contrast  to  that  in  the  glaciated  portions 
(Fig.  172).  The  depressions  in  which  lakes  and  ponds  occur  were 


FIG.  173.  —  The  Fulton  chain  of  lakes  in  the  Adirondacks,  New  York.  First, 
Second,  Third,  Fourth,  Fifth,  Sixth,  and  Seventh  Lakes  are  evidently  the  result  of 
the  partial  filling  of  a  preglacial  river  channel  with  glacial  drift. 


1 86 


PHYSICAL  GEOLOGY 


FIG.   174.  —  Lake  Marjelen,  formed  by  the  damming  of 
a  valley  by  the  Aletsch  Glacier. 


formed  in  several 
ways.  The  rock  may 
have  been  scooped 
out  by  glaciers,  form- 
ing rock  basins  (p. 
145).  Many  such 
exist  in  mountainous 
regions  affected  by 
glaciation.  River 
valleys  may  have 
been  dammed  by 
drift  so  as  to  form 
large  lakes,  such  as 
Lake  Geneva  and 
Lake  Constance  in 
Switzerland.  The 
many  lakes  which  add  so  much  to  the  attractiveness  of  the  Adiron- 
dacks  are  the  result  of  the  repeated  damming  of  old  river  courses  (Fig. 
173).  The  uneven  surface  of  the  drift  is  often  dotted  with  lakes  and 
ponds  which  rest  in  the  inequalities  formed  by  the  irregularly  de- 
posited material.  Basins  may  be  pro- 
duced by  a  combination  of  the  above 
methods.  The  finger  lakes  of  central 
and  western  New  York  (Cayuga  Lake 
and  Seneca  Lake),  Lake  Chelan,  Wash- 
ington, and  Lake  Como,  Italy,  are  the 
result  of  the  deepening  of  old  river 
valleys  which  lay  in  the  direction  of 
the  movement  of  the  ice  and  of  the 
damming  of  their  outlets.  The  bodies 
of  water  thus  formed  are  long  and 
remarkably  deep.  Many  temporary 
lakes  were  formed  between  the  ice  front 
and  moraines  and  also  when  glaciers 
moved  up  a  slope,  thus  preventing 
the  waters  from  taking  their  natural 
course.  Glacial  Lake  Agassiz  is  such 
an  example  (p.  656).  Valley' glaciers 
sometimes  dam  their  tributary  valleys, 
thus  forming  lakes  in  them.  Lake 


FIG.  175.  —  Map  showing  the 
probable  preglacial  course  of  the 
Genesee  River  (shaded)  and  the  pres- 
ent drainage. 


THE  WORK  OF  GLACIERS 


187 


Marjelen,   Switzerland    (Fig.    174),   owes  its  existence  to  the  dam 
formed  by  the  Aletsch  Glacier. 

2.  Rivers.  —  To  form  a  conception  of  the  effect  of  glaciation  on  a 
well-drained  region,  imagine  a  mature  country,  such  as  portions  of 
West  Virginia  (Fig.  95,  p.  no),  invaded  by  glaciers  advancing  from 
the  north.  It  is  evident  that  the  north-south  valleys  would  be  the 
ones  most  likely  to  be  deepened,  since  they  are  in  the  direction  of 
the  movement  of  the  ice,  and  that  the  east-west  valleys  would  prob- 
ably be  entirely  or  partially  filled  with  drift,  leaving  basins  which 
would  be  occupied 
by  lakes  upon  the 
disappearance  of  the 
ice.  In  many  cases, 
the  streams  would 
keep  their  old,  wide, 
preglacial  courses  for 
a  portion  of  their 
length,  but  in  other 
parts  would  occupy 
deep,  narrow,  rock 
gorges  which  they 
had  eroded  after  they 
were  forced  out  of 
their  old  channels  and 
had  cut  down  through 
the  drift  (Fig.  175). 

Waterfalls  and 
rapids  often  occur  at 
the  points  where 
streams  have  been 

diverted    from    their 

u     ,  !     ,        j   -r       preglacial  valley  and  to  cut  a  new  one.     (Modified  after 

old  channels  by  drift    £elfneman  } 

(p.  163).      Many    of 

the  manufacturing  centers  of  New  England  and  other  northern  states 
owe  their  establishment  to  the  existence  of  such  natural  water  power. 
In  portions  of  New  York,  Ohio  (Fig.  176),  Michigan,  Indiana, 
Minnesota,  and  other  northern  states,  and  in  Canada,  the  preglacial 
drainage  has  been  greatly  modified  by  glacial  action.  In  certain 
areas  the  streams  have  new  courses;  the  old  valleys  are  so  filled  with 
drift  that  no  evidence  of  them  is  to  be  obtained  except  by  borings. 


FIG.  176.  —  Map  showing  the  present  course  of  the 
Ohio  River  and  the  course  which  it  had  previous  to 
glacial  times  (shown  by  arrows).  Because  of  the  deposi- 
tion of  drift  the  Ohio  was  forced  to  abandon  its  wide, 


1 88  PHYSICAL  GEOLOGY 


ICEBERGS 

Formation  of  Icebergs.  —  On  account  of  the  muddiness  of  the  water  in  front  of 
a  glacier  which  enters  the  sea  (Fig.  177)  and  also  because  of  the  danger  from  the  frag- 
ments of  ice  which  continually  break  off  from  the  glacier  without  warning,  it  has 
been  impossible  to  determine  definitely  how  great  icebergs  are  formed.  Ice  breaking 
from  that  portion  of  the  front  of  a  glacier  which  is  above  the  water  produces  small 
bergs,  but  large  ones  do  not  usually  have  this  origin.  The  two  figures  (Figs.  178, 


FIG.   177.  —  Nunatak  Glacier,  Alaska,  entering  the  water  of  a  fiord  and 
discharging  icebergs.     (U.  S.  Geol.  Surv.) 

179)  show  two  theories  of  the  origin  of  icebergs.  The  first  theory  (Fig.  178)  holds 
that  near  sea  level  a  glacier  is  cut  back  by  wave  action  and  melting,  leaving  a  pro- 
jecting ice  foot  some  distance  beneath  the  surface  of  the  water,  which  gradually 
thins  toward  the  end.  Great  icebergs  which  suddenly  appear  from  the  water  some 
distance  from  a  glacier  are  believed  by  the  adherents  of  this  theory  to  have  come 
from  the  ice  foot,  from  which  they  had  been  broken  by  the  buoyancy  of  the  water. 
The  second  theory  (Fig.  179)  holds  that  the  upper  part  of  a  glacier  which  enters 
the  sea  will  project  beyond  the  lower  part,  both  because  of  the  more  rapid  movement 
of  the  top  than  the  bottom  and  because  of  the  melting  of  the  ice  by  the  water.  In 
proof  of  this  it  is  stated  that  large  masses,  extending  to  the  very  top  of  the  ice 
front,  shear  off  and  sink  vertically  into  the  water,  disappear  for  a  few  seconds,  and 
then  reappear,  almost  to  their  original  height,  before  they  turn  over.  If  the  glacier 
projected  under  the  water  to  within  300  feet  of  the  surface,  it  would  cause  the 
mass  to  turn  over  at  once.  According  to  this  theory  most  of  the  small  bergs  consist 
of  masses  broken  from  the  ice  precipices ;  larger  ones  are  formed  when  a  piece  shears 
off  and  sinks  into  the  water;  and  ice  detached  under  the  water  may  also  form  bergs. 
A  third  theory  (Fig.  180)  holds  that  the  front  of  the  glacier  is  broken  off  by  the 
buoyancy  of  the  water. 


THE  WORK  OF  GLACIERS 


189 


FIG.  178. 


Size  and  Work  of  Icebergs.  —  Bergs  from  Greenland  seldom  stand 
200  feet  out  of  water,  and  a  height  of  100  feet  is  more  usual;  but 
icebergs  have  been  reported  from  the  Antarctic  which  are  of  great 
size,  being  several  miles  long  and  500  feet  or  more  high.  Icebergs 
vary  greatly  in  shape  (Figs.  181,  182), 
those  of  the  Antarctic  regions  being 
frequently  of  a  tabular  form,  while 
those  from  Greenland  are  usually 
picturesquely  irregular.  If  icebergs 
were  regular  in  shape  and  without 
debris  their  thickness  could  be  easily 
determined,  since  in  the  case  of  solid 
ice  the  part  which  appears  above  the 
water,  is  only  one  ninth  of  the  mass. 

The  .principal  geological  effects  of 
icebergs  are  two:  they  abrade  the 
bottoms  of  the  shallow  seas  where 
they  strand,  and  they  transport  their 
load  of  debris  until  it  is  dropped  as  ***• 
the  ice  melts.  Most  of  the  load  is 
lost  before  it  has  been  carried  100 
miles,  but  some  of  the  debris  of  Green- 
land icebergs  is  deposited  on  the  New- 
foundland Banks.  It  is  stated  that 
in  the  Baltic  Sea  bowlders  which  have 


FIG.  180. 


FIGS.  178-180.  —  Diagrams  illus- 
trating the  theories  of  the  forma- 
tion of  icebergs.  Fig.  178.  —  Ice- 
bergs formed  by  the  breaking  off 
been  dropped  from  icebergs  are  often  and  floating  of  the  foot  of  a  glacier 
found  upon  vessels  which  have  been  as  the  upper  portion  is  eroded  and 
sunk  a  few  vean;  melted  back  by  the  waves.  Fig.  179. 

nk  a  tew  years.  -  Icebergs  formed  by  gravity,  since 

it  is  held  that  the  upper  part  of  a 

GLACIAL   MOVEMENT  glacier  will  project  beyond  the  lower 

r™  .  ,.„.  r        .    .          part,  both  because  of  the  more  rapid 

There  is  great  difference  of  opinion  movement  of  the  top  and  because  of 

concerning   the    mechanics   of  glacial  the  melting  of  the  ice.    Fig.  180.  — 

movement,  and  the  problem  may  be  Icebergs  broken  from  the  glacier  as 

•  i        j  i  i       j  it  enters  the  sea,  by  the  buoyancy 

considered  as  one  yet  to  be  solved.  of  the  water 

(/)    Viscosity   Theory. — One  of  the 

early  theories  held  that  the  motion  of  glaciers  is  due  to  the  semiplastic 
or  viscous  nature  of  ice  (Forbes),  which  permits  it  to  move  upon  a 
slope  very  much  as  do  such  substances  as  thick  tar  or  sealing  wax, 
the  force  which  urges  it  forward  being  its  own  weight.  Experiments 
have  been  performed  which  appear  to  show  that,  in  small  masses,  ice 


190 


PHYSICAL  GEOLOGY 


will  not  yield  to  pressure  without  breaking,  even  when  the  pressure  is 
very  slowly  applied.  If  this  is  true  under  all  conditions  ice  is  not  a 
viscous  substance  as  has  been  supposed,  and  the  theory  fails. 

(2)  Expansion  and  Contraction.  —  According  to  this  theory  a  glacier 

downhill 


moves 


as 


a  son'd  body,  simply 
through  alternations 
of  temperature. 
When  a  mass  suffers 
a  rise  in  temperature 
it  expands,  the  mo- 
tion taking  place  in 
the  direction  of  least 
resistance,  that  is, 
down  the  bed.  When 
the  temperature 
falls,  contraction  will 

ensue;  but  since  gravity  opposes  a  backward  movement  a  gradual 
creeping  down  the  bed  occurs.  The  creep  of  sheet  lead  on  a  roof 
illustrates  this  action.  Since  ice  is  a  poor  conductor  of  heat,  it  is 


FIG.  181.  —  An  iceberg.     The  vessel  gives  an  idea 
of  the  size. 


1 


FIG.   182.  —  Iceberg,  Labrador.     The  dark  bands  of  debris  were  probably 
horizontal  in  the  glacier.     (Photo.  F.  B.  Sayre.) 

evident  that  such  rapid  movement  as  is  often  observed  could  not 
result  from  this  cause. 

(j)  Re  gelation.  —  A  theory  (Tyndall)  based  upon  the  fact  that 
broken  ice  heals  under  pressure,  even  at  melting  temperatures,  holds 
that  the  movement  of  glaciers  is  accomplished  by  the  repeated  frac- 


THE  WORK  OF  GLACIERS 


191 


turing  and  later  freezing  together  (regelation)  of  the  surfaces  of  the 
fractures  when  they  again  come  into  contact.  Under  the  influence 
of  pressure  a  glacier  is  continually  yielding  to  fractures  of  all  sizes, 
but  after  changing  the  position  of  its  parts  as  a  result  of  the  down- 
ward movement  of  the  broken  fragments,  it  is  again  united  by  regela- 
tion. The  effect  of  this  constant  rupture  and  regelation  is  thought 
to  cause  the  glacier  to  behave  like  a  plastic  or  viscous  body.  That  it 
is  not  plastic  is  indicated  by  the  failure  of  the  Mer  de  Glace,  moving 
at  a  rate  of  only  two  feet  a  day,  to  withstand  a  change  of  slope  in  its 
bed  of  even  two  degrees  without  fracturing. 

(4)  Melting  and    Pressure.  —  The   lowering    by    pressure   of  the 
melting  point  of  ice  forms  the  basis  of  another  theory.    (Thompson.) 
At  the  points  of  greatest  pressure  in  a  glacier  melting  occurs,  and  the 
stress  is  relieved.     The  water  thus  formed  moves  to  a  point  where 
there  is  less  pressure  and  immediately  freezes.     The  forward  motion 
of  the  whole  is,  therefore,  if  the  theory  is  correct,  effected  by  a  con- 
tinual process  of  alternate  melting  and  freezing. 

(5)  Growth  of  Granules.  —  Since  the  crystals  or  granules  of  glacial 
ice  vary  from  one  seventh  of  an  inch  or  less  to  an  inch  or  even  four 
inches  in  diameter,  it  has  been  held  that  the  growth  of  the  granules 
of  the  ice  is  a  leading  factor  in  its  movement. 

(6)  Other  theories  of  an  importance  perhaps  equal  to  those  pre- 
sented have  been  suggested,  but  none  seems  to  explain  all  of  the  ele- 
ments of  the  problem.     Some  of  the  difficulties  have  doubtless  arisen 
from  the  desire  to  ascribe  all  of  the  phenomena  of  glacial  motion  to 
a  single  cause.     The  movement  of  glaciers  will  undoubtedly  be  found 
to  be  far  from  simple  and  to  depend  upon  a  number  of  factors,  no 
one  of  which  is  alone  competent  to  produce  the  characteristic  move- 
ment of  large  bodies  of  ice. 

REFERENCES  FOR  GLACIERS 

'.:v-    £.>.  ..     .    «'.  :'J  :.•  '  -.'';.   V^ —  *•"'    •  •,'""    ^:r-fr-" 

EXISTING  MOUNTAIN  GLACIERS 

Folios  of  the  U.  S.  Geol.  Surv.,  in  Montana,  California,  Washington,  Oregon,   and 

Wyoming. 

GILBERT,  G.  K.,  —  Glaciers  and  Glaciation:  Harriman  Alaska  Expedition,  Vol.  3. 
MARTIN,  L.,  —  The  National  Geographic  Society  Researches  in  Alaska  :  Nat.  Geog. 

Mag.,  Vol.  22,  1911,  pp.  537-561. 
RUSSELL,  I.  C.,  —  Glaciers  of  North  America. 

RUSSELL,  I.  C.,  —  Malaspina  Glacier:  Jour.  Geol.,  Vol.  I,  1893,  pp.  219-245. 
SALISBURY  AND  ATWOOD, —  Topographic  Maps:  Professional  Paper,  U.  S.  Geol.  Surv. 

No.  60. 

CLELAND   GEOL.  — 13 


192  PHYSICAL  GEOLOGY 

TARR,  R.  S.,  —  The  Yakutat  Bay  Region,  Alaska :   Professional  Paper,  U.   S.  Geol. 

Surv.  No.  64,  1909. 
TYNDALL,  J.,  —  The  Glaciers  of  the  Alps,  1860. 

PRESENT  CONTINENTAL  GLACIERS 

HOBBS,  W.  H., —  Characteristics  of  Existing  Glaciers,  1911. 
NANS  EN,  F.,  —  The  First  Crossing  of  Greenland,  1890. 
PEARY,  R.  E.,  —  Northward  over  the  Great  Ice,  1898. 
SHACKLETON,  E.  H.,  —  The  Heart  of  the  Antarctic,  1910. 

GLACIAL  EROSION 

CHAMBERLIN,  T.  C.,  —  The  Rock  Scourings  of  the  Great  Ice  Invasions :   Seventh  Ann. 

Rept.,  U.  S.  Geol.  Surv.,  1885,  pp.  174-248. 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  i,  1906,  pp.  281-307. 
DAVIS,  W.  M.,  —  Glacial  Erosion  in  France,  Switzerland,  and  Norway :   Proceedings 

Boston  Soc.  Nat.  Hist.,  Vol.  29,  1900,  pp.  273-322. 

DAVIS,  W.  M.,  —  The  Sculpture  of  Mountains  by  Glaciers :  Geographical  Essays,  1909. 
DAVIS,  W.  M.,  —  Die  Erkldrende  Beschreibung  der  Landformen,  1912. 
GEIKIE,  J.,  —  Earth  Sculpture,  pp.  212-249. 

RESULTS  OF  GLACIATION 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3,  1906,  pp.  327-446. 

GEIKIE,  J.,  —  The  Great  Ice  Age. 

SALISBURY,  R.  D.,  —  Glacial  Geology  in  New  Jersey :  N.  J.  Geol.  Surv.,  Vol.  5,  1902. 

TARR,  R.  S.,  —  The  Physical  Geography  of  New   York  State,  1902,  pp.  103-154. 

WRIGHT,  G.  F.,  —  The  Ice  Age  in  North  America. 

WRIGHT,  W.  B.,  —  The  Quaternary  Ice  Age,  1914. 

DRUMLINS,  ESKERS,  AND  KAMES 

ALDEN,  W.  C.,  —  Drumlins  of  Southeastern  Wisconsin :   Bull.  U.  S.  Geol.  Surv.  No. 

273.  1905- 

FAIRCHILD,  H.  L.,  —  Drumlins  of  Central  Western  New  York :  Bull.  N.  Y.  State  Mu- 
seum No.  in,  1907. 

GREGORY,  J.  W., —  The  Relation  of  Eskers  and  Kames :  Geog.  Jour.,  Vol.  40,  1912, 
p.  169. 

GLACIAL  MOVEMENT 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  i,  pp  308-323. 

OGILVIE,  ALAN  G.,  —  Some  Recent  Observations  and  Theories  on  the  Structure  and 
Movement  of  Glaciers  of  the  Alpine  Type :  Geog.  Jour.,  Vol.  40,  pp.  280-294;  (es- 
pecially for  bibliography). 

PRESTON,  T., —  The  Theory  of  Heat,  pp.  279-300;  (especially  for  the  theories  of 
regelation,  and  expansion  and  contraction). 

REID,  H.  F.,  —  Mechanics  of  Glaciers:  Jour.  Geol.,  Vol.  4,  1896,  pp.  912-928. 


THE  WORK  OF  GLACIERS 


193 


TOPOGRAPHIC  MAP  SHEETS,  U.  S.  GEOLOGICAL  SURVEY,  ILLUSTRATING  GLACIERS 
AND  GLACIAL  EROSION 

Mountain  Glaciers  Cirques  and  Glacial  Valleys 

Shasta,  California.  Chief  Mountain,  Montana. 

Chief  Mountain,  Montana.  Philipsburg,  Montana. 

Glacier  Peak,  Washington.  Cloud  Peak,  Wyoming. 

Cloud  Peak,  Wyoming.  Hayden  Peak,  Utah. 
Mt.  Stuart,  Washington. 
Kintla  Lakes,  Montana. 

TOPOGRAPHIC  MAP  SHEETS  ILLUSTRATING  GLACIAL  DEPOSITION 

Drumlins                                 Moraines  Outwash  Plains 

Sun  Prairie,  Wisconsin.  St.    Paul,    Minnesota.  Brooklyn,  New  York. 

Boston,  Massachusetts.  Harlem  and  Brooklyn,  New  York.     New    York    City    and 

Weedsport,  New  York.  New  York  City  and  Vicinity.  Vicinity. 

Waterloo,  Wisconsin.  Minnetonka,  Minnesota.  Elmira,  New  York. 

Syracuse,  New  York.  Lake  Geneva,  Wisconsin.  Whitewater,  Wisconsin. 
Northville,  South  Dakota. 


CHAPTER  VI 
THE   OCEAN  AND  ITS   WORK 

THE  oceans  and  seas  cover  about  72  per  cent,  of  the  surface  of  the 
earth.  The  average  depth  of  the  oceans  is  about  two  and  one  half 
miles,  that  of  the  Pacific  being  somewhat  greater  than  that  of  the 
Atlantic ;  the  average  height  of  the  continents,  however,  is  only  a 
little  more  than  2000  feet.  If  all  the  dry  land  above  sea  level  were 
washed  into  the  sea,  it  would  fill  only  one  fortieth  of  that  depression. 
Soundings  to  a  depth  of  32,088  feet  have  been  made  in  the  Pacific 
Ocean  near  Mindanao,  P.  L,  and  to  a  depth  of  nearly  28,000  feet 
near  Japan  (Tuscarora  Deep).  Within  70  miles  of  Porto  Rico  the 
ocean  bottom  descends  to  27,366  feet,  and  within  10  miles  of  the 
Bermuda  Islands  depths  of  17,460  feet  are  encountered.  These 
great  depths  are  not  of  wide  extent,  but  are  almost  as  limited  as  are 
the  great  heights  of  the  continents.  Moreover,  the  greatest  depth 
of  the  oceans  is  practically  the  same  as  the  greatest  mountain  height, 
each  being  about  six  miles.  There  are,  however,  few  such  excessive 
differences  in  elevation  in  short  distances  on  the  land  as  there  are 
differences  in  depth  in  the  ocean,  although  Mt.  Everest  (29,002  feet) 
is  within  60  miles  of  the  nearly  sea  level  Ganges  plain,  and  the  vol- 
cano Fuji  in  Japan  rises  12,400  feet  directly  from  sea  level. 

GENERAL  CHARACTER  OF  THE  OCEAN 

Topography  of  the  Ocean  Floor.  —  In  order  to  gain  a  true  concep- 
tion of  the  topography  of  the  ocean  bottom,  it  must  be  borne  in  mind 

(1)  that  stream  erosion,  which  is  continually  at  work  on  the  land  and 
which  tends  to  roughen  its  surface,  is  absent  on  the  ocean  bottom ; 

(2)  that  minor  depressions  which  may  exist  temporarily  tend  to  be 
rapidly  filled  by  the  sediments  brought  to  the  ocean  from  the  land 
and  by  the  material  carried  in  solution,  some  of  which  is  precipitated 
directly  and  some  absorbed  by  animals  to  form  their  shells  and  skele- 
tons, only  to  be  left  upon  the  ocean  floor  after  their  death. 

'-Bordering  practically  all  of  the  shore  lines  of  the  oceans  is  a  belt  of 
water  which  has  a  depth  of  less  than  600  feet  and  is  from  several  miles 

194 


THE  OCEAN  'AND  ITS  WORK 


195 


to  200  miles  wide.  This  gently  sloping  sea  floor  is  known  as  the 
continental  shelf  or  submarine  delta  (Fig.  183).  The  continental  shelf 
is  economically  of  great  importance,  since  the  waters  lying  above  it 
are  the  great  fishing  grounds  of  the  world.  From  its  outer  edge  the 
sea  floor  slopes  abruptly,  so  that  within  a  few  miles  there  are  depths 
as  great  as  6000  feet,  while  beyond  the  slope  is  gentle  but  gradually 
increases,  until  within  a  distance  of  from  50  to  100  miles  it  attains  a 
depth  of  12,000  feet.  At  this  depth  or  lower,  the  greater  part  of  the 
ocean  bottom  is  a  great  monotonous  plain,  so  nearly  flat  that  if  the 


APPALACHIAN 


FIG.  183.  —  Profile  showing  the  continental  surface  from  the  Appalachian 
Mountains  to  the  deep  sea. 

water  were  removed,  the  greater  part  of  it  would  appear  to  the  eye 
to  be  almost  perfectly  smooth. 

Irregularities  of  the  Ocean  Floor.  —  The  irregularities  which  exist 
on  the  ocean  bottom  are  (i)  depressions  on  the  continental  shelf 
which  are  extensions  of  river  valleys  (p.  227) ;  (2)  the  steep  slope  at 
the  outer  edge  of  the  continental  shelf;  (3)  volcanic  cones,  built  up 
from  the  depths  of  the  sea ;  (4)  precipices,  due  to  faulting  (some 
in  the  Mediterranean  being  3000  to  5000  feet  high) ;  (5)  well- 
defined,  wavelike  ridges,  corresponding  to  mountains  on  the  land ; 
and  (6)  broad  plateau  areas  which  rise  several  thousand  feet 
above  the  deeper  portions.  Such  a  plateau  extends  beneath  the 
Atlantic  Ocean  from  Iceland  to  a  point  in  the  South  Atlantic 
almost  opposite  the  southern  extremity  of  Africa.  It  reaches  the 
surface  in  Iceland,  the  Azores,  St.  Paul,  Ascension,  and  Tristan  de 
Cunha  islands,  but,  for  the  most  part,  lies  6000  feet  or  more  below 
the  surface. 

Composition  of  Ocean  Water.  —  The  water  of  the  oceans  contains 
about  three  and  one  half  per  cent,  of  mineral  matter  in  solution,  more 
than  three  fourths  of  which  is  common  salt  (NaCl).  Of  the  total 
mineral  matter  in  solution,  the  salts  of  sodium,  magnesium,  and  cal- 
cium constitute  97  per  cent.  Almost  every  known  element  has  been 
found  by  analysis  to  be  dissolved  in  sea  water,  and  they  are  all  more  or 


196  PHYSICAL  GEOLOGY 

less  radioactive.     The  composition  of  the  salts  which  occur  in  ocean 
water  is  as  follows 1 : 

Common  salt,  NaCl 77-76  Potassium  sulphate,  K2SO4   .  .  .    2.46 

Magnesium  chloride,  MgCl    .     .     .  10.88  Magnesium  bromide,  MgBr2  .  .      .22 

Magnesium  sulphate,  MgSO4     .     .  4.74  Calcium  carbonate,  CaCO3   .  .  .      .34 

Calcium  sulphate,  CaSO4       ...  3.60  100.00 

In  a  discussion  of  the  composition  of  sea  water  not  only  the  dis- 
solved mineral  matter  should  be  considered,  but  the  dissolved  gases 
as  well,  since  oxygen  is  essential  for  the  existence  of  marine  organisms 
and  for  oxidizing  dead  matter  of  organic  origin.  In  addition  to  oxy- 
gen, nitrogen  and  carbon  dioxide  are  present.  In  fact,  the  ocean 
probably  contains  from  eighteen  to  twenty-seven  times  as  much 
carbon  dioxide  as  the  atmosphere  and  is  the  great  reservoir  of  this 
gas.  It  is  not,  however,  equally  distributed,  but  is  more  abundant 
in  polar  seas  than  in  equatorial,  since  cold  water  has  a  greater  capac- 
ity for  it  than  warm. 

Temperature  of  the  Ocean.  —  The  temperature  of  the  surface  of 
the  ocean  varies  with  the  latitude,  from  a  mean  annual  temperature 
of  80°  F.  at  the  equator  to  one  of  40°  F.  at  the  poles.  Since  the  rays 
of  the  sun  do  not  penetrate  the  water  to  great  depths,  it  is  probable 
that  the  seasonal  changes  are  not  felt  below  50  feet.  The  tempera- 
ture at  the  bottom  of  the  ocean  is  surprisingly  cold,  being  about  29° 
F.  at  the  poles  and  35°  F.  at  the  equator.  This  layer  of  cold  water  is 
very  thick;  for  if  we  consider  water  above  40°  F.  as  warm,  the  layer 
of  warm  water  is  nowhere  more  than  4800  feet  thick,  and  is  usually 
considerably  less.  The  low  temperature  of  the  deep  water  is  due  to 
the  movement  of  the  waters  from  the  polar  regions,  which  slowly 
creep  toward  the  equator  along  the  ocean  bottom,  so  that  we  find  in 
the  tropics,  at  great  depths,  the  low  temperatures  which  are  en- 
countered only  on  the  surface  in  the  Arctic  and  Antarctic  regions. 
Exceptions  to  the  rule  that  the  temperature  decreases  from  the  sur- 
face downward  are  found  in  such  seas  as  the  Mediterranean,  the 
Gulf  of  Mexico,  and  the  Red  Sea.  In  these  seas  the  temperature 
of  the  bottom  is  approximately  the  same  as  that  at  the  bottom  of  the 
strait  separating  them  from  the  ocean,  and  the  surface  temperature 
is  almost  constant,  being  practically  the  average  temperature  of  the 
surface  in  winter.  In  the  Mediterranean,  for  example,  the  tem- 
perature at  a  depth  of  6000  feet  is  55°  F.,  while  in  the  Atlantic  at 

1  Data  of  Geochemistry,  Bull.  U.  S.  Geol.  Surv.  No.  491,  p.  23. 


THE  OCEAN  AND   ITS  WORK 


197 


3000 


4000 


FIG.   184.  —  Diagram  showing  the  peculiarity 
of  temperature  of  the  Mediterranean. 


that  depth  it  is  35°  F.     This     ATLANTIC  MEDITERRANEAN 

difference  in  temperature  is 
due  to  the  failure  of  the  cold 
waters  which  slowly  move  on 
the  ocean  bottom  from  the 
poles  toward  the  equator,  to 
reach  the  confined  basin  of 
the  Mediterranean  (Fig.  184). 
Distribution  of  Marine  Life. 
-There  is  little  doubt  that 
the  marine  life  of  the  past  ex- 
isted under  conditions  similar 
to  those  of  the  present,  with 
the  exception,  perhaps,  that 
in  the  early  history  of  the 
world  the  great  depths  were  less  inhabited  than  now. 

In  the  seas  of  to-day  the  greatest  abundance  of  animal  life  is  found 
in  the  shallow  waters  of  the  continental  shelf,  where  food,  supplied 
both  from  the  sediments  brought  in  by  the  streams  and  by  the  plants 
that  grow  there,  is  plentiful.  However,  some  animals  are  able  to 
withstand  the  enormous  pressure  of  the  water  at  great  depths,  al- 
though the  abundance  and  variety  is  small  compared  with  that  which 
flourishes  in  the  shallow  waters.  When  the  oceanic  telegraphic 
cables  are  raised  for  repairs,  marine  animals  are  often  found  attached 
to  them.  Warm  waters  are  more  favorable  to  organisms  than  cold, 
although  even  in  the  waters  bordering  the  Antarctic  Continent  the 
fauna  is  often  varied  and  plentiful.  At  and  near  the  surface  of  the 
ocean  microscopic  and  other  small  organisms  appear  in  great  numbers, 
and  on  the  bottom  numerous  forms  of  life  are  frequently  found,  but 
in  the  thousands  of  feet  of  water  which  lie  between  the  bottom  and  a 
few  hundred  feet  of  the  surface  of  the  deep  seas  there  is  an  almost 
total  absence  of  life.  There  is  no  portion  of  the  land  surface  on  which 
life  is  so  nearly  absent.  This  is  in  contrast  with  the  shallow  waters, 
where  life  is  probably  much  more  abundant  than  on  any  portion  of 
the  dry  land.  Certain  species  are  restricted  to  muddy  bottoms  ;  some 
to  sandy;  some  thrive  best  in  clear,  quiet  waters  out  of  reach  of 
land  sediments ;  while  others  are  most  abundant  where  the  water  is 
in  motion.  Plant  life  is  limited  to  the  depth  to  which  light  penetrates 
and  is,  consequently,  scarce  in  bottoms  lying  at  depths  greater  than 
100  to  200  feet.  Since  the  presence  of  ammonium  carbonate  in  water 


198  PHYSICAL  GEOLOGY 

aids  marine  organisms  in  the  formation  of  their  calcareous  shells  and 
skeletons,  and  since  this  compound  is  most  abundant  in  warm  waters, 
it  is  probable  that  when  the  shells  of  fossils  are  thick,  the  water  in 
which  they  lived  was  warm.  Thus  the  existence  of  thick-shelled, 
Paleozoic  fossils  in  the  rocks  of  the  Arctic  region  indicates  that  when 
they  were  alive,  the  waters  in  that  region  were  much  warmer  than 
now.  It  is  evident  from  the  above  that  in  order  to  understand  the 
life  and  physical  conditions  of  the  remote  past  a  knowledge  of  the 
habits  and  conditions  of  life  of  animals  now  living  is  necessary. 

Age  of  the  Ocean.  —  Attempts  have  been  made  to  estimate  the  age  of  the  ocean 
from  the  quantity  of  salt  dissolved  in  it.  Such  estimates  are  based  on  the  as- 
sumption that  all  of  the  salt  of  the  ocean  has  been  derived  from  the  weathering  and 
erosion  of  rocks  and  has  been  carried  to  the  seas  by  streams.  The  simplest  form  of 
the  problem  assumes  that  the  age  of  the  ocean  may  be  determined  by  dividing  the 
total  amount  of  salt  in  it  by  the  amount  of  this  mineral  carried  to  the  sea  each  year 
by  streams.  The  amount  of  salt  in  the  ocean  can  be  determined  with  considerable 
accuracy,  since  the  composition  of  sea  water  varies  little  in  different  parts  of  the  world, 
and  the  approximate  total  volume  of  the  ocean  is  known.  There  are,  however,  a 
number  of  doubtful  elements  in  the  problem,  (i)  The  amount  of  salt  discharged  by 
rivers  may  have  varied  from  time  to  time.  The  rate  of  discharge  has  undoubtedly 
been  hastened  through  human  agency.  The  importance  of  this  factor  is  seen  in  the 
fact  that  14,500,000  metric  tons  of  common  salt  are  mined  or  extracted  from  salt 
wells  yearly.  If  this  is  annually  returned  to  the  ocean,  it  is  evident  that  the  present 
rate  of  discharge  is  higher  than  in  the  past.  (2)  The  salt  blown  upon  the  land  from 
the  ocean  is  considerable  and  must  be  deducted  from  the  total  carried  in.  (3)  The 
salt  received  by  the  decomposition  of  the  rocks  by  marine  erosion  (p.  202),  and  from 
volcanic  ejectamenta  must  be  subtracted.  (4)  Much  salt  once  in  the  ocean  is  now 
stored  within  the  sedimentary  rocks. 

When  the  known  factors  are  considered,  it  is  "  inferred  that  the  age  of  the  ocean, 
since  the  earth  assumed  its  present  form,  is  somewhat  less  than  100,000,000  years."  1 

The  amount  of  calcium  carbonate  in  the  oceans  cannot  be  used  as  a  basis  for  an 
estimate  of  their  age,  since  some  of  it  is  precipitated  upon  reaching  the  salt  water, 
and  much  of  it  is  used  by  animals  and  plants  for  their  skeletons  and  shells. 

MOVEMENT  OF   THE   WATER 

Wave  Motion.  —  Since  marine  erosion  is  accomplished  chiefly  by 
wave  action,  it  is  important  to  know  something  of  the  theory  of  wave 
motion,  of  the  height  and  force  of  waves,  and  of  the  depth  to  which 
they  are  effective.  Storm  waves  are  set  in  motion  as  a  result  of  the 
friction  between  the  wind  and  water.  The  water  appears  to  move 
forward,  just  as  do  the  waves  in  a  field  of  grain  which  is  agitated  by 
the  wind.  If  a  pebble  is  thrown  into  a  pond  on  a  calm  day,  waves 
Clarke,  F.  W.,  — Bull.  U.  S.  Geol.  Surv.  No.  490,  p.  142. 


THE  OCEAN  AND   ITS  WORK  199 

are  set  in  motion  and  any  floating  object  is  seen  to  rise  and  fall  as  the 
crests  and  troughs  of  the  waves  pass  under  it,  but  it  is  not  borne  along. 
As  each  wave  glides  under  the  object,  it  is  moved  forward  a  short  dis- 
tance, but  as  soon  as  the  crest  has  passed  beneath  it,  it  comes  back 
to  its  former  position.  In  storm  waves,  however,  the  friction  of  the 
wind  drives  some  of  the  surface  water  along  and  thus  produces  sur- 
face currents  (p.  217).  The  height  of  a  wave  is  the  vertical  distance 
between  the  trough  and  the  crest,  and  the  wave  length  is  the  distance 
from  crest  to  crest.  The  wave  length  in  heavy  storms  varies  but 
little  from  600  feet,  although  waves  more  than  twice  that  length  have 
been  observed  in  the  southern  ocean.  Each  particle  of  water  in  a 
wave  moves  in  a  vertical  orbit  (Fig.  185),  i.e.,  if  a  wave  is  ten  feet  in 


f        y        h        a'      b'       c' 

FIG.  185.  —  Diagram  illustrating  the  orbital  movement  of  water  in  waves.  The 
particles  of  water  move  forward  in  the  crests  and  backward  in  the  troughs,  each  particle 
moving  in  a  closed  orbit. 

height  the  diameter  of  the  orbit  is  ten  feet.  In  open  seas  storm  waves 
may  be  20  to  30  feet  high,  and  waves  of  50  feet  have  been  reported ; 
it  is,  perhaps,  doubtful  if  waves  exceeding  50  feet  in  height  are  ever 
developed  in  the  open  ocean.  Waves  10  to  15  feet  high  are  propa- 
gated at  a  rate  of  about  60  miles  an  hour. 

Wave  motion  is  propagated  indefinitely  downward,  but  rapidly  de- 
creases from  the  surface  to  the  bottom  (Fig.  185),  so  that  at  compara- 
tively shallow  depths  even  sand  is  not  disturbed ;  the  force  of  wave 
motion  is  one  fifth  at  65  feet  (20  m.),  one  fiftieth  at  about  190  feet 
(50  m.),  and  perhaps  not  effective  below  230  feet  (70  m.).  The 
depth  to  which  agitation  extends  is  in  the  ratio  which  the  length  bears 
to  the  height.  Thus,  a  wave  30  feet  long  and  10  feet  high  would  move 
the  water  6  inches  vertically  at  a  depth  of  10  feet,  whereas  a  wave  of 
the  same  height  and  three  times  the  length  would  agitate  the  water 
1 8  inches  below  the  bottom  of  the  wave.  (Wheeler.)  In  violent 
storms  it  is  possible  that  there  is  some  motion  at  3000  feet,  but,  in 


2OO 


PHYSICAL  GEOLOGY 


general,  the  mechanical  action  of  the  waves  is  not  perceptible  at 
depths  greater  than  600  feet.  This  last  estimate  is  based  upon  the 
occurrence  of  ripple  marks  to  be  found  upon  the  sand  of  the  ocean 
bottom. 

Storm  waves  sometimes  travel  great  distances,  even  thousands  of 
miles,  preserving  their  length  and  velocity,  but  diminishing  in  height 
until  they  become  gentle  swells. 

The  Breaking  of  Waves.  —  As  a  wave  nears  a  shelving  shore  its 
length  is  decreased  and  its  height  increased.  The  breaking  of  the 

wave  is  the  result  of  friction  with  the 
bottom,  which  retards  the  lower  part, 
while  the  crest,  continuing  with  its 
previous  speed,  finds  itself  without 
support  and  "  breaks."  This  tumbling 
crest  is  called  a  breaker  or  roller. 
Since  waves  of  the  same  height  break 
in  about  the  same  depth  of  water,  a 
line  of  breakers  is  formed.  If  the 
ocean  bottom  descends  gently,  the 
water  of  the  breakers  rushes  upon  the 
shore,  and  gravity  then  draws  it  back 
down  the  beach  and  along  the  bottom 
beneath  the  incoming  wave  as  the  un- 
dertow. On  pebbly  (shingle)  beaches 
FIG.  186.  —  Diagram  showing  the  •  j  •  r  i_  11  i  i_ 

directions  of  the  various  currents  pro-  the  grinding  ot  the  pebbles  as  they 
duced  by  a  wave  moving  in  the  direc-  are  moved  forward  by  the  waves  and 
tion  AB9  a  shore  current  BE,  and  carried  back  by  the  undertow  can 
undertow  BC,  and  a  reflected  wave  .  , 

£/)  be  heard,  even  when  the  waves  are 

small. 

When  waves  strike  a  coast  obliquely,  a  shore  current  (p.  217)  is 
produced  (Fig.  186),  and  on  coasts  where  the  prevailing  direction  of 
the  storms  is  fairly  constant,  the  importance  of  currents  of  this  origin 
in  transporting  sand  and  gravel  is  very  great. 

Force  of  Storm  Waves.  —  The  force  of  waves  varies  with  their 
height,  but  it  is  difficult  to  reduce  the  force  of  impact  with  which  a 
breaking  wave  strikes  a  clifF  to  an  exact  mathematical  calculation. 
Their  strength  is,  however,  influenced  by  the  force  of  the  wind  which 
generates  them,  by  the  depth  of  the  water  over  which  they  have 
moved,  and  by  the  distance  which  they  have  traveled.  Experiments 
at  Cherbourg,  France,  showed  that  the  force  of  storm  waves  on  that 


THE  OCEAN  AND   ITS  WORK  2OI 

coast  varies  from  about  600  to  800  pounds  a  square  foot.  The  force 
of  the  impact  of  waves  10  feet  high  on  certain  harbor  walls  and  piers 
was  determined  to  be  1.36  tons  a  square  foot.  The  average  wave 
pressure  on  the  coast  of  Scotland  for  the  five  summer  months  is  61 1 
pounds  a  square  foot  and  for  the  six  winter  months  is  2086  pounds. 
At  Dunbar  in  the  North  Sea  the  pressure  is  sometimes  three  and  a 
half  tons  a  square  foot. 

Height  of  Storm  Waves.  —  On  an  islet  off  the  coast  of  Oregon  (Tillamook  Rock) 
which  is  exposed  to  the  sweep  of  the  ocean,  the  waves  of  a  storm  in  1912  dashed  against 
the  lighthouse  with  such  force  and  to  such  a  height  as  to  break  the  heavy  glass  of  the 
lantern  132  feet  above  the  sea.  During  another  storm  on  the  same  islet  a  mass  of 
concrete,  weighing  half  a  ton,  was  thrown  to  a  point  88  feet  above  sea  level.  During 
the  construction  of  the  breakwater  at  Plymouth,  England,  blocks  of  stone  weighing 
from  7  to  9  tons  were  removed  from  the  seaward  side  of  the  breakwater  at  low-water 
level,  carried  over  the  top,  a  distance  of  138  feet,  and  piled  upon  the  inside.  During 
a  heavy  gale  three  and  three  quarters  million  tons  of  shingle  are  estimated  to  have 
been  taken  from  Chesil  Bank,  England,  and  carried  seaward  by  the  waves. 

The  height  to  which  the  water  of  storm  waves  is  thrown  is  often  very  great.  At 
Alderney  breakwater,  in  England,  the  spray  from  the  breaking  waves  was  thrown 
upward  to  a  height  of  200  feet.  At  Hastings,  England,  water  was  thrown  as  high  as 
the  top  of  a  large  hotel,  and  pebbles  were  lifted  from  the  beach  and  carried  across  the 
wide  promenade  into  the  bedroom  windows  of  the  houses  fronting  the  sea.  It  is 
stated  that  windows  in  the  Dunnet  lighthouse,  Scotland,  were  broken  at  a  height  of 
300  feet  above  high-water  mark,  by  stones  swept  up  the  cliff  by  sheets  of  sea  water 
during  heavy  gales. 

Tides.  — Tides  must  be  considered  in  a  discussion  of  the  work 
of  the  ocean,  since  they  are  an  important,  though  usually  inconspicu- 
ous agent.  Tides  are  produced  by  a  combination  of  the  attraction 
of  the  sun  and  moon,  and  of  the  rotation  of  the  earth  ;  and  every  part 
of  the  ocean  experiences  two  high  and  two  low  tides  each  day.  Al- 
though the  tide  in  mid-ocean  is  only  about  three  feet  high,  its  height 
becomes  greatly  increased  when  it  approaches  shallow  shores  or  enters 
funnel-shaped  bays  or  estuaries.  In  the  Bay  of  Fundy,  Nova  Scotia, 
for  example,  the  difference  in  height  between  low  and  high  tide  is 
sometimes  greater  than  50  feet.  Because  of  its  effect  on  the  level 
of  the  water,  the  tide  permits  a  wide  vertical  range  for  the  work  of 
waves  on  shores. 

Tidal  Currents.  —  Tidal  races  or  currents,  such  as  that  at  Hell  Gate,  in  the  City  of 
New  York,  are  not  infrequent  in  narrow  straits,  and  are  often  effective  in  erosion. 
The  race  at  Hell  Gate  is  due  to  the  fact  that  the  tide  rises  higher  in  Long  Island  Sound 
than  in  the  bay  of  New  York  harbor,  and  to  the  further  circumstance  that  the  time  of 
the  high  tide  is  different  on  the  two  sides  of  the  strait.  The  inlets  of  barrier  islands 


202  PHYSICAL  GEOLOGY 

(p.  221)  and  the  channels  (thoroughfares)  back  of  them  are  kept  open  largely  by  tidal 
scour,  and  the  deep  waterways  in  some  bays  are  sometimes  maintained  in  the  same 
way.  The  work  accomplished  by  tidal  currents  consists  more  in  the  transportation 
of  material  prepared  by  the  waves  than  in  the  actual  wear  of  the  coast. 

The  importance  of  tides  to  man  is  considerable.  Many  of  the 
important  harbors  of  the  world  could  not  be  entered  without  tides. 
This  is  shown  by  the  fact  that  ships  must  wait  until  the  water  is 
deepened  by  high  tide  before  entering.  The  washing  out  of  harbors 
by  the  tides  twice  a  day  is  of  great  sanitary  importance.  The  produc- 
tion of  power  from  tides  has  not  as  yet  been  financially  successful, 
but  the  possibility  of  the  use  of  tidal  power  in  the  future  in  the  gen- 
eration of  electrical  energy  is  worthy  of  mention. 

Tidal  Bores.  —  When  a  tide  enters  the  mouth  of  a  river  which  is  obstructed  by  the 
form  of  the  entrance  and  by  the  shallows,  its  progress  may  be  so  retarded  that  its  waters 
will,  for  a  while,  be  prevented  from  passing  up  the  valley.  When  its  height  finally 
becomes  great  enough,  it  rushes  up  in  one  or  more  great  waves,  which  are  called  bores. 
In  the  Tsientang  River,  China,  and  in  the  Amazon  River,  Brazil,  waves  20  or  more 
feet  in  height  are  said  to  have  been  developed  at  times  in  this  way.  Smaller  bores 
occur  in  other  rivers.  These  waves  are  characteristic  of  but  few  rivers  and  are  not 
of  daily  occurrence  in  any,  but  in  such  rivers  as  those  cited  they  sometimes  tear  out 
the  banks,  destroy  forests  along  the  shores,  and  wash  away  islands. 

Earthquake  Waves.  —  Because  of  their  great  length,  waves  generated  by  earth- 
quakes (p.  292)  rise  to  great  heights  when  they  reach  shelving  shores.  Such  a  wave 
10  to  30,  or  perhaps  more,  feet  in  height  struck  the  coast  of  Japan  in  1896,  killing 
26,975  people,  destroying  $3,000,000  worth  of  property,  and  changing  the  shore  line 
in  many  places.  Because  of  their  infrequency,  earthquake  waves  are  of  little  impor- 
tance in  marine  erosion  as  compared  with  storm  waves. 

Ocean  Currents.  —  The  great  currents  of  the  ocean,  such  as  the  Gulf  Stream,  per- 
form a  very  slight  work  of  erosion  or  transportation,  but  are  of  vast  importance  in 
modifying  past  and  present  climates  of  those  regions  near  which  they  pass.  This  is 
due  to  the  influence  of  the  winds,  since  they  convey  the  warmth  of  the  poleward  cur- 
rents and  the  cold  of  the  equator-moving  currents  to  the  adjacent  lands. 

MARINE  EROSION 

Factors  in  Marine  Erosion.  —  The  impression  one  receives  on  seeing 
a  wave  strike  a  rocky  shore  is  that  the  blow  and  the  weight  of  the 
water  are  the  only  forces  which  are  important  in  marine  erosion. 
This,  however,  is  an  error,  (i)  When  a  wave  is  dashed  against  a 
cliff,  every  crack  and  cranny  is  more  or  less  filled  with  water,  and  the 
hydrostatic  pressure  exerted  tends  to  force  the  walls  of  the  fissure 
apart.  This  force  sometimes  amounts  to  three  tons  on  the  square 
foot ;  a  force  which,  often  repeated,  must  accomplish  an  important 


THE  OCEAN  AND  ITS  WORK 


203 


work.  (2)  Moreover,  the  air  in  the  fissures,  even  above  the  reach 
of  the  waves,  is  suddenly  compressed  and  forced  into  the  minute 
cracks  as  the  waves  dash  against  the  cliffs.  Upon  the  withdrawal  of 
a  wave  the  pressure  is  suddenly  released,  and  the  air  and  water 
rush  out  with  a  suction  which,  when  frequently  applied,  may  loosen 
and  dislodge  large  blocks  of  rock.  An  often-quoted  example  is  that 
of  the  Eddystone  lighthouse,  England,  in  which  a  securely  fastened 
door  was  driven  outward  as  a  result  of  the  partial  vacuum  produced 
by  the  withdrawal  of  a  wave  during  a  storm  in  1840.  Blocks  of  stone 
in  well-built  sea  walls  are  sometimes  started  from  their  places,  partly 
at  least  in  this  way.  (3)  The  rocks  which  are  broken  or  quarried 
from  sea  cliffs  by  the  impact  of  the  waves  and  in  other  ways  become 
tools  with  which  the  waves  are  able  to  accomplish  their  greatest 
work  of  erosion.  As  these  are  lifted  by  the  waves  and  hurled  against 
the  cliffs,  they  act  as  hammers  which  beat  to  fragments  the  rocks 
against  which  they  strike.  A  high  cliff  is  affected  in  the  same  way  as 
a  lower  one,  but  is  usually  cut  back  more  slowly,  because  as  the  waves 
undercut  it,  the  talus  (p.  29)  falling  from  above  may  accumulate  in 
quantities  greater  than  the  waves  can  quickly  remove.  Under  such 
conditions  the  energy  of  the  waves  may  be  largely  expended  in  grind- 
ing to  pieces  and  removing  the  talus.  Sea  cliffs,  however,  weather 
back  more  rapidly  than  cliffs  inland,  as  they  are  wet  with  spray  and 
are  usually  undermined  by  springs  and  are  comparatively  free  from 
talus.  When  a  cliff  descends  precipitously  into  deep  water  the  waves 
merely  wash  up  and  down  and,  having  no  tools  with  which  to  cut,  wear 
it  back  very  slowly.  (4)  The  spray  thrown  up  by  the  waves  also  has 
an  erosive  effect  upon  certain  rocks,  since  it  washes  away  the  weaker 
ones  and  dissolves  others  which  it  can  affect  chemically.  In  this 
latter  way  silicates  are  broken  down  and  limestones  are  dissolved. 

Shore  Ice.  —  In  high  latitudes  shore  ice  protects  the  shore  during 
the  winter  months,  and  even  when  loosened  by  the  summer  thaw  it 
prevents  the  waves  from  breaking  against  the  coasts  with  their  full 
force.  Shore  ice,  nevertheless,  is  important  in  the  erosion  of  coasts 
in  regions  where  it  forms.  During  the  winter  a  broad  shelf  of  ice 
develops,  whose  thickness  is  usually  much  greater  than  that  which 
would  result  from  the  direct  freezing  of  the  sea,  which  even  in  polar 
regions  seldom  exceeds  8  or  10  feet.  The  thickness  of  30  to  60  feet 
to  which  this  shore  ice  or  ice  foot  forms  is  the  result  partly  of  the  direct 
freezing  of  the  ocean  water,  partly  of  the  accumulation  of  snow  on 
the  ice,  which  is  converted  into  ice  by  the  water  from  the  waves,  and 


204  PHYSICAL  GEOLOGY 

partly  of  the  action  of  storms  in  heaping  up  the  ice.  Shore  ice  may 
hold  a  load  of  pebbles,  both  on  its  upper  surface  and  near  the  bottom  : 
the  former  falling  on  the  ice  from  the  cliffs,  as  a  result  of  the  loosen- 
ing of  the  rocks  by  frost ;  the  latter  being  obtained  from  the  beach 
which  is  frozen  to  the  bottom  of  the  ice.  Therefore  so  far  as  the 
position  of  the  debris  is  concerned,  shore  ice  resembles  glaciers  (p.  156). 
During  storms  this  ice  is  broken  into  great  rafts  or  floes,  and  large 
masses  are  driven  upon  the  shores  by  the  force  of  the  wind  and  waves, 
while  in  calmer  weather  they  are  moved  backward  and  forward  by 
the  tides.  The  stones  embedded  in  the  bottom  of  the  ice  grind  and 
crush  the  rocks  over  which  they  are  pushed,  scratching  and  polish- 
ing rocky  shores  very  much  as  glaciers  polish  and  scratch  the  rocks 
over  which  they  move  (p.  183).  As  in  the  case  of  glaciers,  the  rock 
tools  which  accomplish  this  work  are  themselves  ground  to  powder 
(P-  IS9)-  It  is  probable  that  many  of  the  striations  on  the  rocks  of 
the  coast  of  Labrador,  and  even  on  coasts  as  far  south  as  Newfound- 
land, were  produced  by  floe  ice  and  not  by  glaciers.  The  striae  made 
by  the  former,  however,  seldom  have  a  uniform  direction. 

Ice  in  Lakes.  —  Ice  has  much  the  same  effect  in  protecting  and  erod- 
ing the  shores  of  lakes  as  in  the  seas,  but  the  protection  which  it  af- 
fords is  probably  relatively  greater,  because  the  waves  are  usually 

less  effective.  The 
absence  of  strong 
shore  lines  in  some 
glacial  lakes  (such  as 
those  which  formerly 
existed  in  New  York 

and    Massachusetts) 

FIG.   187.  —  Diagrams  showing  the  effect  or  ice  shove         ,  .  ,  ,  , 

in    producing    "walled    lakes."      (After    Hobbs,    Earth     which  may  have  been 
Features.)  in  existence  for  a  long 

time,  may  have  been 

largely  due  to  a  protecting  fringe  of  ice  which  prevented  the  waves 
from  cutting  back  the  shores. 

If  a  lake  freezes  over  completely  and  is  repeatedly  subjected  to 
considerable  changes  in  temperature,  it  may,  by  the  expansion  of  the 
water  in  refreezing,  produce  a  strong  "  push  "  on  the  shores.  The 
expansion  of  the  ice  which  accomplishes  this  result  is  produced  as 
follows.  Water  freezes  at  32°  F.,  and  in  so  doing  expands  one  ninth 
in  volume,  but  when  the  temperature  of  the  ice  becomes  lower  than 
32°  it  contracts.  This  causes  the  ice  to  pull  away  from  the  shores 


THE  OCEAN  AND   ITS  WORK 


205 


or  to  crack.  The  water  which  rises  in  the  cracks  soon  freezes,  and 
when  the  temperature  is  again  raised,  the  ice  will  expand  so  that  the 
surface  will  be  too  large  for  the  lake  in  an  amount  equal  to  the  width 
of  the  cracks,  and  will  either  override  the  shores  or  push  them  up  by 
horizontal  pressure.  If  the  shores  are  marshy  they  may  be  ridged 
or  arched  up  into  gentle  folds.  Such  a  push  may  make  a  ridge  or  wall 
about  a  lake  if  the  shores  are  of  sand  and  gravel.  "  Walled  lakes  " 
are  not  uncommon  in  Canada  and  in  the  northern  United  States 
(Fig.  187).  The  ridging  may  be  increased  to  some  extent  by  the  ice 
driven  up  by  the  waves  in  the  spring. 

RESULTS  OF  MARINE  EROSION 

The  erosive  work  of  the  ocean  is  constant ;  in  storms  the  waves 
strike  with  great  violence,  at  other  times  more  gently,  but  always 
some  work  is  being  accomplished.  The  conspicuous  work  of  the 
waves  is  on  the  cliffs  which  border  the  sea.  The  rapidity  with  which 
cliffs  are  worn  back  and  the  sharpness  of  their  profile  depend  upon  a 
number  of  factors  :  (i)  the  hardness  or  softness  of  the  rock,  (2)  the 
presence  of  cracks  and  joints,  (3)  the  position  of  the  beds,  (4)  the 
depth  of  the  water,  and  (5)  the  height  of  the  waves,  the  work  of  the 
waves  being  confined  to  a  belt  extending  a  little  above  high  tide  and 
slightly  below  low  tide. 

If  the  water  at  the  base  of  a  cliff  is  deep,  the  incoming  waves  do 
not  break.  Moreover,  since  no  rock  fragments  are  available  for 
battering  the  shore,  such  a  wall  may  endure  many  centuries  with  little 
change.  On  those  portions  of  the  Outer  Hebrides  where  no  gravel 
exists,  barnacles  are  said  to  be  as  abundant  on  the  wave-swept  cliffs 
after  a  storm  as  before.  Since  seaweeds  often  flourish  upon  the 
shores  where  the  waves  are  very  active,  they  are  important  in  protect- 
ing the  rocks  upon  which  they  grow. 

Effect  of  Erosion  on  Different  Materials.  — It  is  evident  that  soft 
chalk  or  glacial  drift  will  be  worn  back  much  more  rapidly  than  hard 
granite.  At  Cape  de  la  Heve,  France,  where  the  chalk  cliffs  are  300  feet 
high,  the  shore  is  being  cut  back  at  a  rate  of  about  one  yard  a  year,  and 
the  lighthouse  stationed  there  has  been  twice  set  back.  The  annual 
loss  of  these  cliffs,  for  a  distance  of  142  miles,  is  estimated  to  be  about 
five  and  one  half  million  cubic  yards.  (Wheeler.)  So  effective  is  the 
marine  erosion  of  some  chalk  cliffs  that  some  of  the  streams  flowing 
over  them  are  unable  to  deepen  their  beds  with  sufficient  rapidity  to 


206 


PHYSICAL  GEOLOGY 


FIG.   188.  —  Chalk  cliffs  on  the  coast  of  France.     The  waves  have  cut  back  the 
cliffs  so  rapidly  that  the  streams  enter  the  sea  from  hanging  valleys. 


keep  pace  with  the  wearing  back  of  the  cliffs  and  consequently  fall 
over  them  from  hanging  valleys  (Fig.  188).  The  wear  on  granite 
cliffs,  on  the  other  hand,  is  often  so  slight  that  the  battering  of  the 
waves  for  a  century  is  scarcely  perceptible.  Along  the  coast  of  Marble- 
head,  Massachusetts,  granite,  well  within  reach  of  the  waves,  still 

bears  glacial  striae, 
showing  that  thou- 
sands of  years  of 
wave  wear  have  not 
been  effective  on  this 
hard  rock.  Since 
low-lying,  sandy 
shores  are  apt  to  lie  in 
places  where  sand  is 
accumulating,  they 
usually  suffer  less 
than  rocky  and  pre- 
cipitous shores.  On 
such  a  coast,  how- 
ever, a  slight  change 
in  the  currents  such 
as  that  due  to  un- 
usual or  prolonged 

storms  may  cause  the  shores  to  be  cut  away  rapidly,  as  has  been  true 
of  Coney  Island,  New  York,  and  along  the  New  Jersey  coast,  where 
the  former  sites  of  houses  and  hotels  are  now  covered  by  the  sea. 


FIG.  189.  —  Undercutting  of  massive  granite  by 
wave  action. 


THE  OCEAN  AND  ITS  WORK 


207 


Influence  of  Joints  and  Other  Planes  on  Erosion.  —  The  profile 
of  a  cliff  is  largely  determined  by  the  nature  and  trend  of  the  divi- 
sional planes  of  the  rock  of  which  it  is  composed  (Fig.  189),  especially 
of  the  stratification  planes  and  joints. 

If  stratified  rock  is  not  strongly  jointed  and  dips  toward  the  sea  (Fig.  190  A],  the  cliff 
formed  will  incline  in  the  same  direction.  In  such  a  case  the  wave  moves  up  the  slope 
with  little  resistance,  since  an  overhanging  cliff  is  absent.  When  the  strata  dip  gently 


FIG.   190.  —  Ay  cliff  formed  in  seaward-dipping  strata  without  strong  joints. 
B,  cliff  formed  in  strongly  jointed  seaward-dipping  strata. 


SI 


SI 


A  B 

FIG.   191.  —  Ay  cliff  formed  in  landward-dipping  strata  without  strong  joints. 
B,  cliff  formed  in  strongly  jointed,  landward-dipping  strata. 


SI 


X" 


JL 

II        1  I 

PL 

11     1  1 

SI. 

1  1       II 

__-r-r—  /I     '    ' 

II       I  ' 

/yf 

1     1     1    '     1    1  1  1 

/f  1   '  ' 

I    |    i    1     I'M 

II     1  ' 

A  B 

FIG.   192.  —  A,  profile  of  a  cliff  formed  in  horizontal  strata  without  strong  joints. 
By  profile  of  a  cliff  formed  in  strata  with  strong  joints. 

FIGS.   190-192.  —  Diagrams  showing  the  profiles  of  cliffs  formed  by  wave  erosion. 

towards  the  sea  and  a  porous  stratum  rests  on  an  impervious  one,  landslides  may  occur, 
when  the  porous  stratum  is  undermined.  If  the  strata  are  inclined  towards  the  land 
(Fig.  191  A)  overhanging  cliffs  will  be  formed,  since  as  one  layer  is  worn  back  another 
equally  overhanging,  is  exposed.  It  is  on  such  cliffs  that  the  waves  are  most  effective. 
If  the  strata  are  horizontal  the  base  of  the  cliff  is  excavated,  but  as  the  upper  part  is  in 
the  form  of  a  stair  (Fig.  192  A),  the  waves  have  little  effect.  It  should  be  borne  in  mind 
in  this  connection  that  if  the  joints  of  the  rock  are  better  developed  than  the  stratifi- 
cation planes,  the  profile  of  the  cliff  will  depend  largely  upon  their  direction,  so  that 

CLELAND    GEOL.  —  14 


208 


PHYSICAL  GEOLOGY 


an  overhanging  cliff  (Figs.  190  B,  193)  will  be  the  result  of  joints  inclining  inland;  a 
sloping  cliff,  of  joints  that  incline  toward  the  sea  (Fig.  191  E)  ;  and  a  vertical  cliff,  of 
vertical  joints  (Fig.  192  B). 

In  overhanging  cliffs  the  dismemberment  sometimes  begins  at  the 
top  of  the  cliff,  where  the  agents  are  not  the  waves,  but  the  rain,  frost, 

etc.  In  such  cases  the  work 
of  the  sea  consists  largely  in 
keeping  the  base  of  the  cliff 
free  from  talus.  The  height 
of  a  sea  cliff  depends,  to  some 
extent,  upon  the  rapidity 
of  marine  erosion,  since  if 
weathering  is  more  rapid 
than  the  work  of  the  ocean, 
talus  will  accumulate  at  its 
base  and  protect  the  shore. 
In  this  connection  the  im- 
portance of  springs  and  seep- 
age from  underground  water 
FIG.  193.  —  The  effect  of  marine  erosion  on  111  i  i  i  j 

strongly  jointed  beds.  Nantucket,  Massachu-  should  not  be  overlooked, 
setts.  (Photo.  S.  Powers.)  for  they  often  assist  in  under- 

mining cliffs.   Loose  material, 

such  as  sand  or  glacial  deposits,  will  not  form  cliffs  unless  the  erosion 
is  very  rapid. 

Coves  and  Headlands. — The  irregularities  which  result  from 
marine  erosion  may  in  general  be  classed  as  headlands  and  crescent- 
shaped  beaches  called  coves,  and  are  brought  about  (i)  by  the  un- 
equal resistance  of  the  rock,  the  softer  being  cut  away  more  rapidly 
than  the  harder.  Such 
a  condition  results 
when  vertical  or 
steeply  dipping  strata, 
composed  of  hard  and 
soft  beds,  lie  at  right, 

Or    at     considerable          ^IG>    I94>  —  Block  diagram  showing  coves  formed  in 
.  ,  weak  strata,  a  harder  stratum  and  a  lava  dike  projecting 

angles    to    the    coast    as  headlands. 

(Fig.    194),    and    a 

similar  shore  line  is  produced  when  a  rock  is  much  more  jointed  or 

fractured   in  one   portion   than  in   another.     (2)    Where   the   force 

of  the  waves  is  greater  on  certain  parts  of  shores  than  on  others, 


THE  OCEAN  AND  ITS  WORK 


209 


coves  and  headlands  may  also  result.  Coves  are  not  cut  back 
indefinitely,  since  after  a  time  the  headlands  protect  them  from 
the  full  force  of  the  waves  and  equilibrium  is  established.  When 
this  condition  is  attained,  the  headlands  and  coves  are  worn  back 
at  an  equal  rate.  It  is  evident  from  the  above  that  wave  action 
is  able  to  develop  small  irregularities  of  coast  line,  but  not  great 
ones. 

Sea  Caves  and  Blowholes.  —  Caves  (Fig.  195)  are  often  developed 
on  rocky  shores  where  the  rock  is  strong  enough  to  form  a  roof.  If 
the  rock  is  weak,  chasms 
develop.  Such  chasms  or 
gullies  sometimes  extend 
across  narrow  headlands, 
converting  the  outermost 
parts  into  islands.  Caves 
occur  at  the  bases  of  cliffs 
and  are  formed  in  one  of 
several  ways,  or  by  a  com- 
bination of  them  :  (i)  by  the 
beating  of  the  waves,  espe- 
cially if  the  water  near  the 
shore  is  neither  too  deep  nor 
too  shallow  and  if  there  is  a 
supply  of  debris  which  can 
be  used  in  the  work  of  ex- 
cavation ;  (2)  by  quarrying 
along  joints.  (3)  Since  the 
level  of  underground  water 
near  the  coast  is  sea  level, 
solution  caves  are  not  un- 
common at  bases  of  cliffs  in 


FIG.  195.  —  Sea  cave,  Watermouth,  England. 
The  sea  worked  along  some  fault  or  plane  of  weak- 


limestone  strata.    Such  caves  ness  m  tne  slate.     The  enlargement  of  the  cave 

are    often    enlarged    by    the  was  assisted  by  the  cleavage  planes^    (E  A.  N. 

'  y  Arber,  The  Coast  Scenery  of  North  Devon.) 

waves.     (4)  If  a  weak  bed  of 

horizontal  rock  is  at  sea  level  and  is  subjected  to  the  attack  of  the 
waves,  it  affords  especially  favorable  conditions  for  excavation  by 
waves.  In  the  development  of  caves  hydrostatic  pressure  and  the 
compression  and  expansion  of  the  air  are  important  forces.  Fingal's 
Cave  has  been  thus  quarried  out  of  the  lava  of  the  south  shore  of  the 
island  of  StafFa.  It  extends  inland  200  feet,  the  floor  being  below 


210 


PHYSICAL  GEOLOGY 


FIG.    196.  —  Perce  Rock,  Gaspe,  Canada.     (Photo.  S.  Powers.) 

sea  level  and  the  roof  more  than  50  feet  above.     Sea  caves  are  ex- 
cellent indicators  of  ancient  sea  levels  (p.  214). 

Sea  caves  occasionally  extend  inland  and  open  to  the  surface  of  the 
ground,  sometimes  behind  headlands  one  hundred  or  more  feet  in 
height,  and  at  considerable  distances  from  the  shore.  During  quiet 
weather  these  openings  appear  on  the  surface  as  deep  wells,  but  during 
storms  the  water  is  sent  through  them  with  great  force,  sometimes 
throwing  spray  high  into  the  air,  and  they  are  consequently  known 

as  blowholes,  spouting 
horns,  etc.  Blow- 
holes are  sometimes 
formed  simply  by  the 
landward  extension 
of  sea  caves  whose 
bottoms,  as  well  as 
roofs,  usually  have  a 
strong  upward  in- 
clination inland. 
They  are  also  formed 
when,  in  the  land- 
ward cutting  of  a 
cave,  a  vertical  joint 
is  encountered  which, 

when  enlarged    by  hydrostatic   pressure   and    the  compression   and 
expansion  of  air,  is  drilled  to  the  surface. 

Arches  are  not  uncommon  features  on  some  coasts.  They  are 
formed  (i)  by  the  uniting  of  two  caves  on  opposite  sides  of  a  head- 


FIG.   197.  —  A  sea  arch.     When  the  roof  falls  the  point 
will  become  an  island.      (De  Martonne.) 


THE  OCEAN  AND   ITS  WORK 


211 


FIG.  198.  —  Stacks,  Skye,  Scotland. 


land,  as  is  illustrated  by  Perce 
Rock  in  Quebec  (Figs.  196, 
197)5  <>r  (2)  by  the  partial 
collapse  of  the  roof  of  a  cave. 

Stacks. — Waves  sometimes 
quarry  along  strong  joints, 
leaving  isolated  portions  of 
cliffs  in  the  form  of  chimneys 
or  stacks  (Fig.  198).  Stacks 
are  also  formed  by  the  falling 
in  of  the  top  of  a  sea  arch  (Fig. 
199).  High  stacks  and  chim- 
neys are  most  common  in  hori- 
zontal or  gently  inclined  beds, 

where  the  strike  (p.  253)  coincides  with  the  general  trend  of  the  coast. 
The  Old  Man  of  Hoy  on  the  coast  of  the  Orkney  Islands  is  a  well- 
known  example.  This  is  an  angular  column  of  red  and  yellow  sand- 
stone, more  than  600  feet  high.  Many  examples  of  such  structures 

are  to  be  seen  on 
the  rocky  shores  of 
New  England  and 
Nova  Scotia,  in  the 
Bermuda  Islands, 
and  on  the  shores  of 
Lake  Superior.  If 
the  rock  is  resistant, 
the  stacks  withstand 
the  battering  of  the 
sea  for  many  years, 
and  as  the  sea  cliffs  re- 
treat, may  be  left  be- 
hind as  rocky  islets. 
Marine  Terraces. 
—  As  waves  cut  back 
a  shore,  they  develop 
a  submarine  terrace 
(Fig.  200)  which  ex- 

.  tends  from  the  base 

tic.    199.  —  The  Burgermeister  Gate:   A  in  1864,  and  r   ,        ..—          ,    , 

B  in    1899   after   the   arch    had    fallen    leaving    a   stack,  of  the  cliffs  and  slopes 

(Drawing  after  Andersson.)  gently  seaward   unfl 


212 


PHYSICAL  GEOLOGY 


FIG.   200.  —  Plain  of  marine  denudation,  Yorkshire,  England. 
(Photo.  J.  W.  Gregory.) 

it  ends  abruptly  in  deep  water.  The  width  of  such  a  terrace  depends 
upon  the  distance  that  the  waves  have  cut  into  the  land  —  the  wave- 
cut  terrace  —  and  the  distance  to  which  the  terrace  has  been  built 
out  by  the  material  worn  from  the  cliff  and  carried  out  to  the  edge 
of  the  rock  terrace  by  the  undertow  —  the  wave-built  terrace  (Fig.  201). 
The  depth  of  the  water  over  the  outer  edge  of  the  "  cut  and  built " 
terrace  depends  upon  the  size  of  the  waves  which  prevailingly  beat 

against  the  shore.   In 
small  lakes  it  is  slight, 
while  in  larger  lakes 
it  may  be  twenty  or 
more  feet  in   depth. 
The  floor  of  the  North 
Sea    between    Great 
FIG.  201. —  Section  showing  the  wave-cut  terrace  CB,     Britain    and    Europe 
and  the  wave-built   terrace  EC,  the  whole  constituting  i       r     i        At-]ontj 

the  wave  cut  and  built  terrace. 

a  few  miles  west  of 

Ireland  is  believed  by  some  geologists  to  be  a  plain  of  marine 
denudation.  In  eastern  Patagonia,  southern  Australia,  and  other 
places  the  sea  beats  against  cliffs  from  200  to  1000  feet  high,  a  fact 
which  implies  that  marine  erosion  has  cut  them  back  tens  or  perhaps 
scores  of  miles. 


THE  OCEAN  AND  ITS  WORK 


213 


Striking    Examples    of 
Marine      Erosion.  —  The 

almost  complete  destruc- 
tion by  the  sea  of  the 
village  of  Dunwich,  Eng- 
land, within  historic  times, 
affords  an  excellent  ex- 
ample of  rapid  marine 
erosion  under  favorable 
conditions.  The  village 
was  built  upon  sand  and 
gravel  which  formed  at 
the  shore  a  cliff  50  feet 
high.  In  the  time  of 
Henry  II  the  village  is 
described  as  "  of  good  note 
and  abounding  with  much 
riches,"  but  in  Queen 


FIG.  203.  —  Map  of 
Sharp's  Island,  Chesa- 
peake Bay.  In  1848  it 
contained  438  acres  and 
supported  a  summer  re- 
sort and  a  number  of 
people  throughout  the 
year  ;  in  1900  (lines)  the 
area  was  91  acres,  and 
in  1910  (solid  black)  54 
acres.  If  the  rate  of 
erosion  continues,  the 
island  will  disappear  be- 
fore 1930.  (U.  S.  Geol. 
Surv.) 


FIG.  202.  —  Map  showing  how  rapidly  the  island  of 
Helgoland  has  been  destroyed  by  the  sea.  The  length  of 
the  shore  line  at  different  times  is  given.  This  island  is 
used  by  Germany  as  a  naval  base,  and  its  shores  are 
artificially  protected. 

Elizabeth's  time  it  was  reduced  to  one  fourth  of  its  former 
size.  Records  show  that  "  at  one  time  a  monastery,  at 
another  several  churches,  then  the  old  port,  then  four 
hundred  houses  at  once,  and  gradually  the  jail,  the  town 
hall,  the  high  roads,  and  even  the  ancient  cemeteries,  the 
coffins  of  which  were  for  sometime  exposed  in  the  cliff,  were 
all  swept  away  by  the  devouring  sea."  The  erosion  of  the 
cliff  has  now  ceased,  as  it  is  protected  by  a  bank  of  shingle. 
The  port  of  Ravenspur,  England,  where  Henry  Boling- 
broke  landed  in  1399  to  depose  Richard  II,  has  entirely 
disappeared,  and  no  one  knows  exactly  where  it  stood. 
Portions  of  the  English  coast,  where  the  cliffs  are  from  200 
to  250  feet  high,  have  receded  at  an  annual  rate  of  14  feet. 
In  1831  a  volcanic  island  —  called  Graham's  Island  — 
composed  of  volcanic  ash,  appeared  above  the  Mediter- 
ranean Sea  near  Sicily.  After  reaching  a  height  of  200  feet 
above  the  sea  and  a  diameter  of  a  mile,  the  volcano  became 
extinct,  and  so  rapidly  and  thoroughly  have  the  waves  worn 
it  down,  that  not  even  a  shoal  remains  to  indicate  its 
former  position. 

On  Cape  Charles,  Virginia,  it  has  been  necessary  to  build 
three  successive  lighthouses  on  account  of  the  encroach- 
ment of  the  sea.  The  first  was  built  in  1827,  700  feet 
from  the  shore  line  of  that  time;  this  was  abandoned  in 
1863,  and  the  whole  site  has  now  been  washed  into  the  sea. 


2I4 


PHYSICAL  GEOLOGY 


The  second  was  built  in  1864,  also  about  700  feet  from  the  shore,  but  this  now  stands 

on  the  edge  of  the  water  and  has  been  abandoned  for  a  new  tower  still  further  inland. 

A  remarkable  case  of  marine  erosion  is  exemplified  in  an  island  in  the  North  Sea, 

Helgoland,  whose  circumference  has  been  reduced  from  120  miles  in  the  ninth  century 

to  45  in  the  fourteenth,  8  miles 
in  the  seventeenth,  and  to  an 
islet  only  3  miles  in  circumfer- 
ence at  present.  The  remnant 
has  probably  survived  because 
of  its  greater  height  (170  feet) 
and  because  of  the  somewhat 
more  resistant  character  of  the 
rock  (Figs.  202,  203). 

Sea-captured  Streams. 
—  When  streams  on  ap- 
proaching the  seashore 
turn  and  run  parallel  to 
it  for  some  distance  be- 
fore entering  it,  they  are 
sometimes  cut  in  two  as 
a  result  of  the  more  rapid 
erosion  of  the  coast  at 
some  one  point  (Fig.  204 
A,  B).  Streams  which 
have  been  recently  af- 
fected in  this  way  enter 
the  sea  over  falls. 

Raised  Beaches.  - 
Shores  that  have  been 
raised  (Fig.  205)  are 
sometimes  marked  by  sea 
cliffs,  beaches  (Fig.  206), 
sand  spits,  and  bars,  un- 
less the  elevation  took 
place  so  long  ago  that 
stream  erosion  and  the 


FIG.  204  A,  B.  —  Block   diagrams   showing  how  a 
stream  may  be  captured  by  marine  erosion. 


weather  have  destroyed 
these.  On  the  coast  of 
Scotland  the  beaches  rise  one  above  another  to  a  height  of  100  feet, 
and  the  old  sea  caves  are  sometimes  used  as  stables.  The  raised 
beaches  of  Norway  and  Scotland  are  occupied  by  villages,  and 
without  them  the  shores  would  often  be  deserted.  On  the  coast  of 


THE  OCEAN   AND   ITS  WORK 


215 


FIG.   205.  —  A  raised  beach  showing  the  coarse  bowlders  of  the  old  beach  and 
the  ancient  sea  cliff.     (Photo.  F.  B.  Sayre.) 

California  well-marked  marine  terraces  are  found  1500  feet  above  sea 
level.  On  the  coasts  of  South  America  and  elsewhere  the  recent 
elevation  of  the  land  is  also  proved  by  their  presence. 

Ancient  Plains  of  Marine  Denudation. —  In  Labrador  and  Cali- 
fornia ancient  marine  terraces  are  well  marked.  The  ancient  plain 
of  marine  denudation  on  the  east  coast  of  India  is  an  unusually 
fine  example  of  such 
a  plain  which  has  re- 
sulted from  the  long- 
continued  action  of 
the  waves.  The  two 
striking  features  (Fig. 
207)  of  the  plain  are 
(i)  the  evenness  of 
the  surface  and  (2) 
the  steep-sided  hills, 
the  former  islands, 
which  rise  above  it. 
As  the  ancient  shore 
is  approached,  the 
outliers  (p.  106)  (ancient  islands)  are  more  numerous ;  those  which 
are  near  the  old  shore  are  tied  to  it  by  old  sand  and  pebble  bars. 
Moreover,  sea  caves  (Fig.  208)  are  not  uncommon  in  the  ancient 
sea  cliffs.  Seaward  of  the  marine  plain  is  the  coastal  plain  in  which 
cuestas  (p.  225)  have  been  developed. 


FIG.    206.  —  A   plain  of  marine  erosion  with   ancient 
islands,  east  coast  of  India.     (Photo.  S.  W.  Gushing.) 


2l6 


PHYSICAL  GEOLOGY 


FIG.  207.  — An  ancient  plain  of  marine  denudation,  with  the  former  islands  stand- 
ing above  the  plain  as  hills,  is  shown  in  the  diagram.  The  accordant  level  of  the  hills 
indicates  an  ancient  peneplain.  (Modified  after  S.  W.  Gushing.) 

The  New  England  Marine  Plain.  —  As  has  been  seen  (p.  215),  the  presence  of  cliffs 
at  the  shore  line  shows  that  locally  marine  erosion  may  become  more  effective  than 
subaerial  (the  work  of  weather,  wind,  and  streams).  The  sea,  however,  can  work  only 
against  the  shore,  while  the  effect  of  stream  erosion  and  of  the  weather  is  to  reduce  the 
whole  surface  of  the  land  (p.  114).  The  work  of  the  sea,  though  powerful,  is  limited 
to  the  shore  line,  and  plains  produced  by  marine  erosion  are  of  small  extent  compared 

with  the  extensive  plains  carved  by 
the  subaerial  agencies.  It  has  been 
suggested,  however,  that  at  certain 
times,  planation  by  the  sea  may 
become  more  effective  than  usual 
over  much  broader  areas.  After  a 
region  has  been  worn  down  to  such 
an  extent  that  the  soft  beds  of  rock 
are  reduced  to  base  level,  leaving  the 
harder  as  hills,  subaerial  erosion 
works  very  slowly,  and  the  amount 
of  sediment  carried  to  the  sea  by  the 
streams  is  so  small  that  the  littoral 
currents  expend  little  energy  in 
moving  it.  Under  such  conditions 
the  surface  of  the  land  is  lowered 
very  slowly,  while  marine  erosion  is 
relatively  much  more  effective.  If 
such  a  stage  is  combined  with  some 
submergence  the  sea  has  an  added 
advantage,  and  its  action  is  concen- 
trated on  the  residual  hills  and 

T-  o         A  L        r          TU  uplands    remaining    from    subaerial 

tic.  208. —  An  ancient  shore  line.      1  he  rim        v    .  1111- 

of  the  cave  is  65   feet   high.     East   coast  of    erosion'      The   land    bordering    the 
India.     (Photo.  S.  W.  Gushing.)  Atlantic  coast  of  North  America  is 

thought  by  some  to  have  been  under 

conditions  such  as  these  during  a  long  period  of  time  (Cretaceous  and  Tertiary).1 
The  uplands  of  New  England  and  New  Jersey  and  the  resistant  ridges  of  the  Appa- 

1  Barrell,  J.,  —  Bull.  Geol.  Soc.  Am.,  Vol.  24,  No.  4,  1913,  pp.  688-696. 


THE  OCEAN  AND  ITS  WORK  217 

lachian  Mountains  presented  an  irregular  front  to  the  sea,  upon  which  marine  erosion 
was  concentrated.  As  a  result,  a  plain  of  marine  denudation  many  miles  wide  was 
cut.  Upon  subsequent  oscillatory  elevation,  with  many  halts,  lower  plains  were  cut. 
Consequently,  in  traveling  from  western  Massachusetts  to  Long  Island  Sound,  instead 
of  a  much-dissected,  gently  sloping  peneplain,  one  finds  first  the  high,  rugged  moun- 
tains which  were  not  attacked  by  the  sea;  then  a  deeply  dissected,  slightly  sloping 


FIG.  209.  —  New  England  plains  of  marine  denudation,  according  to  Barrell.     The 
dotted  lines  A,  B,  C,  D  are  the  successive  levels  of  the  sea. 

high  plain,  now  almost  completely  destroyed ;  and,  successively,  lower  plains,  better 
preserved,  until  the  sea  is  reached.  The  highest  plain  if  restored  would  reach  an 
elevation  of  from  2300  to  2400  feet  in  western  Massachusetts ;  and  a  total  of  seven 
originally  well-developed  plains  may  be  recognized,  the  lowest  at  a  height  of  700 
feet.  Below  this  are  four  plains  of  fainter  development.  If  this  theory  is  correct, 
the  so-called  New  England  peneplain  (p.  114)  is  really  a  combination  of  several 
surfaces  of  marine  denudation  (Fig.  209). 


TRANSPORTATION 

Littoral  or  Shore  Currents.  —  The  sediment  carried  into  the  ocean 
by  streams,  as  well  as  that  eroded  from  the  shores  by  waves,  is 
usually  soon  carried  away  by  currents  produced  by  waves,  wind,  and 
tides.  When  a  wave  strikes  a  shore  at  right  angles  to  its  trend,  the 
water  thrown  upon  the  shore  returns  as  the  undertow  (p.  200)  and 
may  carry  the  sediment  to  great  depths.  The  debris  at  the  foot  of 
the  cliffs  is,  however,  not  immediately  transported  to  the  deep  water, 
but  is  moved  back  and  forth  by  the  waves  and  the  undertow,  and  is 
thus  ground  finer  and  finer  with  time.  Since  the  velocity  of  the 
undertow  rapidly  decreases  with  the  depth  of  the  water,  only  the 
finer  sand  can  be  carried  a  considerable  distance.  Consequently, 
one  usually  finds  coarse  pebbles  (shingle)  near  shore,  and  progressively 
finer  sediment  farther  out. 

When  a  wave  strikes  a  shore  obliquely,  a  portion  of  the  water 
returns  immediately  as  undertow  (Fig.  1 86,  p.  200),  and  a  portion 
moves  along  the  shore  and  forms  a  littoral  or  shore  current.  The 
zone  of  breaking  waves  is  the  road  of  shore  drift,  and  it  often  happens 
that  it  is  the  waves  produced  by  storms  rather  than  those  of  the 


2i8  PHYSICAL  GEOLOGY 

prevailing  winds  which  determine  the  direction  of  the  greatest  shore 
drift.  Large  particles  are  not  carried  far  by  the  shore  currents,  but 
the  finer  sand  may  be  transported  many  hundreds  of  miles. 

Tidal  Currents.  —  Tidal  currents  are  often  of  great  importance  in 
the  removal  of  sediment  (p.  221).  When  the  tide  flows  through 
narrow  passages,  as  between  islands,  or  in  V-shaped  bays,  swift  cur- 
rents are  developed  which  erode  and  carry  away  the  mud,  sand,  and 
gravel  which  come  within  their  reach.  Some  tidal  currents  run  so 
strongly  that  divers  are  unable  to  stand  against  them.  The  out- 
going tide  has  greater  power  than  the  inflowing,  since  the  latter  mov- 
ing in  as  a  great  wave  fills  the  bays  above  their  normal  level  and 
backs  up  the  water  of  the  rivers,  often  for  long  distances.  On  ac- 
count of  this  accumulation  of  water  an  outflowing  current  begins 
along  the  bottom  before  the  tide  is  wholly  in,  and  when  the  tide  changes 
this  adds  to  the  strong  current  which  has  already  begun.  Such 
strong,  outflowing  currents  tend  to  keep  the  channels  deep  and  open, 
and  carry  the  mud  and  sand  into  deeper  water. 

The  transporting  and  erosive  powers  of  the  outgoing  and  incoming  tides  are,  how- 
ever, sometimes  almost  equally  strong,  as  was  shown  by  an  examination  of  a  steamer 
which  was  sunk  off  the  mouth  of  the  Gironde  River  in  France.  The  vessel  rested  on 
her  keel  in  36  feet  of  water.  At  the  end  of  the  ebb  tide  the  sands  were  so  scoured  as 
to  leave  the  hull  supported  only  in  the  middle,  but  at  the  end  of  the  flood  tide  the 
vessel  was  again  completely  covered,  the  sand  beds  extending  100  yards  fore  and  aft 
of  the  vessel  and  50  yards  from  each  side.  (Partiot.) 

Tides  not  only  scour  out  channels  but  may  also  cause  the  deposi- 
tion of  the  sediment  which  the  rivers  are  carrying  to  the  sea.  It 
often  happens  that  sand  flats  are  formed  at  the  entrances  of  bays. 
If  a  point  projects  on  the  side  of  the  river  mouth  first  reached  by  the 
incoming  tide,  the  tidal  flow  may  carry  the  sediment  far  beyond  the 
mouth  of  the  river ;  but  if  no  such  point  exists,  the  entrance  may  be- 
come more  or  less  choked. 

FEATURES  RESULTING  FROM  TRANSPORTATION 

Beaches.  —  When  the  sea  has  cut  a  rock  terrace  so  wide  that  a  strip 
of  sand  and  gravel  is  left  between  the  cliff  and  the  sea,  a  beach  is 
formed.  Along  coasts  exposed  to  strong  waves  the  breadth  of  the 
wave-cut  terrace  must  be  much  wider  before  sand  is  left  on  it  to  form 
a  beach  than  in  quiet  water,  since  in  the  former  the  sand  and  gravel 
may  be  swept  away  as  fast  as  formed  even  when  the  terrace  is  several 
hundred  feet  wide.  Wide  beaches  are  usually  first  formed  within 


THE  OCEAN  AND  ITS  WORK 


219 


slight  recesses  of  the  coast,  where  the  littoral  currents  deposit  their 
load.  Such  beaches  are  crescent-shaped.  Near  the  base  of  a  sea  cliff 
bowlders  or  coarse  gravel  will  be  found,  but  as  one  goes  from  the  cliff 


FIG.  210.  —  A  bayhead  beach.     Conception  Bay,  Newfoundland.     (U.  S.  Geol.  Surv.) 

the  material  of  the  beach  is  seen  to  become  pebbly  and  finally  to 
consist  of  fine  sand.  The  horizontal  distance  over  which  a  pebble 
travels  before  it  is  ground  to  sand  is  very  short. 

Bayhead  Beaches.  —  The  detritus  worn  by  the  waves  from  the 
cliffs  and  from  the 
bottom  where  the 
water  is  shallow,  and 
that  brought  to  the 
sea  by  streams  is  in 
part  carried  into  deep 
water,  where  it  is  im- 
mediately deposited 
and,  in  part,  is  swept 
along  the  beach  by 

shore  or  littoral   cur-  pIG>  2ii.  —  Lagoon  inclosed  by  a  storm  ridge. 

rents.     As  the  waste  (Photo.  De  Martonne.) 


220 


PHYSICAL  GEOLOGY 


is  carried  along  it  does  not  conform  closely  to  the  shore  unless  the 
indentations  are  comparatively  slight.  When  it  is  swept  into  a 
shallow,  sheltered  bay  or  cove,  it  may  form  a  bay  head  beach  (Fig.  210). 
When  such  a  beach  is  attacked  by  storm  waves  a  ridge  is  sometimes 
thrown  up  on  the  seaward  edge,  forming  a  dam  behind  which  a  shallow 
lake  or  marsh  is  formed  (Fig.  21 1).  An  interesting  fact  in  connection 
with  these  pebble  beaches  is  that  sometimes  during  a  single  gale  an 
entire  ridge  may  be  moved  as  much  as  30  feet. 

Bars  and  Spits.  —  When  littoral  drift  reaches  an  abrupt  bend  in  a 
shore,  as,  for  example,  at  the  entrance  of  a  bay  which  extends  some 

distance  inland,  it  does  not 
follow  the  bend,  but  usually 
continues  in  the  direction  in 
which  it  has  been  moving. 
It  therefore  passes  from  shal- 
low water  to  deep,  where  it 
drops  its  load.  Since  the 
portion  of  the  littoral  cur- 
rent that  carries  the  sand  is 
usually  narrow,  the  material 
dropped  into  the  deep  water 
is  gradually  built  up  in  the 
form  of  an  embankment,  like 
a  railroad  fill,  which  may,  in 
time,  extend  entirely  across 
FIG.  212.  — Map  showing  an  incomplete  bar  the  bay.  Currents  do  not 
almost  shutting  the  larger  lake  from  the  sea,  build  bars  above  the  level  of 
and  a  complete  bar  across  the  smaller  lake.  the  w  but  wayes  do 

Delta  filling  is  well  shown.     (Atwood.) 

so  by  washing  the  sand  and 

gravel  from  the  slopes  of  the  bar  to  the  top.  As  soon  as  a  portion  of 
the  sand  is  exposed  above  the  water,  it  may  be  blown  into  dunes 
by  the  wind.  Since  dune  topography  is  rough,  such  sandy  stretches 
often  have  an  uneven  surface. 

Often  a  bar  is  never  completed  (Fig.  212),  since  the  rivers  flowing 
into  the  bays  have  sufficient  volume  and  current  to  keep  a  channel 
open.  The  scouring  action  of  tidal  currents  may  also  be  able  to  re- 
move sediment  to  deep  water  as  rapidly  as  it  is  brought  in  by  the 
shore  currents.  Incomplete  bars,  when  built  above  the  surface 
of  the  water  by  waves,  are  called  spits,  and  when  curved  by  the 
force  of  the  tidal  current  at  right  angles  to  the  drift  are  called  hooks 


Contoxn-  interval  10  feet 


THE  OCEAN  AND  ITS  WORK 


221 


(Fig.  213).  Sometimes  the  end  of  a  hook  is  curved  so  far  around 
as  to  form  a  loop.  Provincetown  harbor,  Massachusetts,  is  an 
example. 

Bars  are  often  of  great  disadvantage  to  navigation,  since  they  so 
shallow  the  water  that  vessels  are  compelled  to  wait  until  high  tide 
before  they  can  pass 
over  them.  In  other 
cases  constant  dredg- 
ing, maintained  at 
great  expense,  is  nec- 
essary  to  keep  a 
channel  open.  Spits 
and  hooks  sometimes 
serve  as  breakwaters 
and  are  of  consider- 
able value  to  ship- 
ping in  time  of 
storm. 

The  effect  of  the 
formation      of      bay-          FIG.    213.  —  Hook   Bay  near  the  north   entrance  to 
head   beaches  and  of     Chignik  Bay,  Alaska.     The  hook  was  formed  by  shore 

currents.     (Atwood,  U.  S.  Geol.  Surv.) 
bars    by    the    shore 

currents  is  to  shorten  the  coast  line  and  give  it  a  smoother  outline. 

Sand  Reefs  or  Barrier  Beaches.  —  When  the  water  offshore  is 
shallow,  the  waves  drag  bottom  and  build  up  a  ridge  of  sand  or  gravel 
some  distance  from  the  shore,  which  is  as  high  as  the  storm  waves  can 
lift  the  material.  After  the  surface  is  reached  the  height  is  further 
increased  through  the  piling  up  of  sand  dunes  by  the  wind.  Sand 
reefs  or  barrier  beaches  (Fig.  214  A,  B)  are  therefore  formed  on  shelv- 
ing shores,  along  a  line  to  which  material  is  brought  seaward  by  the 
undertow  and  landward  by  the  drag  of  the  waves.  Such  sand  reefs 
are  separated  from  the  mainland  by  narrow  lagoons,  or  if  they  have 
been  in  existence  for  a  long  time  by  marshes  (p.  223).  Sand  reefs 
are  approximately  parallel  to  the  low  shores  which  they  border.  They 
are  seldom  continuous  for  many  miles  (Fig.  215),  but  are  broken  by 
"  inlets  "  which  are  kept  open  by  tidal  scour  or  by  water  which  is 
brought  into  the  lagoons  by  the  streams,  or  by  a  combination  of  the 
two.  Inlets  occur  at  intervals  of  from  two  to  twenty  miles  on  the 
Atlantic  coast  of  the  United  States.  After  a  sand  reef  is  formed,  it 
sometimes  happens  that  a  second  reef  is  built  up  in  front  of  it,  leaving 


'PHYSICAL  GEOLOGY 


FIG.  214.  —  Formation  of  barrier  islands  or  sand  reefs.  These  are  built  near  the 
line  of  breakers,  off  shallow,  sandy  shores.  The  lagoon  in  214  B  is  shown  to  be  nearly 
filled  with  sediment  and  organic  matter. 

a  lagoon  between  it  and  the  first  reef.  One  of  the  most  remarkable 
barrier  beaches  is  off  the  coast  of  Texas  and  extends  without  a  break 
for  a  hundred  miles. 

On  the  Atlantic  coast  of  North  America,  from  New  Jersey  south, 
the  barrier  beaches  are  so  well-developed  that  it  has  been  proposed 
to  make  a  protected  waterway  by  deepening  the  lagoons  back  of  them. 
If  this  is  accomplished,  vessels  will  be  able  to  sail  from  New  York  to 

Florida,  practically  free 
from  storm  waves,  being 
protected  almost  the  en- 
tire distance  by  sand 
reefs.  The  barrier 
beaches  off  the  coast  of 
New  Jersey  are  especially 
favored  as  pleasure  re- 
sorts because  of  their 
mild  temperature  in  win- 
ter and  cooling  breezes  in 
summer.  In  some  places 
the  barriers  are  growing 
and  in  others  they  are 
being  washed  away. 
Whether  they  grow  or 
waste  depends  upon 


FIG.  215.  —  Barrier  beaches  on  the  coast  of 
Texas.  Matagorda  Bay  has  been  formed  by  a 
barrier  beach,  and  Galveston  is  situated  on  one. 


THE  OCEAN  AND  ITS  WORK 


223 


whether  or  not  the  supply  of  sand  is  too  great  for  the  waves  to 
remove. 

The  lagoons  back  of  sand  reefs  are  gradually  filled  by  the  sediment 
carried  in  by  streams  from  the  mainland,  by  the  sand  blown  in  by 
the  winds,  and  by  the  accumulation  of  marsh  vegetation.  Back  of  the 
sand  reef  on  which  Atlantic  City,  New  Jersey,  is  situated,  peat  has 
accumulated  to  a  depth  of  one  or  more  feet  over  a  wide  extent.  In 
time  a  marsh-filled  lagoon  will  become  dry  land,  and  the  sand  reef 
will  be  joined  to  the  mainland. 

Sand  reefs  are  sometimes  hardened  by  the  deposition  of  lime  car- 
bonate between  the  sand  grains  until  they  form  rock  reefs.  A  no- 
table case  of  this  kind  occurs  off  the  coast  of  Brazil. 

Tied  Islands.  —  Islands  are  sometimes  tied  to  the  mainland  by 
sand  and  gravel  brought  by  littoral  currents.  This  is  accomplished 
in  one  of  two  ways,  (i)  If 
littoral  currents  exist  which 
move  parallel  to  the  shore  in 
opposite  directions,  some- 
times simultaneously  and 
sometimes  successively  so 
that  they  carry  material  to  the 
same  point,  which  is  gener- 
ally a  strait  separating  an 
island  from  the  mainland,  a 
tongue  of  land  consisting  of 
sand  and  gravel  may  unite 
the  island  to  the  mainland. 
(2)  Islands  are  also  tied  to 
the  mainland  (Fig.  216)  by 
the  extension  of  sand  spits 
from  either  the  mainland  or 
the  island  or  from  both. 
Many  examples  of  islands 
tied  to  the  mainland  in  one 

of  these  ways  might  be  cited.  Gibraltar,  an  island  tied  to  Spain  by 
a  narrow  sand  beach  called  the  "  neutral  ground,"  and  Nahant,  off 
the  coast  of  Massachusetts,  are  familiar  examples. 

Examples  of  the  Constructive  Work  of  the  Sea.  —  The  work  of 
the  sea,  as  we  have  seen,  is  constructive  as  well  as  destructive.  It  is 
stated  that  on  one  portion  of  the  coast  of  England  (the  estuary  of 

CLELAND   GEOL. — 15 


FIG.  216.  —  Tied  island,  southern  Italy. 


224  PHYSICAL  GEOLOGY 

the  Humber)  about  290  square  miles  have  been  added  to  the  coast, 
while  on  another  (Fens  of  Lincolnshire),  the  area  of  the  land  has 
been  increased  more  than  1000  square  miles.  It  is  stated  that  for 
every  square  mile  washed  away  from  portions  of  this  coast,  three 
square  miles  have  been  added  on  others.  Moreover  the  sea-built 
land  is,  on  the  whole,  richer  than  that  which  was  destroyed.  A  tele- 
graph pole  erected  at  a  point  on  the  English  coast  in  1873  was  3°° 
feet  inland  in  1902.  At  Atlantic  City,  New  Jersey,  portions  of  the 
sand  reefs  are  being  built  out  while  others  are  retreating.  Hotels 
have  had  to  be  moved  forward  so  as  to  be  kept  near  the  sea.  The 
history  of  the  town  of  Rye,  England,  is  instructive  as  showing  that 
the  land  may  be  attacked  by  the  sea  at  one  time  and  later  be  increased 
at  the  same  point  and  by  the  same  agent.  This  town  was  once  de- 
stroyed by  the  sea,  but  the  site  is  now  two  miles  inland. 

SHORES 

The  shores  of  the  oceans  may,  in  general,  be  classed  topographi- 
cally as  smooth  or  rough,  or  according  to  origin  as  those  resulting 
from  elevation  or  from  submergence.  To  understand  the  configura- 
tion of  a  shore  one  must  keep  in  mind  (i)  that  the  effect  of  deposition 
on  the  ocean  bottoms  is  to  smooth  out  all  inequalities  and  to  produce 
a  monotonous  plain  which  slopes  gently  from  the  beach  to  the  edge 
of  the  continental  shelf,  and  (2)  that  the  effect  of  erosion  on  high  land 
is  first  to  roughen  it. 

Smooth  Shores. — When  a  sea  bottom  on  which  sediment  has  long 
been  accumulating  is  raised  to  form  land,  a  smooth,  approximately 
flat  plain  will  be  exposed.  The  low,  level  plain  of  Yucatan,  which 
slopes  beneath  the  water  so  gently  that  vessels  cannot  approach 
in  safety  nearer  than  three  miles  from  the  coast,  so  that  all  freight 
must  be  taken  to  land  in  shallow  boats,  is  a  good  example.  The  land 
bordering  the  Atlantic  and  Gulf  coasts  of  the  United  States  south 
of  New  York  is  a  somewhat  broken,  level  plain,  through  which 
streams  flow  sluggishly  to  the  sea.  The  underlying  strata  dip  gently 
towards  the  sea  and  are  composed  of  unconsolidated  sands  and  clays 
containing  marine  shells.  This  plain  varies  in  width  from  a  fraction 
of  a  mile  to  500  miles,  extending  from  the  Fall  Line  on  the  west  to 
the  shore  on  the  east. 

The  Fall  Line  marks  the  boundary  between  the  new,  unconsolidated 
sands  and  clays  of  the  Coastal  Plain  and  the  harder,  ancient  rocks  of 


THE  OCEAN  AND  ITS  WORK 


225 


the  Piedmont  Plateau  (p.  91).  The  name  indicates  that  the  streams 
flow  over  falls  or  rapids  where  they  pass  from  the  hard  rocks  of  the 
old  land  to  the  easily  eroded  sediments  of  the  Coastal  Plain. 

The  greatest  coastal  plain  in  the  world  forms  the  north  and  west 
parts  of  Siberia  and  has  a  maximum  width  of  more  than  1000  miles. 
The  plain  is  low  and  poorly  drained. 

If  England,  eastern  Europe,  and  the  intervening  sea  floors  were 
raised  300  feet,  England  would  be  united  to  the  mainland,  the  Baltic 
would  be  changed  to  a  chain  of  lakes,  and  the  North  Sea  would  be 
reduced  to  a  gulf.  If  this  should  happen,  the  ancient  shores  could  be 
readily  determined  by  the  elevated  sea  cliffs,  sea  beaches,  wave-cut 
terraces,  and  sand  spits ;  while  the  raised  sea  bottoms  would  consti- 
tute coastal  plains.  The  new  shores  would  be  smooth  with  few  in- 
dentations. 

Cuestas.  —  The  material  of  land  newly  raised  from  the  sea  has  a  dip  seaward, 
due  both  to  the  original  inclination  of  the  sediments  and  also  to  that  which  was  brought 
about  during  the  process  of  uplift.  If  the  beds  of  recently  raised  coastal  plains  differ 
somewhat  in  resistance,  the  streams  will  in  time  give  a  zonal  character  to  the  topog- 
raphy, the  harder  beds  standing  higher  than  the  softer  ones.  There  will  thus  result 


FIG.  217.  —  Diagrams  A  and  B  illustrate  the  development  of  cuestas.  As  the  weak 
stratum  of  the  coastal  plain  was  cut  away  more  rapidly  than  the  firm,  the  latter 
formed  rather  steep  slopes  facing  inward,  and  long,  gentle  slopes  towards  the  coast. 


226 


PHYSICAL  GEOLOGY 


alternating  bands  of  lowland  and  highland,  the  lowland  being  bordered  on  the  sea- 
ward side  by  infacing  cliffs  formed  by  the  harder  beds.  The  low  ridges  thus  developed 
have  a  steep  descent  on  one  side  and  a  gentle  slope  on  the  other  and  are  called  cuestas. 
Examples  of  coastal  plains  with  this  banded  arrangement  are  not  uncommon.  In 
Alabama  the  Appalachian  Mountains  are  bordered  by  the  "  Black  Prairie,"  a  belt 
of  lowland  formed  in  easily  eroded  limestone.  Next  to  this  is  a  ridge  (cuesta)  which 
ascends  rather  abruptly  200  feet  above  the  lowland,  composed  of  more  resistant  lime- 
stone (Fig.  217  Ay  B).  The  geological  structure  of  the  Ghats  in  India  (Fig.  207. 
p.  216)  shows  the  formation  and  characteristics  of  such  topography.  Very  ancient 
coastal  plains  with  resulting  cuestas  constitute  a  large  part  of  New  York,  Ohio,  and 
other  states. 


Rough  Shores.  —  By  marine  erosion  a  shore  may  be  slightly  rough- 
ened, but  it  is  not  possible  for  waves  unaided  to  make  irregular  shores 

like    those   of    the   coast    of 

Maine,  Nova  Scotia,  Wash- 
ington, northern  Europe, 
British  Columbia,  and  the 
coast  of  the  Adriatic.  Such 
shores  are  formed  by  the 
sinking  of  the  land  or  the 
raising  of  the  sea  level.  When 
a  region  is  partially  sub- 
merged the  higher  hills  be- 
come islands  or  peninsulas, 
and  the  valleys  become  estu- 
aries or  bays.  Consequently, 
rugged  coasts  bordered  by 
high,  rocky  islands  (Fig.  218), 
are  evidences  of  subsidence. 
An  interesting  example  is  to  be 
found  in  northeastern  North 
America,  where  the  coast  line 
between  New  Brunswick  and 
Portland,  Maine,  is  2000  miles 
long,  although  a  straight  line 
between  the  points  is  only 
200  miles  in  length. 

Another  characteristic  of  a  sunken  coast  is  the  existence  of  sub- 
marine valleys.  On  the  coasts  of  Europe  and  North  America  sound- 
ings have  shown  that  the  valleys  of  rivers  extend  far  out  on  ancient 
coastal  plains  (Fig.  219),  now  the  sea  bottom.  The  Hudson  River 


FIG.  218.  —  Portions  of  the  coast  of  Maine, 
showing  the  effect  of  subsidence.  The  valleys 
have  become  bays,  and  the  hills  peninsulas  and 
islands. 


THE  OCEAN  AND  ITS  WORK 


227 


(Fig.  220),  for  example,  formerly  extended  across  the  continental 
shelf  into  the  deep  sea,  as  is  shown  by  its  deep,  submarine  channel. 
The  St.  Lawrence,  Potomac,  and  other  rivers  also  have  submarine 

channels.  The  bays  of  the 
Coastal  Plain  are  the  result  of 
a  slight  subsidence  after  the 
newly  made  land  had  been  cut 
up  to  some  extent  by  streams, 
and  are  consequently  merely 
drowned  valleys.  Bays  are 
sometimes  formed  by  the  ele- 
vation of  the  sea  bottom  on 


SCALE  OF  MILES./'! 
1*0    20     30    40     50 


FIG.  219.  —  Chesapeake  and  Delaware 
bays  are  drowned  river  valleys,  the 
ancient  submerged  channels  of  which  can 
be  traced  out  to  sea.  (After  Dryer.) 


FIG.  220.  —  Map  showing  the  sub- 
merged channel  of  the  Hudson  River. 
This  channel  can  be  traced  about  125 
miles  beyond  the  present  mouth  of  the 
river.  (After  Dryer.) 


one  or  two  sides  of  an  area  in  which  there  was  no  such  movement. 
The  Gulf  of  California  had  such  an  origin.  Bays  are  made  also  by 
the  settling  of  great  blocks  (fault  blocks,  p.  267),  as  is  true  on  the 
coast  of  the  Red  Sea. 

Examples  of  Irregular  Coasts.  —  The  character  of  irregular  coasts  depends  upon 
several  factors. 

Coasts  of  Folded  Regions.  —  If  the  region  is  folded,  with  the  axes  of  the  folds  parallel 
to  the  coast,  the  bays  and  islands  will  have  a  like  direction.  A  typical  example  of 
such  a  coast  is  to  be  found  on  the  northeast  shore  of  the  Adriatic  Sea  (Fig.  221), 
with  its  elongated  islands,  its  constricted  straits,  and  narrow  bays;  all  of  which  are 
parallel  to  the  coast.  When  the  folds  are  perpendicular  to  the  shore,  a  rugged 
coast  with  projecting  points  and  deep  indentations  results  (Finisterre,  Spain). 


228 


PHYSICAL  GEOLOGY 


Fiord  Coasts.  —  The  coasts  of  high,  glaciated  regions  are  characterized  by  narrow, 
branching  bays  of  great  depth  (p.  226,  and  Fig.  150),  with  precipitous,  almost  vertical 

sides,  called  fiords  (p.  166). 
It  has  been  shown  (p.  167) 
that  fiords  are  valleys 
greatly  deepened  by  glacial 
erosion,  which  have  prob- 
ably been  drowned  by  a 
sinking  of  the  land. 
Fiords  occur  only  in  high 
latitudes. 

Deeply  Indented  Coasts 
in  Non-glaciated  Regions. 
—  In  northwestern  Spain, 
Brittany,  Ireland,  and 
elsewhere  are  fiord-like 
coasts  which,  however, 
have  not  suffered  from 
glacial  erosion.  These 
funnel-shaped  bays  were 
produced  by  the  drowning 
of  deep  valleys  formed  by 
stream  erosion.  Such  in- 
dentations differ  from 
fiords,  not  only  in  their 
origin,  but  also  in  their 
V-shaped  cross  section  and 
in  the  fact  that  they  gradu- 
ally deepen  seaward,  while 
the  deepest  portions  of 
fiords  are  some  distance 
inland. 

Coasts  of  Slightly    Sub- 
merged Coastal   Plains.  — 
The  coastal  plains  of  the 
United   States  have   been 
described  (p.    224)  as    re- 
cently raised   portions  of  the  ocean  bottom.     After  having   been  cut  up  to  some 
extent  by  erosion  a  slight  submergence  occurred,  which  drowned  the  valleys,  thus 
forming  Chesapeake  Bay,  Delaware  Bay,  etc. 

Proofs  of  Elevation  and  Depression.  — Although  the  coast  of  Maine 
is  quite  typically  that  of  a  region  of  submergence,  there  is  evidence 
that  considerable  elevation  followed  the  period  of  greatest  sinking. 
This  evidence  is  to  be  seen  in  the  marine  clays  which  are  found  above 
the  present  sea  level,  as  well  as  in  the  abandoned  shore  lines  which 
are  now  far  above  the  tide. 


FIG.  221.  —  Portion  of  the  east  coast  of  the  Adriatic. 
The  folds  of  the  rock  largely  determine  the  direction  of 
the  straits,  islands,  and  peninsulas. 


THE  OCEAN  AND  ITS  WORK 


229 


The  Island  of  Capri,  off  the  coast  of  Italy,  offers  an  unusual  ex- 
ample of  submergence  within  historic  times  (Fig.  222).  In  ancient 
times  a  sea  cave,  now  known  as  the  Blue  Grotto,  was  used  by  the 
Romans  as  a  resort  from  the  oppressive  heat  of  certain  seasons.  In 
order  to  obtain  light 
an  opening  was  cut 
in  the  roof.  Since 
that  time  the  land 
has  sunk  so  that 
even  the  artificial 
opening  is  now  partly 
submerged.  The 
blue  color  of  the 
grotto  is  due  to  the 

refraction  of  the  sun's        ,-,  _     .        f  „ 

,  *IG.    222. —  Section   of   the   Blue   Grotto,   Island   of 

rays  in  the  water,  by    Capri,   showing   proof  of  subsidence.     (Modified    after 
means  of  which  the    Von  Knebel.) 
red  rays  are  lost. 

In  some  of  the  caves  of  the  Bermuda  Islands  (Fig.  223),  stalactites 
hang  from  the  roof  and  extend  into  the  sea  water  which  partially  fills 
the  caves.  Stalactites  obviously  could  not  have  been  formed  in  water 
and  therefore  prove  the  former  greater  elevation  of  the  island. 

The  temple  of  Jupi- 
ter Serapis  at  Poz- 
zuoli,  near  Naples, 
proves  that  the  coast 
has  suffered,  first  an 
elevation,  then  a  de- 
pression, and  finally 
a  reelevation  almost 
to  its  former  level. 
The  evidence  is  to  be 
found  in  three  col- 
umns of  the  temple, 
whose  surfaces  have 
been  roughened  for 
a  height  of  from  12  to  21  feet  above  the  base  by  boring  mollusks 
(Lithodomus)  which  live  only  in  sea  water.  The  temple  was,  of 
course,  built  on  land.  It  was  then  submerged  by  the  sinking  of 
the  coast,  so  that  the  columns  were  immersed  in  the  sea  to  a  height 


FIG.  223.  —  Diagrammatic  section  through  a  cave  in 
the  Bermuda  Islands,  showing  one  proof  that  subsidence 
has  taken  place.  Stalagmites  and  stalactites  are  formed 
only  in  the  air. 


230 


PHYSICAL  GEOLOGY 


of  21  feet  above  the  base.  At  this  time  the  Lithodomi  bored  into 
the  stone  and  made  their  homes  there.  The  lower  12  feet  of  the 

columns  were  buried  in  sedi- 
ment and  therefore  escaped 
damage.  Later  the  land  was 
again  raised,  and  the  columns 
are  now  some  distance  from 
the  shore  (Fig.  224).  Such 
evidence,  although  interest- 
ing as  showing  recent  changes 
in  sea  level,  is  of  minor  im- 
portance as  compared  with 
the  occurrence  of,  strata  con- 
taining marine  shells  at 
heights  of  14,000  feet  or 
more  above  the  sea. 

The  Stability  of  the  Atlantic  Coast 
of  North  America. — The  statement 
often  made  that  the  coasts  of  Nova 

Scotia,  New  England,  and  New  Jer- 
FIG.  224.  —  Three  columns  of  the  temple  or  ,  ,          ,  ,     , 

T     .        o         •  AT     i          TL     j    iT      j     sey  have  recently  undergone  a  gradual 

Jupiter  Serapis  near  Naples.      Ihe  dark  and         ,    ..  .     .         .  . 

rough  band  above  the  figure  is  the  portion  ^^dence  and  that  this  movement 
which  was  perforated  by  boring  mollusks.  The  1S  stlU  m  progress,  rests  upon  the 
lower  portion  of  the  columns  was  protected  by  following  evidence.1  Stumps  of 
mud  and  the  upper  portion  projected  above  trees  are  found  in  salt  marshes ;  salt 
the  sea.  water  is  found  overlying  fresh-water 

peat ;  marshes  have  increased  in  size ; 

dikes  erected  to  keep  out  the  tide  are  themselves  covered  at  high  tide;  a  bench 
mark  at  Boston  is  now  three-fourths  of  a  foot  nearer  the  mean  level  of  the  sea  than 
when  it  was  placed  there  three  quarters  of  a  century  ago.  When  each  case  is  care- 
fully studied  it  is  found  either  that  the  apparent  sinking  is  due  to  local  causes,  or 
that  no  definite  conclusions  can  be  drawn.  The  evidence  from  marshes  is  especially 
uncertain,  because  when  drained  they  settle ;  when  sand  dunes  encroach  upon  them, 
they  are  compacted  and  their  surface  is  consequently  lowered ;  when  a  bar  behind 
which  fresh-water  marshes  and  forests  exist  is  cut  through  by  waves  (Fig.  225),  the 
marsh  will  be  invaded  by  sea  water,  the  trees  will  be  killed,  and  salt-water  peat  may 
in  time  cover  the  fresh-water  peat.  Changes  in  the  direction  or  velocity  of  ocean 
currents  may  also  bring  about  local  differences  in  sea  level.  The  apparent  lowering 
of  the  bench  mark  near  Boston  is  doubtless  due  to  the  narrowing  of  the  bay  as  a  result 
of  the  artificial  filling  in  of  the  marshes.  Such  a  constriction  of  the  channel  would 

1  For  a  more  complete  statement  see  :  D.  W.  Johnson,  Science,  Vol.  32,  1910,  pp.  721-723, 
and  Fixite  de  la  Cote  Atlantique  de  VAmerique  du  Nord,  Annales  de  Geographic,  Vol.  21, 
1912,  pp.  195-212. 


THE  OCEAN  AND  ITS  WORK 


231 


<Se<3 


FIG.  225.  —  Map  showing  the  conditions 
which  sometimes  give  rise  to  the  belief  that  a 
coast  has  sunk.  If  the  bar  is  cut  through  by 
the  waves,  salt  water  will  invade  the  fresh  water 
marsh  and  kill  any  trees  which  may  be  growing 
on  it,  and  salt-water  peat  may  in  time  cover 
the  fresh-water  peat.  (After  D.  W.  Johnson.) 


increase  the  height  of  the  tides.  A 
recent  study  of  the  evidence  for  and 
against  subsidence  of  the  Atlantic 
coast  indicates  that  a  subsidence  of 
one  foot  during  the  last  century  is 
impossible. 

In  other  parts  of  the  world 
submergence  and  elevation 
are  certainly  taking  place. 
In  Sweden  careful  measure- 
ments show  that  certain  por- 
tions are  rising  and  others 
sinking. 

Cycle  of  Shore  Erosion.  — 
If  one  takes  into  account  the 
combined  effects  of  erosion 
and  accumulation  on  coasts, 
it  will  be  seen  that  all  coasts  tend  to  become  simple.  A  coast 
recently  formed  by  the  advance  of  the  sea  (submergence  of  the 
land),  as  has  been  seen  (p.  226),  has  many  irregularities,  with 
promontories  corresponding  to  the  hills  and  bays  to  the  depressions. 
At  first  the  effect  of  the  waves  is  to  render  the  coast  even  rougher 
than  it  originally  was,  by  the  formation  of  stacks  and  rocky  islets. 
The  effect  of  difference  in  hardness  is,  however,  of  short  duration. 
A  hard  stratum  may  be  isolated  for  a  time,  but  it  is  an  unstable 
situation,  and  the  islet  or  point  thus  formed  is  destined  after  a 
short  delay  to  disappear;  marine  erosion  is  incapable  of  penetrating 
several  miles  inland  by  the  excavation  of  a  softer  stratum.  In 
these  earlier  stages  of  marine  erosion  (Fig.  226  A,  B},  the  coast 
may  also  for  a  time  be  made  more  irregular  by  the  formation  of 
sand  spits,  or  incomplete  bars,  but  their  further  development  tends, 
as  has  been  seen,  to  the  formation  of  a  smoother  coast  by  cutting 
off  the  indentations  which  are  thus  converted  into  lagoons  and 
later  into  marshes.  In  this  early  stage,  which  may  be  compared  to 
the  stage  of  youth  in  the  evolution  of  land  surfaces,  the  wave-cut 
terrace  is  narrow,  and  much  of  the  shore  drift  is  carried  into  deep 
water,  out  of  reach  of  the  littoral  currents.  The  coast  of  Maine  is 
in  general  in  the  youthful  stage.  The  east  coast  of  Scotland  is 
also  in  early  youth,  while  the  Baltic  coast  of  Germany  is  typical  of 
later  youth. 


232 


PHYSICAL  GEOLOGY 


After  prolonged  erosion 
cutting  back  of  the  shore 
terrace,  and  the  littoral 


FIG.  226.  —  Three  block  diagrams  showing  the  effect 
of  marine  erosion.  Diagram  A  is  the  shore  which  would 
result  if  a  portion  of  western  New  England  were  lowered 
1000  feet.  Diagrams  B  and  C  are  later  stages  of  the  same 
region,  showing  the  supposed  effect  of  marine  erosion  on 
such  a  coast.  Since  the  shore  shown  in  C  is  relatively 
stable  it  is  said  to  be  mature. 


the  marine  terrace  is  widened,  both  by  the 
and  by  the  outbuilding  of  the  wave-built 
currents  have  a  broad  road  over  which 
to  move  the  shore 
waste.  As  a  result 
the  heads  of  the 
smaller  bays  are 
filled  with  sand  and 
pebbles,  the  larger 
bays  are  closed  by 
bars  back  of  which 
delta  deposits  are 
built  out,  and  the 
rocky  islets  and 
stacks  are  cut  away. 
When  the  coast  is 
straighter  and  the 
marine  terrace  is  so 
wide  that  the  waves 
have  lost  their  abil- 
ity to  continue  effec- 
tively their  work  of 
cutting  back  the 
land,  the  shores  may 
be  said  to  be  in  old 
age. 

The  intermediate 
stage,  maturity  (Fig. 
226  C),  is  reached 
when  the  effective 
work  of  the  waves 
is  at  its  height;  that 
is,  when  the  sea  is 
attacking  the  land 
along  a  continuous 
and  nearly  straight 
line  of  cliffs  such  as 
one  finds  on  the 
northwest  coast  of 
France  to-day.  Such 


THE  OCEAN  AND  ITS  WORK  233 

coasts,  however,  are  somewhat  irregular,  because  the  shores  are 
usually  composed  of  heterogeneous  materials,  and  also  because  all 
parts  are  not  equally  attacked  by  the  waves. 

The  rate  at  which  coasts  develop  depends  both  upon  the  character 
of  the  rocks  of  which  they  are  composed  and  upon  their  exposure  to 
the  waves.  The  rate  is  also  affected  by  the  amount  of  sediment 
carried  in  by  streams,  since  if  the  quantity  is  large,  so  much  of  the 
energy  of  the  waves  and  currents  is  expended  in  removing  it  that  the 
shore  is  but  slightly  attacked.  An  excellent  example  of  possible 
difference  in  the  rate  of  erosion  is  to  be  found  on  the  coast  of  New 
England,  where  the  rocky  coast  of  Maine  is  still  in  youth,  while  the 
coast  of  Cape  Cod,  composed  of  soft  glacial  material,  has  been  at- 
tacked more  effectively  and  is  well  advanced  toward  maturity. 

DEPOSITION  IN  SEAS  AND  LAKES 

Source  and  Extent  of  Land-derived  Sediments. — The  sediments 
carried  to  the  ocean  by  the  streams  and  the  fragments  broken  from 
the  shores  by  the  waves  are  soon  deposited  on  the  sea  bottom  and 
for  the  most  part  do  not  reach  a  greater  distance  from  the  shore 
than  ten  miles,  although  some  are  carried  to  the  edge  of  the  conti- 
nental shelf.  Some  material  for  these  deposits  is  also  furnished  by 
glaciers  and  the  winds.  Opposite  great  rivers  such  as  the  Amazon 
and  Ganges,  however,  sediments  are  swept  out  by  their  currents  and 
deposited  several  hundred  miles  from  their  mouths,  as  is  shown  by 
fine  sediments  derived  from  the  sea  bottom  200  to  800  miles  from  the 
shore.  There  are  two  principal  reasons  for  this  comparatively  narrow 
belt  of  deposition.  The  most  important,  as  already  noted  (p.  130), 
is  that  all  sediments  sink  shortly  after  reaching  quiet  water;  and  the 
other,  that  fine  sediments  settle  more  rapidly  in  salt  water,  very  fine 
silts  settling  in  salt  water  in  one  fifteenth  the  time  that  they  do  in 
fresh  water. 

Stratification.  —  Deposits  in  any  one  place  seldom  accumulate  to  a 
great  thickness  under  exactly  similar  conditions  and  are  consequently 
in  layers  (Fig.  227) ;  that  is,  they  are  stratified.  Stratification  is  pro- 
duced (i)  usually  by  a  change  from  time  to  time  in  the  character  or 
composition  of  the  sediment.  For  example,  if  the  deposition  of  clay 
is  interrupted  for  a  short  time  during  which  currents  bring  in  sand, 
the  beds  of  clay  will  be  separated  by  layers  of  sand.  (2)  If,  after  a 
layer  of  sand  has  been  laid  down,  deposition  ceases  for  a  time  and  the 


234 


PHYSICAL  GEOLOGY 


grains  of  sand  become  cemented  together  to  some  extent,  the  succeed- 
ing layer  of  sand  will  be  separated  from  the  underlying  by  a  surface 
which  in  this  case  will  divide  two  beds  of  similar  character.  These 
planes  are  called  bedding  planes.  Slight  and  frequent  changes  in  the 
character  of  the  sediments  during  deposition  produce  thin  layers 
called  lamince.  Laminae  are  often  rendered  distinct  by  the  weather- 
ing of  the  rock.  Stratification  is  so  characteristic  of  sedimentary 


FIG.   227.  —  Stratified  limestone.      Auburn,  New  York.      (Photo.  H.  L.  Fairchild.) 

rocks  that  "  stratified  "  and  "  sedimentary  "  are  used  as  synonymous 
terms  in  describing  rocks  of  this  origin. 

Cross  or  False  Bedding.  —  When  sand  moved  either  by  air  or  water 
currents  is  carried  along  a  surface  which  terminates  in  a  slope, 
the  greater  part  of  the  material  will  roll  down  the  slope  and  come  to 
rest  at  a  steep  angle,  a  steeper  slope  being  made  by  coarse  than 
by  fine  sand.  If  now  the  direction  and  velocity  of  the  currents 
vary,  the  inclined  laminae  will  slope  or  dip  in  different  directions  and 
meet  at  various  angles,  producing  cross-bedding.  Cross-bedding  is 
especially  well-developed  in  wind-blown  deposits  (p.  48),  where  the 
shifting  winds  blow  the  sand  in  one  direction  over  an  abrupt  slope 
at  one  time  and  in  a  different  direction  at  a  later  time  (p.  47).  It  is 
also  common  in  delta  deposits,  where  the  distributaries  of  the  river 
vary  from  time  to  time  (p.  131).  Currents  produce  cross-bedding 
near  shores  and  or*  bars,  since  the  sand  which  they  carry  over  the  end 


THE  OCEAN  AND   ITS  WORK 


235 


of  the  embankments  is  brought  in  from  slightly  different  directions 
at  different  times.  The  variation  in  the  direction  of  the  waves  of 
succeeding  storms  often  produces  a  cross-bedding  very  characteristic 
of  littoral  deposits.  When  the  direction  of  an  air  or  water  current 
changes,  the  tops  of  the  cross-bedded  layers  are  often  eroded  away. 
Later  other  cross-bedded  layers  may  be  laid  down  on  this  erosion 


FIG.  228.  —  Cross-bedded  sandstone.     (U.  S.  Geol.  Surv.)  , 

surface.  The  upper  and  lower  surfaces  of  a  bed  of  sand  may  be 
parallel  with  each  other,  as  well  as  with  the  bottom  upon  which  they 
rest,  yet  the  laminae  of  which  it  is  composed  may  be  inclined  at  an 
angle  of  30°,  or  more  (Fig.  228). 


LITTORAL  DEPOSITS 

Extent.  —  Littoral  deposits  are  those  which  accumulate  on  that 
portion  of  the  shore  which  is  exposed  between  high  and  low  tide,  and 
during  exceptional  storms  or  tides  above  high-water  mark.  They 
are  most  extensive  in  estuaries  where  the  salt  marshes  are  flooded 
at  low  tide,  the  breadth  depending  upon  the  slope  of  the  bottom  and 
the  height  of  the  tide.  The  average  width  of  the  beaches  of  the 
world  does  not  exceed  one  half  mile,  and  it  is  estimated  that  the  littoral 


236 


PHYSICAL  GEOLOGY 


belt  covers  an  area  of  62,000  square  miles.  The  seaward  limit  is 
seldom  sharply  differentiated,  since  the  deposits  grade  into  those 
of  the  shallow  sea,  although  sand,  gravel,  and  shingle  usually  mark 
the  outer  surface  of  the  beach. 

Character  of  Littoral  Deposits.  —  The  deposits  of  this  zone  vary 
greatly  on  different  parts  of  the  same  coast.  Muds  are  most  common 
in  lagoons  and  in  sheltered  spots,  and  bowlders  and  shingle  prevail 
along  rocky  shores.  The  sand  of  beaches  is  usually  composed  of 
quartz  grains,  since  this  is  the  most  common  mineral  of  the  rocks  of 
the  earth's  crust.  Another  reason  for  the  predominance  of  quartz 
is  the  fact  that  when  rock  fragments  are  rolled  about  by  the  waves, 
the  softer  minerals  of  which  they  are  composed  are  soon  ground  to 
fine  powder  and  carried  away  by  even  slight  currents,  and  finally 
deposited  as  clay,  the  harder  constituents  only  being  left  as  sand. 

Locally,  sands  of  other  composi- 
tions than  quartz  occur.  Occa- 
sionally garnet  or  magnetite 
grains  constitute  the  chief  ma- 
terial of  beaches;  in  the  Bay  of 
Naples  the  sand  is  made  up  of 
the  olivine  and  feldspar  derived 
from  volcanic  rocks ;  the  sand  of 
the  Bermuda  Islands  is  composed 
of  minute  shell  fragments. 

Distinguishing  Characteristics 
of  Littoral  Deposits.  —  Certain 
features  are  characteristic  of 
littoral  deposits  and  are  due  to 
the  fact  that  these  are  alternately 
covered  by  water  and  exposed  to 
the  sun  and  wind.  Ripple  marks 
(Fig.  229),  made  by  the  wind  or 
water;  rill  marks  (Fig.  230), 
formed  by  the  water  as  it  flowed 
back  down  the  beach  at  low  tide ; 
rarely  sun  cracks  (Fig.  231), 
formed  by  the  drying  of  the  mud  ;  raindrop  impressions,  footprints  of 
animals,1  and  fossils  of  land  and  sea  animals  and  plants  characterize 

1  Sun  cracks,  raindrop  impressions,  and  footprints  are  more  common  on  flood  plains 
and  playas. 


FIG.   229.  —  Wave  ripples  on  sandstone. 
(Photo.  H.  L.  Fairchild.) 


THE  OCEAN  AND  ITS  WORK 


237 


deposits  of  this  origin.  Such  impressions  cannot  be  retained  on  a  beach 
unless  deposits  are  accumulating  on  it ;  otherwise  the  record  of  one  day 
would  be  obliterated  by  the  tide  or  waves  of  the  next.  The  presence 
of  these  characteristics  of  littoral  deposits  affords  evidence  that  certain 
ancient  rocks  were  deposited  in  the  littoral  belt. 


FIG.  230.  —  Rill  marks  on  a  modern  beach.  They 
resemble,  and  in  ancient  beds  have  sometimes  been  mis- 
taken for,  seaweed  impressions.  (U.  S.  Geol.  Surv.) 

The  thickness  to  which  littoral  deposits  accumulate  depends  upon 
whether  the  coast  is  stationary  or  is  sinking.  If  the  subsidence  is 
slow  and  of  long  duration,  the  deposit  may  accumulate  to  a  great 
thickness  ;  hundreds  and  even  thousands  of  feet  of  sediments  having 
been  deposited  in  this  way  in  the  past. 


SHOAL-WATER  DEPOSITS 

Extent  and  Character  of  Deposits.  —  Shoal-water  deposits  extend 
from  the  outer  face  of  the  littoral  deposits  to  a  depth  of  about  600 


238 


PHYSICAL  GEOLOGY 


feet,  and  to  a  distance  of  100  or  more  miles  from  shore,  that  is 
to  the  outer  edge  of  the  continental  shelf  (p.  195) ;  and  cover  an 
area  of  about  10,000,000  square  miles.  They  pass  almost  im- 
perceptibly on  the  one  hand  into  the  coarser  littoral  deposits,  and 

on  the  other  into  the  fine 
deposits  of  the  deep  sea. 
They  are  similar  in  character 
to  the  littoral  deposits,  but 
are  finer.  Shoal-water  de- 
posits, in  common  with  those 
of  the  littoral  zone,  are  often 
ripple-marked  and  preserve 
the  tracks  of  such  animals  as 
worms  and  shellfish.  Sun 
cracks  and  the  tracks  of  land 
animals  are  absent.  Cross- 
bedding  (p.  235)  is  often  well 
developed  in  the  sand  near 
shore  where  horizontal  strati- 
fication was  interfered  with 
by  currents  (Fig.  228).  In 
general  it  may  be  said  that 
these  sediments  are  coarsest 
near,  shore  and  become  pro- 
gressively finer  away  from  it.  The  reason  for  this  is  evident,  as  the 
following  example  shows.  When  a  river  enters  the  sea  the  force 
of  its  current  is  immediately  checked,  and  the  coarser  sediment 
which  it  carries  is  deposited,  sand  is  swept  out  to  a  greater  distance 
and  spread  over  a  wider  area,  while  the  fine  clay  travels  still  farther 
and  covers  a  much  larger  tract  of  the  sea  bottom.  The  sediments 
moved  by  the  waves  and  ocean  currents  are  similarly  affected; 
shingle  and  gravel  accumulate  close  to  shore,  sand  is  carried  farther 
out,  and  clay  is  most  widely  spread. 

Limestone.  —  Beyond  the  reach  of  the  clay  lime  ooze  accumulates. 
This  statement  should  not  be  taken  to  mean  that  limestone  may  not 
accumulate  near  shore.  Near  coral  reefs  lime  carbonate  is  accumu- 
lating to-day,  and  there  is  much  reason  to  believe  that  during  certain 
periods  of  the  past,  limestone  of  great  thickness  was  deposited  near 
shores  which  bordered  lands  so  low  that  the  streams  were  able  to 
bring  to  the  sea  little  besides  the  lime  carbonate  which  they  carried 


FIG.  231.  —  Mud  or  sun  cracks. 
(U.  S.  Geol.  Surv.) 


THE  OCEAN  AND  ITS  WORK  239 

in  solution.  Although  lime-secreting  organisms  are  found  at  all 
depths  of  the  ocean,  yet  the  most  important  and  abundant  are  not 
found  at  depths  greater  than  light  can  penetrate  (p.  197). 

Mud  and  sand  are  mechanical  or  clastic  (Greek,  clastos,  broken) 
sediments ;  that  is,  they  are  derived  from  the  decay  of  rocks  and  are 
brought  directly  to  the  sea  by  streams  or  by  waves.  All  of  the 
calcium  carbonate  which  has  accumulated  to  form  limestone  was, 
on  the  other  hand,  brought  to  the  ocean  in  solution.  Some  of  it  was 
precipitated  directly  from  the  water,  since  salt  water  is  capable  of 
holding  a  smaller  quantity  of  calcium  carbonate  in  solution  than 
fresh  water.  The  massive  gray  submerged  limestone  off  the  south 
coast  of  England  contains  modern  shells,  proving  that  precipitation 
is  now  taking  place.  Calcium  carbonate  is  also  precipitated  by  the 
ammonium  carbonate  derived  from  the  decay  of  organisms. 

The  most  common  limestones  are  formed  from  the  accumulations 
of  the  remains  of  mollusks,  corals,  sea  urchins,  starfish,  crinoids 
(p.  430),  Foraminifera  (p.  523),  and  other  marine  animals,  and  of  cer- 
tain plants  (calcareous  algae).  One  sometimes  sees  ledges  of  limestone 
almost  completely  made  up  of  a  jumble  of  shells  of  one  or  two  species 
of  mollusks.  Limestone  often  shades  imperceptibly  into  shale  or 
fine  sand. 

We  consequently  find  in  ancient  rocks  that  conglomerates  (p.  249) 
usually  occur  in  relatively  narrow  belts,  and  sandstones  often  cover 
wide  areas,  while  shales  and  limestones  have  a  still  wider  distribution. 

Lens-shaped  Sediments.  —  All  sedimentary  deposits  are  roughly 
lens-shaped.  They  are  thickest  as  well  as  coarsest  near  the 
source  of  supply,  and  become  finer  and  thinner  away  from  it.  This  is 
most  noticeable  in  conglomerates  (p.  249)  which  in  a  distance  of  even 
two  or  three  miles  may  decrease  from  a  thickness  of  perhaps  several 
hundred  feet  to  that  of  a  few  feet,  or  may  disappear  entirely.  Sand- 
stones have  a  similar  character,  but  usually  thin  out  much  less  rapidly ; 
while  muds,  or  their  equivalents,  shale  and  limestone,  may  extend 
many  miles  with  slight  variation  in  thickness.  Beds  of  limestone 
only  a  few  feet  thick  can  sometimes  be  traced  over  an  area  of  several 
hundred  square  miles. 

Dovetailing  of  Sediments.  —  If  a  boring  were  made  a  few  miles 
from  shore,  through  sediments  which  had  accumulated  to  a  consider- 
able thickness,  it  would  seldom  penetrate  a  single  kind  of  rock  for  a 
great  depth,  but  would,  for  example,  first  pass  through  sandstone, 
then  shale,  then  sandstone  again,  and  perhaps  through  limestone. 

CLELAND   GEOL.  —  1 6 


240 


PHYSICAL  GEOLOGY 


If  these  beds  were  traced  from  the  shore  outward  it  would  be  found 
that,  as  in  the  diagram  (Fig.  232),  the  sandstones  projected  as  a  thin 
wedge  between  layers  of  shale.  This  "  dovetailing  "  is  due  to  tem- 
porary changes  in  the  conditions  of  sedimentation.  During  violent 
storms  sand  or  even  gravel  may  be  carried  out  much  farther  from 


FIG.  232.  —  Dovetailing  of  sediments.  As  sediments  are  traced  from  the  shore 
their  character  usually  changes  gradually.  The  dovetail  structure  is  due  to  shifting 
conditions;  heavy  storms  carry  coarse  material  to  an  unusual  distance,  and  calm 
weather  permits  the  deposition  of  fine  sediment  close  to  shore. 

shore  than  usual,  but  when  calmer  weather  prevails  mud  will  be  laid 
down  on  the  sand  and  gravels.  During  severe  floods  rivers  also  bring 
down  an  enormous  quantity  of  sediment  which  is  swept  a  much 
greater  distance  into  the  sea  than  normally. 

Basal  Conglomerates.  —  If  a  coast  is  sinking  more  rapidly  than  it  is 
filled  by  sediments  the  shore  will  gradually  retreat  inland,  with  the 
result  that  the  beach  of  one  period  becomes  deep  water  later.  The 

•Sea  I  eve  2 


FIG.  233.  —  Basal  conglomerate.  When  the  old  land  surface  (i)  was  slowly  sub- 
merged, gravel  (2)  was  deposited  along  the  shore  and  as  a  result  of  progressive  sub- 
sidence covered  the  ancient  land  surface.  The  strata  (3),  (4),  and  (5)  were  deposited 
upon  the  gravel  (basal  conglomerate)  as  the  distance  from  the  shore  increased.  Con- 
temporaneous deposits  are  shown  by  the  dotted  lines  3  and  4. 

coarse  sediment  of  the  submerged  beaches  will  be  covered  by  finer  sand, 
muds,  and  lime,  the  nature  of  the  deposit  depending  largely  upon  the 
distance  from  shore.  These  gravels  which  cover  the  old  land  surface, 
when  hardened  (indurated),  are  called  basal  conglomerates  (Fig.  233). 


THE  OCEAN  AND  ITS  WORK 


241 


Subsidence  Necessary  for  Great  Accumulations. —  Sediments  do 
not  accumulate  to  a  great  thickness  unless  the  sea  bottom  upon 
which  they  are  being  laid  down  is  subsiding.  This  has  not  been 
an  uncommon  condition  in  the  past,  as  is  shown  by  the  occurrence 
of  stratified  rock  four  and  even  more  miles  in  thickness.  Many 
of  these  deposits  are  of  shallow-water  or  of  continental  origin,  as  the 
ripple  and  rill  marks,  the  coarseness  of  some  of  the  ingredients,  and 
the  fossils  show. 


DEEP-SEA  DEPOSITS 

Deep-sea  deposits  cover  about  three  fifths  of  the  sea  bottom,  and 
are  found  beyond  the  limit  of  the  sediments  derived  from  the  land. 

Blue  Mud.  —  Since  an  ocean  depth  greater  than  600  feet  is  usually 
more  than  10  miles  from  the  shores,  only  those  sediments  which  are  so 
fine  that  they  can  be  carried  in  suspension  for  a  long  distance  are  found 
in  such  situations.  The  sediments  are  usually  of  a  bluish  gray  color 
and  are  classed  roughly  as  blue  muds.  The  bluish  gray  color  is  due 
to  the  fact  that  the  contained  organic  matter  prevents  the  oxidation 
of  the  iron  in  the  deposits.  They  cover  an  area  of  approximately 
15,000,000  square  miles  of  the  ocean  bottom,  or  five  times  the  extent 
of  the  United  States.  They  surround  all  coasts,  beyond  the  shoal 
deposits,  and  cover  the  deeper  parts  of  such  inland  seas  as  the 
Mediterranean.  The  depth  of  water  in  which  they  occur  varies  from 
about  750  feet  to  16,800  feet. 

Globigerina  Ooze.  —  This  ooze  is  a  deposit  consisting  of  30  to  90 
per  cent,  of  the  shells  of  Foraminifera  (Fig.  234),  of  which  the  most 
abundant  genus  is  Globigerina.  These 
unicellular  animals  seldom  attain  a  size 
greater  than  that  of  a  pinhead,  and 
secrete  a  shell  (test)  of  calcium  car- 
bonate. They  are  extremely  simple  in 
structure,  but  the  shells  as  seen  through 
a  microscope  are  very  beautiful.  They 
live  in  countless  millions  in  the  surface 
and  subsurface  waters  of  the  ocean  and  FIG  234.  ~  Globigerina  ooze, 
.  greatly  magnified.  (After 

upon  their  death  rain   down  on  the  sea    Shimer.) 

floor.     It  is  not  to   be   understood   that 

these  organisms   are  the  only  ones  whose  remains   constitute  this 

widespread  deposit.     Other  small  forms  of  life,  which   also  live  in 


242  PHYSICAL  GEOLOGY 

great  abundance  at  the  surface,  likewise  add  to  the  deposit  after 
their  death.  Foraminifera  are  not  more  abundant  over  the  deeper 
waters  than  over  those  nearer  shore,  but  the  deposits  formed 
from  their  remains  are  not  recognizable  in  the  latter,  because  of  the 
large  percentage  of  land  sediments  with  which  they  are  mixed.  Glo- 
bigerina  ooze  is  seldom  found  in  water  more  than  one  to  two  miles  deep. 
Below  a  depth  of  15,000  feet  the  proportion  of  calcareous  deposits 
diminishes,  owing  to  the  increase  in  the  percentage  of  carbon  dioxide 
in  the  water  which  dissolves  the  shells.  However,  many  millions  of 
square  miles  (probably  49,520,000)  of  the  ocean  floor  in  temperate 
and  tropical  regions  are  being  covered  by  deposits  of  globigerina  ooze 
to-day.  The  chalk  of  England  and  of  the  western  United  States  is 
composed  largely  of  the  remains  of  Foraminifera,  although  the  re- 
mains of  other  animals  are  not  uncommon  (p.  249). 

Radiolarian  Ooze.  —  Other  unicellular  animals  which  secrete  sili- 
ceous shells  (Radiolaria)  form  siliceous  oozes,  but  are  found  in  bottoms 
at  a  greater  depth  than  the  globigerina  ooze,  in  some  cases  where 
the  ocean  is  five  miles  deep. 

Red  Clay.  — At  depths  greater  than  15,000  feet,  as  has  been  seen, 
the  calcium  carbonate  of  the  Foraminifera  is  dissolved  ;  consequently 
below  this  depth  enormous  areas  of  the  ocean  floor  are  covered  with 
extremely  fine,  reddish  clay,  composed  of  the  insoluble  portion  of  Fo- 
raminifera, volcanic  dust,  pumice,  ash,  and  minute  meteorites.  Ra- 
diolarian ooze  and  red  clay  shade  into  each  other  in  certain  places, 
the  deposit  being  called  radiolarian  ooze  when  these  organic  remains 
constitute  25  per  cent,  of  the  mass.  The  color  of  red  clay  is  due  to 
the  presence  of  iron  oxide  (Fe2Os)  formed  by  the  oxidation  of  iron 
and  iron  compounds.  This  deposit  covers  an  area  of  51,500,000 
square  miles,  four  fifths  of  which  is  in  the  Pacific  Ocean,  the  smaller 
area  in  the  Atlantic  being  due  to  its  lesser  depth. 

The  slowness  with  which  red  clay  has  been  deposited  in  the  past  is 
perhaps  best  shown  by  the  number  of  sharks'  teeth  that  are  dredged 
from  the  bottom.  In  a  single  haul  in  the  south  Pacific  1500  sharks' 
teeth  were  brought  to  the  surface,  many  of  which  were  of  extinct  species. 
It  has  even  been  suggested  that,  were  all  of  the  sharks  alive  whose 
teeth  rest  upon  certain  areas  of  the  ocean  floor,  the  ocean  immediately 
above  these  deposits  would  be  filled  from  top  to  bottom  with  living 
flesh.  The  fact  that  meteoric  dust  which  gathers  with  extreme  slow- 
ness can  be  detected  in  these  deposits  is  a  further  evidence  of  the 
great  slowness  with  which  the  red  clay  accumulates. 


THE  OCEAN  AND  ITS  WORK 


243 


It  is  interesting  to  note  that  no  deposits  of  this  sort  have  ever  been 
found  on  the  continents,  showing  perhaps  that  the  great  depths  of 
the  ocean  have  never  been  raised  to  form  dry  land.  It  is  rare  that 
any  rock  is  found  on  the  continent  which  implies  water  deeper  than  a 
few  hundred  feet. 

CORAL  REEFS  AND  ISLANDS 

Coral  islands  have  long  excited  the  interest  of  manners,  both  be- 
cause of  their  location  far  from  land  and  because  of  their  beauty. 
They  are  also  of  great  scientific  interest  because  of  their  origin. 

Reef-building  corals  grow  best  in  seas  (i)  with  a  minimum  tem- 
perature of  not  less  than  60°  F. ;  (2)  at  a  depth  of  not  more  than  150 
feet;  (3)  where  the  salt  water  is  free  from  sediment;  and  (4)  where 
they  are  exposed  to  the  dash  of  the  waves.  Free  exposure  to  the 
waves  is  of  advantage,  since  the  profusion  of  life  on  a  coral  reef  soon 
exhausts  the  oxygen  needed  for  respiration  and  the  calcium  carbonate 
necessary  for  their  stony  structure.  Since  they  do  not  thrive  in 
muddy  or  fresh  waters  they  are  not  developed  near  the  mouths  of 
rivers. 

In  tropical  regions  where  the  above  favorable  conditions  prevail, 
the  shores  of  the  continents  are  bordered  by  coral  reefs,  called  fring- 
ing reefs,  which  have  _  i 

a  steep  slope  of  50° 
to  60°  on  the  sea- 
ward side.  In  many 
cases  in  addition  to 
the  fringing  reef 
there  is  another  reef, 
surrounding  the  is- 
land or  paralleling 
the  land,  several  miles 
from  shore.  Such  a 
reef  is  termed  a 
barrier  reef  (Fig. 
235).  Circular  reefs 
or  atolls,  without 
islands  in  the  center,  and  lagoonless  coral  islands  also  occur. 

The  geological  importance  of  coral  animals  lies  in  the  fact  that  they 
have  the  power  of  extracting  calcium  carbonate  from  sea  water  and 
depositing  it  within  their  own  bodies.  Upon  the  death  of  the 


FIG.   235.  —  Barrier  reef  off  the  coast  of  the  island 
of  Curafao,  Dutch  West  Indies. 


242  PHYSICAL  GEOLOGY 

great  abundance  at  the  surface,  likewise  add  to  the  deposit  after 
their  death.  Foraminifera  are  not  more  abundant  over  the  deeper 
waters  than  over  those  nearer  shore,  but  the  deposits  formed 
from  their  remains  are  not  recognizable  in  the  latter,  because  of  the 
large  percentage  of  land  sediments  with  which  they  are  mixed.  Glo- 
bigerinaooze  is  seldom  found  in  water  more  than  one  to  two  miles  deep. 
Below  a  depth  of  15,000  feet  the  proportion  of  calcareous  deposits 
diminishes,  owing  to  the  increase  in  the  percentage  of  carbon  dioxide 
in  the  water  which  dissolves  the  shells.  However,  many  millions  of 
square  miles  (probably  49,520,000)  of  the  ocean  floor  in  temperate 
and  tropical  regions  are  being  covered  by  deposits  of  globigerina  ooze 
to-day.  The  chalk  of  England  and  of  the  western  United  States  is 
composed  largely  of  the  remains  of  Foraminifera,  although  the  re- 
mains of  other  animals  are  not  uncommon  (p.  249). 

Radiolarian  Ooze.  —  Other  unicellular  animals  which  secrete  sili- 
ceous shells  (Radiolaria)  form  siliceous  oozes,  but  are  found  in  bottoms 
at  a  greater  depth  than  the  globigerina  ooze,  in  some  cases  where 
the  ocean  is  five  miles  deep. 

Red  Clay.  — At  depths  greater  than  15,000  feet,  as  has  been  seen, 
the  calcium  carbonate  of  the  Foraminifera  is  dissolved  ;  consequently 
below  this  depth  enormous  areas  of  the  ocean  floor  are  covered  with 
extremely  fine,  reddish  clay,  composed  of  the  insoluble  portion  of  Fo- 
raminifera, volcanic  dust,  pumice,  ash,  and  minute  meteorites.  Ra- 
diolarian ooze  and  red  clay  shade  into  each  other  in  certain  places, 
the  deposit  being  called  radiolarian  ooze  when  these  organic  remains 
constitute  25  per  cent,  of  the  mass.  The  color  of  red  clay  is  due  to 
the  presence  of  iron  oxide  (Fe2Os)  formed  by  the  oxidation  of  iron 
and  iron  compounds.  This  deposit  covers  an  area  of  51,500,000 
square  miles,  four  fifths  of  which  is  in  the  Pacific  Ocean,  the  smaller 
area  in  the  Atlantic  being  due  to  its  lesser  depth. 

The  slowness  with  which  red  clay  has  been  deposited  in  the  past  is 
perhaps  best  shown  by  the  number  of  sharks'  teeth  that  are  dredged 
from  the  bottom.  In  a  single  haul  in  the  south  Pacific  1500  sharks' 
teeth  were  brought  to  the  surface,  many  of  which  were  of  extinct  species. 
It  has  even  been  suggested  that,  were  all  of  the  sharks  alive  whose 
teeth  rest  upon  certain  areas  of  the  ocean  floor,  the  ocean  immediately 
above  these  deposits  would  be  filled  from  top  to  bottom  with  living 
flesh.  The  fact  that  meteoric  dust  which  gathers  with  extreme  slow- 
ness can  be  detected  in  these  deposits  is  a  further  evidence  of  the 
great  slowness  with  which  the  red  clay  accumulates. 


THE  OCEAN  AND   ITS  WORK 


243 


It  is  interesting  to  note  that  no  deposits  of  this  sort  have  ever  been 
found  on  the  continents,  showing  perhaps  that  the  great  depths  of 
the  ocean  have  never  been  raised  to  form  dry  land.  It  is  rare  that 
any  rock  is  found  on  the  continent  which  implies  water  deeper  than  a 
few  hundred  feet. 

CORAL  REEFS  AND  ISLANDS 

Coral  islands  have  long  excited  the  interest  of  mariners,  both  be- 
cause of  their  location  far  from  land  and  because  of  their  beauty. 
They  are  also  of  great  scientific  interest  because  of  their  origin. 

Reef-building  corals  grow  best  in  seas  (i)  with  a  minimum  tem- 
perature of  not  less  than  60°  F. ;  (2)  at  a  depth  of  not  more  than  150 
feet;  (3)  where  the  salt  water  is  free  from  sediment;  and  (4)  where 
they  are  exposed  to  the  dash  of  the  waves.  Free  exposure  to  the 
waves  is  of  advantage,  since  the  profusion  of  life  on  a  coral  reef  soon 
exhausts  the  oxygen  needed  for  respiration  and  the  calcium  carbonate 
necessary  for  their  stony  structure.  Since  they  do  not  thrive  in 
muddy  or  fresh  waters  they  are  not  developed  near  the  mouths  of 
rivers. 

In  tropical  regions  where  the  above  favorable  conditions  prevail, 
the  shores  of  the  continents  are  bordered  by  coral  reefs,  called  fring- 
ing reefs,  which  have  _  i 

a  steep  slope  of  50° 
to  60°  on  the  sea- 
ward side.  In  many 
cases  in  addition  to 
the  fringing  reef 
there  is  another  reef, 
surrounding  the  is- 
land or  paralleling 
the  land,  several  miles 
from  shore.  Such  a 
reef  is  termed  a 
barrier  reef  (Fig. 
235).  Circular  reefs 
or  atolls,  without 
islands  in  the  center,  and  lagoonless  coral  islands  also  occur. 

The  geological  importance  of  coral  animals  lies  in  the  fact  that  they 
have  the  power  of  extracting  calcium  carbonate  from  sea  water  and 
depositing  it  within  their  own  bodies.  Upon  the  death  of  the 


FIG.   235.  —  Barrier  reef  off  the  coast  of  the  island 
of  Curasao,  Dutch  West  Indies. 


244  PHYSICAL  GEOLOGY 

animal  this  "  skeleton  "  is  left  as  a  firm  calcareous  deposit.  The 
reef-building  corals  live  principally  in  colonies  and  because  of  this 
assume  many  forms ;  some  are  in  great  head-like  masses  (brain  corals), 
others  are  branching  like  trees  (staghorn  corals),  while  others  are 
in  flat  masses. 

Coral  reefs  are  not  built  up  entirely  of  the  remains  of  coral  animals, 
but  a  large  part  is  contributed  by  other  lime-secreting  organisms 
which  live  in  association  with  the  corals.  These  reefs  are  not  built 
above  the  level  of  the  water  by  the  coral  animals,  but  by  storm  waves 
which  tear  masses  of  coral  from  the  reef  and  pile  them  up  above  sea 
level.  The  building  above  the  sea  is  thus  seen  to  be  accomplished 
in  the  same  way  as  is  the  formation  of  sand  reefs  (p.  221).  As  soon 
as  the  broken  coral  rock  is  above  the  sea,  some  of  it  is  dissolved  by 
rain  water  and  spray,  and  upon  being  redeposited  cements  the  frag- 
ments into  firm  rock.  The  coral  reefs  thus  built  above  the  sea  are 
consolidated  into  compact  limestone. 

The  most  extensive  barrier  reef  in  the  world  is  the  Great  Barrier 
Reef  of  Australia  which  borders  the  coast  of  that  continent  for  about 
1000  miles  at  a  distance  of  20  to  50  miles  from  the  mainland.  Its 
breadth  beneath  the  surface  of  the  sea  varies  from  10  to  90  miles, 
although  but  little  is  exposed  above  the  water.  The  channel  between 
the  Great  Barrier  Reef  and  the  shore  is  from  60  to  240  feet  deep,  but 
the  outside  of  the  reef  has  a  steep  slope,  so  that  in  short  distances 
depths  of  1800  feet  are  encountered.  This  difference  of  slope  on 
the  two  sides  is  due  to  the  fact  that  growth  on  the  outside  is  better 
assured,  as  it  receives  the  full  sweep  of  the  waves,  so  that  aeration 
is  better  realized  there,  food  is  more  abundant,  and  the  washing 
away  of  dead  parts  more  quickly  accomplished.  This  rapid  growth 
toward  the  open  sea  causes  a  very  jagged  contour  and  an  abrupt 
slope. 

Coral-reef  Problem.  —  The  origin  of  fringing  reefs  is  evident, 
since  all  of  the  conditions  favorable  to  coral  growth,  such  as  a  warm 
temperature,  a  depth  of  water  not  greater  than  150  feet,  and  free  ex- 
posure to  the  waves,  are  present  on  portions  of  the  shores  of  the 
islands  and  continents  of  the  tropics,  or  where  the  ocean  currents  bring 
water  with  a  temperature  of  at  least  68°  F.  The  origin  of  barrier 
reefs,  atolls,  and  lagoonless  islands  is  not  so  clear,  since  barrier  reefs 
are  separated  from  the  land  by  channels  sometimes  several  miles  wide 
and  from  120  to  180  feet  deep,  and  the  lagoons  of  the  atolls  some- 
times have  a  depth  of  350  feet. 


THE  OCEAN  AND  ITS  WORK 


245 


Subsidence  Theory  of  Darwin.  —  This  theory  holds  that  on  a  slowly 
sinking  island  (Fig.  236)  a  fringing  reef  would  be  built  to  the  surface 
by  the  accumulation  of  the  calcareous  remains  of  the  corals  and  other 
animals,  and  plants  which  flourish  under  similar  conditions.  It  is 
apparent  that  since  corals  grow  best  on  the  outside  of  a  reef  where 
the  waves  beat  freely  and  there  is  an  almost  complete  absence  of  sedi- 
ment, a  fringing  reef  would  in  time,  if  the  sea  bottom  slowly  sub- 
sided, become  a  barrier  reef,  separated  from  the  island  by  a  lagoon. 


FRINGING   REEF  STAGE 


ENCIRCLING    REEF     STAGE 


PLAN 


FIG.  236.  —  Diagram  illustrating  Darwin's  theory  of  coral  islands,  showing  the 
fringing  reef  stage  before  subsidence,  the  encircling  reef  stage  after  some  subsidence 
(dotted  line),  and  the  atoll  stage  after  the  island  had  been  completely  submerged. 

It  is  also  readily  seen  that  under  such  conditions  the  lagoons  would 
become  deeper  and  wider  as  the  subsidence  proceeded.  The  distance 
of  the  barrier  reef  from  the  island,  under  these  conditions,  would  de- 
pend upon  the  slope  of  the  island  and  fhe  amount  of  sinking.  If  sub- 
sidence continued,  the  peak  of  the  original  island  would  eventually 
disappear  and  an  atoll  would  be  left.  The  following  have  been  offered 
as  proofs  of  this  theory.  Islands  surrounded  by  barrier  reefs  are 
characterized  (i)  by  an  embayed  shore  line  which,  as  has  been  seen 
(p.  228),  indicates  subsidence;  (2)  by  the  absence  of  delta  plains  in 
the  indentations,  such  as  would  be  present  if  the  island  had  stood  at 
the  same  level  for  a  long  period  of  time;  and  (3)  by  ridges  that  do 


246  PHYSICAL  GEOLOGY 

not  end  in  sea  cliffs,  as  would  be  the  case  if  a  volcanic  island 
had  been  cut  back  by  the  waves  to  form  a  marine  platform  upon 
which  the  corals  grew.  (4)  It  has  also  been  found  that  bor- 
ings of  1000  or  more  feet  penetrate  materials  like  those  of  the 
superficial  layers  of  the  reef.  Also,  according  to  this  theory,  the , 
barrier  reef  gradually  contracts  as  subsidence  continues,  resulting, 
if  the  sinking  has  been  long  continued,  in  the  complete  drowning 
of  the  island  and  the  formation  of  an  atoll  and  finally  of  a  lagoonless 
island.1 

Submarine  Bank  Theory  of  Murray  and  Others.  —  This  theory 
holds  that  barrier  reefs  and  atolls  may  be  explained  without  postu- 
lating subsidence  of  the  sea  floor.  The  supporters  of  this  theory  be- 
lieve that  banks  may  be  built  up  by  the  accumulation  of  the  remains 
of  marine  animals  until  a  depth  of  water  suitable  for  coral  growth  is 
attained,  or  a  platform  for  the  corals  may  be  formed  by  volcanic 
cinder  cones  (such  as  that  of  Graham's  Island). 

Since  coral  growth  is  most  rapid  on  the  outer  margin  of  such  a  bank 
or  cone,  a  ring  arises  with  a  lagoon  within.  Waves  break  through  the 
ring,  separating  it  into  a  series  of  islets,  and  the  solvent  action  of  sea 
water  together  with  the  erosion  of  currents  deepens  and  widens  the 
lagoon.  According  to  this  theory  the  coral  ring  grows  larger;  ac- 
cording to  the  subsidence  theory  it  becomes  continually  smaller.  In 
support  of  this  theory  it  is  pointed  out  that  elevated  atolls  are  some- 
times mere  skins  on  older  volcanic  rocks  and  are  not  of  great  thick- 
ness. In  many  cases,  moreover,  coral  atolls  rest  upon  limestone  and 
volcanic  rock  which  have  been  cut  down  by  erosion  and  have  not 
sunk.  The  most  serious  objection  to  this  theory  is  that  sediments 
carried  into  the  lagoons  by  streams  from  the  islands  have  not  built 
delta  plains,  as  would  be  the  case  had  the  region  suffered  no  subsid- 
ences. Some  atolls  have  probably  been  formed  in  this  way,  but  the 
general  application  of  the  theory  is  not  justified. 

Change  in  Sea  Level  Due  to  Glaciation,  or  the  Glacial-control 
Theory. — An  ingenious  theory  elaborated  by  Daly  is  based  upon 
the  lowering  of  the  level  of  the  sea  in  the  tropics,  due  to  the  with- 
drawal of  the  water  in  those  regions  by  evaporation,  and  its  later 
precipitation  in  the  north  as  snow  during  the  formation  of  the  great 
(Pleistocene)  ice  sheets ;  this  withdrawal  being  increased  by  the 
attraction  of  the  water  by  the  great  mass  of  ice  in  the  Arctic 
regions.  "  The  ice  sheets  (Pleistocene)  which  have  since  melted 

1  Davis,  W.  M., — Nature,  Vol.  90,  1913,  pp.  632-634. 


THE  OCEAN  AND   ITS  WORK 


247 


away  had  a  combined  area  of  at  least  6,000,000  square  miles,  with  an 
average  thickness  of  probably  more  than  3000  feet.  The  removal 
of  enough  water  to  form  that  ice  tended  to  lower  sea  level  all  around 
the  globe  at  least  150  feet.  The  gravitative  attraction  of  the  ice 
caps  must  have  further  lowered  the  equatorial  seas  by  amounts 
ranging  from  30  to  50  feet.  The  net  shift  of  level  in  the  equatorial 
zone  was,  therefore,  at  least  180  feet.  Conversely,  the  melting  of 
the  full  6,000,000  square  miles  of  ice  must  have  raised  sea  level  in 
that  zone  about  180  feet."  x  The  cooling  of  the  climates  and  waters  of  J 
the  world  during  the  Glacial  Period  (p.  644)  retarded,  or  entirely  I 
stopped,  the  growth  of  reefs  over  a  large  part  of  the  world.  Having 
lost  their  defending  reefs  by  this  temporary  change  in  climate,  the 
islands  were  vigorously  attacked  by  the  powerful  breakers  of  the 


FIG.  237.  —  Diagram  illustrating  the  glacial-control  theory  of  coral  islands. 
The  platform  rock  is  shown  by  dots,  the  coral  reef  and  calcareous  debris  by  solid  black. 
The  level  platform  is  thought  to  be  a  plain  of  marine  denudation  cut  when  the  sea 
level  was  lowered  by  the  withdrawal  of  water  to  form  the  great  ice  sheets  of  Europe  and 
North  America.  The  coral  islands  were  slowly  built  up  as  the  level  of  the  sea  was 
raised  upon  the  melting  of  the  glaciers.  (After  Daly.) 

open  sea,  resulting  in  their  planation  at  a  depth  of  a  few  fathoms 
below  the  level  of  the  sea  of  that  time  (Fig.  237).  With  the  ameliora- 
tion of  the  climate,  the  ice  caps  of  the  high  latitudes  began  to  melt, 
and  the  surface  temperature  of  the  equatorial  ocean  was  soon  raised 
to  a  point  which  permitted  the  coral  polyps  to  flourish.  These 
animals  speedily  colonized  the  eroded  platforms  and  developed  the 
atoll  form  as  the  result  of  the  slow  rise  of  sea  level. 

As  proof  of  this  theory  it  is  pointed  out  that  the  platforms  upon 
which  the  reefs  rest  are  remarkably  flat,  as  if  planed  by  marine  erosion, 
and  that  they  have  a  nearly  uniform  depth  of  275  feet,  i.e.,  a  depth 
of  about  95  feet  below  the  level  of  the  seas  of  glacial  times.  It  will 
be  seen  that  Darwin's  and  the  Penck-Daly  theories  differ  principally 
in  that,  in  the  former,  the  land  is  believed  to  have  slowly  sunk, 
while  in  the  latter,  the  sea  level  is  thought  to  have  been  gradually 
raised.2 

1  Daly,  R.  A.,  —  Pleistocene  Glaciation  and  the  Cord  Reef  Problem,  Am.  Jour.  Sci.,  Vol.  30, 
1910,  pp.  297-308. 

4  Daly,  R.  A.,  —  Science  Conspectus,  Vol.  i,  1911,  pp.  120-123. 


248  PHYSICAL  GEOLOGY 

CONSOLIDATION  OF  SEDIMENTS 

The  greater  part  of  the  surface  of  the  continents  of  the  world  is 
composed  of  sedimentary  rocks  which  were  originally  laid  down  in 
the  sea.  These  were  at  first  largely  unconsolidated  gravels,  muds, 
and  calcareous  oozes,  but  are  now  usually  thoroughly  consolidated. 
In  general,  it  may  be  said  that  the  more  recent  rocks  are  not  as  firm  as 
those  of  greater  age.  For  example,  the  rocks  l  of  the  coastal  plains 
of  the  United  States  (p.  574)  are  sands  and  clays  which  are  almost 
or  quite  as  soft  as  when  first  laid  down ;  while  in  other  portions  of 
the  continent,  where  the  sedimentary  rocks  are  older,  they  are  usually 
hard.  The  consolidation  of  sedimentary  rocks  is  brought  about  by 
(i)  cementation,  (2)  pressure,  and  (3)  heat. 

Cementation.  —  Loose  (incoherent)  deposits  are  consolidated  either 
by  direct  cementation  or  by  the  formation  of  interlocking,  fibrous 
crystals  which  hold  the  grains  firmly  together.  Some  recently  built 
sand  reefs,  such  as  those  which  border  the  coast  of  Pernambuco,  Brazil, 
are  already  converted  into  sandstone  by  the  deposition  of  calcium  car- 
bonate between  the  sand  grains.  At  the  mouths  of  rivers  the  sedi- 
ments are  sometimes  consolidated  by  calcium  carbonate,  precipitated 
from  the  fresh  water  as  it  mingles  with  the  sea  water,  as  rapidly 
as  they  are  laid  down.  In  deposits  composed  of  fragments  of  shells, 
calcium  carbonate  also  constitutes  the  cementing  material.  In  this 
case,  as  has  been  seen  (p.  51),  the  sediment  furnishes  its  own 
cement,  which  is  first  dissolved  from  the  calcareous  fragments  and 
later  redeposited  a  short  distance  beneath  the  surface  of  the  de- 
posit, thus  forming  a  more  or  less  compact  limestone.  In  this  way 
the  limestone  of  Bermuda  and  the  "  coquina  "  limestone  of  the  coast 
of  Florida  were  formed. 

Sediments  are  also  cemented  by  iron  oxide  which  is  derived  from 
the  soluble  salts  of  iron  carried  into  seas  and  lakes.  These  iron 
compounds  upon  oxidation  sink  to  the  bottom  and  firmly  cement 
the  sand. 

Sands  composed  of  quartz  grains  are  sometimes  cemented  by 
silica  and  form  extremely  hard  quartzites  (p.  344). 

Effect  of  Pressure.  —  When  subjected  to  the  great  pressure  of 
overlying  sediments,  muds  are  compacted  and  thus  hardened  into 
shale.  The  compactness  of  shale  is  also  sometimes  increased  by  the 

1  The  word  rock,  used  technically,  does  not  necessarily  imply  compactness,  but  includes 
loose  sands  as  well  as  granites. 


THE   OCEAN  AND  ITS  WORK 


249 


deposition  of  some  mineral  matter  about  the  grains.     Other  sedi- 
mentary rocks  are  also  compacted  in  the  same  way. 

Effect  of  Heat.  —  Sediments  usually  become  more  compact  when 
subjected  to  heat,  as  will  be  seen  in  a  later  discussion  (Metamor- 
phism,  p.  341). 

CLASSIFICATION  OF  SEDIMENTARY  ROCKS 

Limestones.  —  The  rocks  of  this  class  are  composed  either  of 
calcium  carbonate  (CaCO3)  or  of  calcium  and  magnesium  carbonate 
(CaCO3-MgCO3),  and  are  formed  in  one  of  the  following  ways  :  (i)  as 
a  result  of  chemical  precipitation,  (2)  by  the  accumulation  of  the 
calcareous  coverings  or  skeletons  of  animals  and  plants,  and  (3)  from 
fragments  of  limestone  which  have  been  re-cemented  into  a  solid  mass. 
Chalk  (p.  523)  is  a  limestone  composed,  for  the  most  part,  of  the 
remains  of  microscopic  animals  (Foraminifera).  Oolitic  limestone 
consists  of  minute  spherical  concretions  having  much  the  appearance 
of  a  fish  roe,  hence  the  name  oolitic  (p.  78).  When  the  little  con- 
cretions are  broken  open  and  examined  with  a  microscope  a  grain 
of  sand  is  often  found  in  the  center,  around  which  concentric 
layers  of  calcium  carbonate  have  been  added.  Oolitic  limestone  is 
widely  used  for  building  purposes  in  the  United  States,  England,  and 
France.  Travertine  is  deposited  from  solution  near  places  where 
springs  laden  with  calcium  carbonate  emerge.  In  the  Dinaric 
Alps  travertine  forms  thick  beds  and  partially  fills  the  basins.  Such 
deposits  are  not  uncommon  in  North  America  in  valleys  of  lime- 
stone regions,  where  they  occasionally  attain  a  thickness  of  more 
than  100  feet.  Dolomite  is  a  limestone  composed  of  calcium  and 
magnesium  carbonate  in  varying  proportions.  It  is  widespread  and 
often  of  great  thickness. 

Sandstones.  —  Under  this  term  are  included  sandstones  and 
conglomerates.  They  were  derived  from  the  land,  and  their  coarse- 
ness or  fineness  (texture)  depends  upon  their  nearness  to  or  remote- 
ness from  their  source,  or  upon  the  swiftness  of  the  currents  which 
transported  them.  A  sandstone  is  composed  of  fine  grains,  while  a 
conglomerate  is  made  up  of  gravel  and  shingle.  A  breccia  (Italian, 
pronounced  brech'a)  is  a  rock  composed  of  angular  fragments  larger 
than  sand,  and  was  formed  by  the  cementing  together  of  the  particles 
of  a  much  fractured  or  crushed  rock,  or  of  talus  which  had  been  trans- 
ported but  a  short  distance  and  therefore  was  not  worn  and  rounded. 


250  PHYSICAL  GEOLOGY 

Shales.  —  These  are  consolidated  muds  which  were  formerly  de- 
rived from  the  decomposition  of  the  feldspars  of  igneous  rocks  (p.  330) 
under  the  action  of  the  water.  They  are  usually  finely  laminated 
(P-  234). 

Sandstones  and  shales,  when  traced  for  some  distance,  may  become 
more  and  more  calcareous,  gradually  shading  into  pure  limestone, 
and  vice  versa. 

Deposits  in  Lakes  and  Deserts.  —  The  deposits  of  lakes  will  not 
be  treated  separately,  since  they  consist,  as  in  the  seas,  of  clays, 
sand,  and  occasionally  of  gravel,  and  when  consolidated  are  dis- 
tinguished with  difficulty  from  marine  deposits.  Because  of  their 
limited  extent  and  the  small  chance  of  preservation,  they  are  usually 
of  little  importance  when  compared  with  the  widespread  and  thick 
marine  deposits.  In  certain  regions  ancient  lake  sediments  several 
thousand  feet  deep  occur.  The  conglomerates  and  sandstones  of 
desert  regions  have  been  discussed  elsewhere  (p.  52). 

Influence  of  Sedimentary  Rocks  upon  Topography.  —  Firmly 
cemented  conglomerates  and  sandstones  are  important  hill  and 
mountain  makers.  The  "  rock  cities  "  of  southwestern  New  York 
and  many  of  the  ridges  of  the  Appalachian  Mountains  stand  in  relief 
because  of  the  presence  of  strata  of  resistant  sandstones  or  conglom- 
erates. When  pure  sandstones  disintegrate,  they  form  barren  soils 
which  even  in  populous  regions  are  usually  covered  with  forests, 
since  they  are  too  poor  for  agriculture.  The  scenery  of  limestone 
regions  has  already  been  described  (p.  73).  In  such  a  land  wide 
joints,  swallow  holes,  and  caves  are  usually  common,  and  the  drainage 
may  be  entirely  underground.  When  limestone  is  massive  it  may  be 
cut  down  by  the  streams  less  rapidly  than  the  neighboring  strata, 
and  form  high  cliffs  and  mountains.  The  Helderberg  escarpment 
of  eastern  New  York  is  a  conspicuous  line  of  limestone  cliffs  stretch- 
ing for  many  miles. 

Regions  underlain  by  shale  are  usually  low  and  flat.  The  wide 
Mohawk  valley  of  New  York ;  the  level,  fertile  plains  of  Ontario ; 
the  northern  Middle  States  of  the  United  States ;  and  the  Black 
Belt  of  Alabama  are  examples. 

REFERENCES  FOR  OCEANS  AND  LAKES 

ARBER,  E.  A.  N.,  —  The  Coast  Scenery  of  North  Devon. 

CORNISH,  V.,  —  Sea  Waves. 

DAVIS,  W.  M.,  —  The  Outline  of  Cape  Cod:  Geographical  Essays,  pp.  690-725. 


THE  OCEAN  AND   ITS  WORK 


251 


DAVIS,  W.  M.,  —  Home  Study  of  Coral  Reefs,  Bull.  Am.  Geog.  Soc.,  Vol.  46,  pp.  561- 

677;  641-654;  721-739. 

DE  MARTONNE,  E.,  —  Geographie  Physique,  pp.  673-706. 
FENNEMAN,  N.  M.,  —  Lakes  of  Southeastern  Wisconsin:   Bull.  Wis.  Geol.  and  Nat. 

Hist.  Surv.  No.  8,  1902. 
GEIKIE,  J.,  —  Earth  Sculpture,  pp.  315-335. 

GILBERT,  G.  K.,  —  Lake  Bonneville:  Monograph,  U.  S.  Geol.  Surv.,  Vol.  i,  1890. 
GILBERT,  G.   K.,  —  The   Topographic  Features  of  Lake  Shores:    Fifth  Ann.  Rept., 

U.  S.  Geol.  Surv.,  1885,  pp.  63-123. 
HUNTER,  J.  F.,  —  Erosion  and  Sedimentation  in  Chesapeake  Bay :  Professional  Paper, 

U.  S.  Geol.  Surv.  No.  90-6,  1914. 
JOHNSON,  W.  D.,  —  The  Atlantic  Coast. 
SHALER,  N.  S., —  The  Geological  History  of  Harbors:  Thirteenth  Ann.  Rept.,  U.  S. 

Geol.  Surv.,  1893,  pp.  93-209. 
WHEELER,  W.  H.,  —  The  Sea  Coast. 

TOPOGRAPHIC  MAP  SHEETS,  U.  S.  GEOLOGICAL  SURVEY,  ILLUSTRATING  OCEAN  AND 

LAKE  SHORES 

Bars  and  Tied  Islands  Drowned  Coasts 

Tamalpais,  California.  Boothbay,  Maine. 

Duluth,  Minnesota.  Seattle,  Washington. 

Marthas  Vineyard,  Massachusetts.  Boston,  Massachusetts. 

Boston,  Massachusetts.  Charlestown,  Rhode  Island. 

Coos  Bay,  Oregon.  Tolchester,  Maryland. 

Point  Lookout,  Maryland. 
Sandy  Hook,  New  Jersey. 


CHAPTER  VII 
THE   STRUCTURE   OF  THE  EARTH 

UNLESS  subsequently  disturbed,  the  sedimentary  rocks  of  the  earth 
are  in  the  approximately  horizontal  position  which  they  had  when 
they  were  first  deposited.  For  example,  we  find  the  sedimentary 
strata  covering  the  vast  territory  between  the  Appalachian  and  Rocky 
mountains,  for  the  most  part,  in  the  horizontal  position  which  they 
had  when  they  were  outspread  in  sheets  on  the  ocean  floor. 

STRUCTURAL  FEATURES  OF  ROCKS 

Dip  and  Strike.  —  In  such  regions  as  the  Appalachians,  New 
England,  eastern  Canada,  the  Rocky  Mountains,  and  the  Sierra 
Nevadas,  however,  the  strata  are  often  inclined  at  angles  varying 
from  horizontal  to  vertical.  The  strata  thus  tilted  are  sedimentary, 


fen  fo/cf  \         '•••-.......-••''  '••-.- 

fsoc//'ne 

FIG.  238.  —  Section  through  a  folded  region  showing  a  symmetrical  fold,  an 
isocline,  and  a  fan  fold. 

and  their  present  altitude  is  the  result  of  compressional  forces  to 
which  they  were  subjected.  As  a  result  of  weathering  and  erosion 
their  upper  portions  were  denuded,  leaving  the  inclined  beds  outcrop- 
ping at  the  surface.  In  Fig.  238  the  position  of  the  strata  with  refer- 
ence to  the  present  surface  is  shown,  and  the  portion  carried  away  by 
erosion  is  indicated  by  dotted  lines.  Two  terms  are  used  to  describe 
an  inclined  bed  :  dip  and  strike.  The  meaning  of  the  terms  can  best 
be  understood  from  an  illustration.  If  one  side  of  the  roof  of  a  house 
is  taken  to  represent  an  inclined  stratum,  the  downward  inclination, 
that  is,  the  course  which  water  poured  on  the  roof  would  take,  is  the 
direction  of  the  dip,  the  angle  of  the  dip  being  the  departure  from  the 
horizontal.  Thus,  if  a  roof  is  inclined  at  an  angle  of  30°  to  a  level 

252 


THE  STRUCTURE  OF  THE  EARTH 


253 


FIG.   239.  —  Diagram  illustrating  dip 


plane,  it  has  a  30°  dip  ;  if  the  angle  is  40°,  the  dip  is  40°.  The  ridge- 
pole of  the  house  extends  at  right  angles  to  the  dip  of  the  roof 
and  corresponds  to  the  strike  of  a  stratum  which  is  defined  as 
the  direction  at  right  angles  to  the  dip.  If,  for  example,  a  bed  dips 
to  the  west,  the  strike  is  north  and  south  (Fig.  239).  The  importance 
of  ascertaining  the  dip  and  strike 
of  beds  is  evident  in  such  cases  as 
the  following.  Suppose  a  land- 
owner finds  that  a  valuable  coal 
seam  outcrops  a  few  hundred  feet 

east  of  his  property.  If  the  dip  of  and  strike t  the  dip  being  the  angle  of  the 
the  bed  were  found  to  be  15°  west,  greatest  inclination  of  a  bed  (shown  by 
i  !  j  ,  i  i  i  arrow),  and  the  strike  the  direction  at 

he  would  know  that  the  coal  seam   right  angles  to  the  dip 

could  also  be  encountered  on  his 

property  by  sinking  a  shaft,  and  the  exact  depth  at  which  it  would  be 

found   could   be   determined   by  a  simple  mathematical  calculation 

(Fig.  240).     As  the  angle  BAG  (dip)  is  15°  and  the  angle  ABC,  a 

right  angle,  the  angle  BCA  must  be  75°.     The  length  of  the  side  BA 

n/^» 

can  be  ascertained  by  the  trigonometric  formula,  tan  BAG  =  —— -,  or 

AB 

BC  =  tan  BAG  X  AB.  If,  however,  it  were  found  that  the  strike 
of  the  coal  seam  is  east  and  west,  the  landowner  would  know  that 

in  this  case  also  the 
coal  seam  probably 
extended  on  his  land 
and  was  merely  hid- 
den from  view  by  soil 
or  glacial  drift.  A 
knowledge  of  the  dip 
and  strike  of  a  water- 


FIG.  240.  —  Block  diagram  showing  the  effect  of  dip 
upon  the  width  of  the  outcrop  of  strata.  The  strata  at 
the  right  are  nearly  four  times  as  wide  as  those  at  the 
left,  although  the  thickness  is  the  same. 


bearing  stratum, 
such  as  the  Dakota 
sandstone  in  Nebraska  and  South  Dakota,  has  enabled  those  in 
search  of  artesian  water  to  estimate  the  depth  to  which  it  is  neces- 
sary to  bore  and  whether  or  not  a  well  would  flow.  It  is,  however, 
in  determining  the  structure  of  a  region  that  the  dip  and  strike  are 
especially  used. 

Effect  of  Dip  and  Strike  upon  Outcrop.  —  When  a  series  of  strata 
are  horizontal,  only  the  uppermost  appears  at  the  surface;  but  when 
the  beds  are  inclined  and  eroded,  each  bed  in  succession  outcrops 


256 


PHYSICAL  GEOLOGY 


distances,   but   is   either  gently   arched,  or   dips    and    pitches,  the 
slant   of  the    axis    being    called    its    pitch    (Figs.    244,   245). 

One  sometimes  sees 

..•'..-• -,'' .  anticlines      unaccom- 

panied by  synclines, 
but  more  often  the 
two  occur  together. 
Notable  examples  of 
series  of  great  parallel 
folds  are  to  be  seen  in 
the  Appalachians  of 


FIG.  246.  —  An  anticlinorium.     (After  J.  Geikie.) 


Pennsylvania  and  in  the  Juras  of  Switzerland.     In  greatly  folded 
regions,  indeed,  the  conspicuous  anticlines  may  be  considered  as  minor 


FIG.  247. — The  Mt.  Greylock,  Massachusetts,  synclinorium.     (U.  S.  Geol.  Surv.) 

crumplings  of  a  great,  complex   anticline  (Fig.  246)  called  an  anti- 
clinorium, or  of  a  complex  syncline  called  a  synclinorium  (Fig.  247). 
A  form  of  fold  which,  though  simple,  cannot  be  included  in  the  term 
anticline  is  the  monocline, 
or   step-fold    (Fig.   248). 
It  occurs  where  horizon- 
tal rocks  suddenly  bend 
downward.     In  portions 
of  Utah    and    Arizona 
monoclinal  folds  are  not 
uncommon    and    often 
merge  into  or  give  place 
to  faults. 

Effect  of  Folding  on 
Competent  and  Incom- 
petent Strata.  —  If  the  FIG.  248.  —  Monoclinal  flexure  and  fault. 


THE  STRUCTURE  OF  THE  EARTH 


257 


strata  which  are  being  folded  are  composed  of  beds  of  firm  sandstone 
or  limestone,  they  will  be  thrown  into  anticlines  and  synclines. 
Such  strata  are  called  competent.  If  the  strata,  however,  consist  of 
shale  or  mud,  they  may  not  be  strong  enough  to  form  arches  and  will 
be  squeezed  and  crumpled  together,  and  are  called  in  consequence 
incompetent  strata.  The  effect  of  horizontal  compression  upon  beds 
of  rock  differs  under  different  conditions.  For  example,  if  a  bed  is 
near  the  surface,  it  may  upon  being  subjected  to  lateral  pressure 
be  able  to  form  an  anticline;  but  if  it  is  deeply  buried,  the  pressure  of 
the  overlying  strata  may  be  so  great  that  it  will  be  crumpled  into 
many  small  folds.  No  one  has  ever  seen  strata  in  the  process  of 
folding,  but  through  experimentation  (p.  361)  and  a  study  of  folded 
strata  the  means  by  which  it  is  produced  have  become  known. 

How  the  Structure  of  a  Region  is  Determined.  —  In  order  to 
determine  the  structure  of  a  region  in  which  the  strata  have  been 
greatly  folded  and 
eroded,  it  is  necessary 
that  a  geological  map 
of  the  region  be  made, 
in  which  the  areas 
underlain  by  the  vari- 
ous rocks  are  indicated 
and  the  dip  and  strike 
of  the  outcrops  re- 
corded. When  such 
a  map  as  that  shown 
in  Fig.  249  is  com- 
pleted, we  find  that 

not  only  does  the  strike  of  the  beds  vary,  but  that  the  dip  as  shown 
by  the  arrows  varies.  By  combining  all  of  the  data  attainable, 
the  cross  section  also  shown  on  the  side  of  the  diagram  can  be 
constructed. 

Origin  of  Folds.  —  The  origin  of  folds  will  be  discussed  more  fully 
under  mountains  (p.  358).  It  will  be  sufficient  at  this  time  to  state 
that  the  various  folds  which  have  been  discussed  (with  the  exception 
of  monoclinal  folds)  are  in  general  due  to  lateral  compression  (p.  364). 

Warping.  —  Besides  the  folding  described  above  there  are  move- 
ments of  the  earth's  crust  which  result  in  the  warping  of  its  surface ; 
that  is,  there  are  vertical  movements  which  raise  the  surface  in 
certain  places  and  depress  it  in  others.  This  is  well  shown  along  the 


FIG.  249.  —  Block  diagram  showing  the  manner  in 
which  geological  maps  are  prepared.  The  various  rock 
formations  are  mapped  and  the  dip  and  strike  of  the 
strata  (shown  by  arrows)  recorded,  in  order  that  the 
geological  section  (shown  on  the  sides  of  the  diagram) 
can  be  determined. 


258  PHYSICAL  GEOLOGY 

Atlantic  coast  of  Canada,  where  the  ancient  shore  line  stands  575 
feet  above  tide  at  St.  Johns,  Newfoundland,  and  declines  to  250  feet 
in  northern  Labrador.  The  warping  of  the  eastern  part  of  the  United 
States  in  the  Appalachian  region  has  resulted  in  the  high  mountains  of 
North  Carolina  and  the  lower  areas  of  Pennsylvania.  Portions  of 
Sweden  are  sinking  while  others  are  rising.  Proofs  of  such  move- 
ments are  not  to  be  seen  in  folds,  since  the  rocks  under  these  condi- 
tions are  not  compressed  ;  but  can  be  ascertained  by  careful  measure- 
ments extending  over  many  years,  and  especially  by  a  study  of 
elevated  marine  terraces  and  old  land  surfaces.  The  movement  may 
be  extremely  slow,  often  being  only  a  few  inches  a  century. 

Zones  of  Flow  and  Fracture.  —  The  rock  of  the  earth's  crust  is 
much  fractured  at  and  near  the  surface.  Such  fracturing,  however, 
does  not  extend  indefinitely  downward,  although  at  a  depth  of  at 
least  eleven  miles  1  empty  cavities  may  exist  in  granite,  and  if  the 
cavities  are  filled  with  water,  gas,  or  vapor,  they  may  exist  at  even 
greater  depths.  It  is  evident,  however,  that  at  depths  of  even  eleven 
miles  many  rocks  less  strong  than  granite  will  be  unable  to  withstand 
the  enormous  weight  of  the  overlying  mass  and  will  yield  to  the  pres- 
sure, not  by  fracturing  but  by  flowing,  after  the  manner  of  wax. 
The  portion  of  the  crust  which  yields  to  pressure  by  fracturing  and 
in  which  fractures  consequently  exist  is  called  the  zone  of  fracture ; 
that  portion  of  the  crust  below  this,  in  which  the  rock  yields  by 
flowage,  is  called  the  zone  of  flow.  In  this  zone  cavities  are  absent 
and  the  particles  of  rock  occupy  the  minimum  space.  Since  rocks 
vary  greatly  in  strength,  it  is  evident  that  the  depth  to  which  the 
zone  of  fracture  extends  will  vary  with  the  rock,  and  that  conse- 
quently the  upper  surface  of  the  zone  of  flow  is  very  irregular. 
Between  the  zone  of  flow  and  the  zone  of  fracture  is  an  intermediate 
zone  in  which  the  soft,  incompetent  beds  flow,  and  the  hard  or  com- 
petent beds  fracture.  This  is  called  the  zone  of  flow  and  fracture. 

JOINTS 

Attention  has  been  called  to  the  division  planes  or  joints 
by  which  all  rocks  near  the  earth's  surface  are  more  or  less  broken 
into  angular  blocks  (p.  25).  This  structure  can  be  well  studied 
in  almost  any  quarry  or  cliff  (Figs.  I,  5,  85).  Joints  approach 

1  Adams,  F.  D.,  —  An  Experimental  Contribution  to  the  Question  of  the  Depth  of  the  Zone  of 
Flow  in  the  Earth's  Crust:  Jour.  Geol.,  Vol.  20,  1912,  p.  97. 


THE   STRUCTURE  OF  THE  EARTH 


259 


verticality  in  horizontal  rocks,  but  are  inclined  at  various  angles 
in  strata  which  have  been  folded.  In  horizontal  strata  it  is  usually 
found  that  two  vertical  systems  of  joints  are  present  at  right  angles 
to  each  other  (Fig.  I,  p.  23),  although  more  than  two  systems  are  often 
present.  Two  remarkable  features  of  the  joints  of  undisturbed, 
sedimentary  rocks  are  (i)  the  horizontal  extent  of  the  joints  which 
stretch  many  hundreds  of  feet  in  some  cases,  and  (2)  the  smoothness 
of  their  faces.  One  often  finds  that  the  calcareous  concretions  and 
even  the  quartz  pebbles  contained  in  some  beds  are  broken  in  two 
where  joints  cross  them,  with  faces  as  smooth  as  if  cut  by  a  saw,  and 
the  faces  of  joints  exposed  in  cliffs 
are  often  seen  to  be  as  smooth  as 
a  plastered  wall.  The  distance  be- 
tween joints  varies  from  a  fraction 
of  an  inch  to  many  yards. 

Joints  extend  to  considerable 
depths  but  cannot  exist  below  the 
zone  of  fracture.  They  frequently 
end  when  a  stratum  of  a  different 
character  is  reached  ;  for  example, 
a  joint  which  extends  through  a 
limestone  may  end  when  it  reaches 
a  shale.  In  such  case  other  joints 
of  a  different  interval  may  extend 
through  the  lower  stratum. 

Joints  are  taken  advantage  of  in 
quarrying,  but  if  they  are  very 
close  together  the  blocks  may  be 
too  small  for  building  purposes, 
and  if  too  far  apart  may  make  the 
profitable  quarrying  of  the  rock 
impossible.  Some  rich  ore  veins 
are  developed  in  joints  (p.  370). 

Origin  of  Joints.  — The  origin  of 
joint  planes  in  sedimentary  rocks 

is  not  fully  understood.  It  is  generally  believed,  however,  that  they 
are  the  result  of  movements  of  the  earth's  crust  and  have  been  pro- 
duced by  powerful  mechanical  stresses  and  strains  which  are  the  result 
either  (i)  of  torsion,  or  (2)  of  compression  brought  about  by  crustal 
movement.  A  suggestive  experiment  (Fig.  250),  in  which  plates  of 


FIG.  250.  —  A  plate  of  ice  fractured  by 
twisting  movement,  the  cracks  produced 
being  chiefly  nearly  at  right  angles  to 
each  other.  Some  joints  seem  to  have 
a  similar  origin.  (After  Daubree.) 


260 


PHYSICAL  GEOLOGY 


ice  and  of  other  brittle  substances  were  twisted  by  applying  force  at 
the  two  ends,  produced  cracks  at  right  angles  to  each  other,  running 
diagonally  from  the  corners. 

The  formation  of  columns  in  fine-grained,  igneous  rock  is  probably 
due  to  cooling  and  contraction,  as  has  been  explained  (see  also  p.  333). 

Effect  of  Joints  on  Topography.  —  The  influence  of  joints  in  de- 
termining the  courses  of  streams  is  especially  noticeable  in  small 
streams.  The  brooks  that  flow  into  the  lakes  of  central  New  York, 
for  example,  have  their  directions  determined  to  an  important  degree 
by  the  systems  of  joints  into  which  the  strata  are  broken.  The 
course  of  the  Zambezi  River  in  South  Africa  above  Victoria  Falls  is  a 


FIG.  251. — A  shore  whose  configuration  has  been  greatly  influenced  by  jointing. 
Holsteenborg,  West  Greenland.     (After  Kornerup.) 

remarkable  example  of  a  large  river  which  follows  joints  for  a  con- 
siderable distance.  For  many  miles  before  it  plunges  over  the  fall 
the  river  is  confined  in  a  narrow  gorge  whose  direction  is  a  series  of 
zigzags,  this  angular  course  being  determined  by  the  joints  of  the 
lava  plateau  in  which  the  gorge  is  cut.  The  contours  of  coasts 
sometimes  show  the  effect  of  jointing  by  the  existence  of  angular  bays 
and  promontories  (Fig.  251)  where  the  rock  is  unequally  jointed, 
permitting  the  waves  to  work  faster  in  some  portions  than  in  others 
(p.  207).  Joints  also  allow  of  more  rapid  work  by  weathering  (p.  29), 
more  rapid  broadening  of  stream  valleys  (p.  88),  the  circulation  of 
ground  water,  and  the  deepening  of  glacial  valleys  by  plucking 
(p.  157)-  In  limestone  regions  joints  are  often  widened  by  solution 
and  therefore  have  a  marked  effect  on  the  topography  of  such  districts. 


THE   STRUCTURE  OF  THE  EARTH 


261 


FAULTS 

When  beds  are  displaced  along  joints,  bedding,  or  fracture  planes, 
the  beds  are  said  to  be  faulted  (Fig.  252).  The  fracture  along  which 
the  movement  occurred,  usually 
is  not  an  open  crack.  It  some- 
times consists  of  a  single,  clean- 
cut  fracture,  but  more  often  of  a 
number  of  closely  parallel  frac- 
tures. A  fault  dies  out  at  its  ends 
and  varies  along  its  course  in  the 
amount,  direction,  and  character 
of  the  displacement.  The  fault 
line,  moreover,  is  not  always 


FIG.  252.  —  Normal  fault.  The  hori- 
zontal displacement  BC  is  the  heave;  the 
vertical  displacement  AB is  the  throw;  the 
angle  BAG  is  the  hade;  AC  is  the  slip. 


straight  and  single,  but  is  often 

irregular  and  in  many  cases  splits 

into  one  or  more  branches.     Faults  are  seldom  vertical,  but  when 

followed  downward  in  mines  are  usually  found  to  be  inclined,  the 

degree  of  inclination 
often  varying  from 
point  to  point.  The 
general  inclination  of 
a  fault  from  the  verti- 
cal is  called  the  hade 
(Fig.  252). 

There  are  three 
principal  types  of 
faults:  (i)  normal, 
(2)  reverse  or  thrust, 
and  (3)  vertical. 

Normal  or  Gravity 
Faults.  —  A  normal 
fault  (Figs.  252,  253) 
is  the  simplest  type. 
In  faults  of  this  class 
it  is  convenient  to 
consider  one  side  as 

FIG.  253.  —  Faults  in  sand,  Adirondacks.     Since  the  having  moved    down 
faulting,  lateral  movement  has  occurred  and  has  changed  •      r       j    r 

the  direction  of  the  fault  planes  so  that   the  footwall  an  mchned   fracture, 

seems  to  be  on  the  downthrow  side.  and  the  Other  to  have 


262 


PHYSICAL  GEOLOGY 


remained  stationary  or  to  have  moved  up.  The  vertical  displace- 
ment or  the  vertical  distance  between  the  ends  of  a  dislocated 
stratum  is  called  the  throw;  the  horizontal  displacement,  the  heave. 
The  distance  a  stratum  has  moved  on  the  fault  surface  is  indicated 
by  the  term  slip.  The  upthrow  side  is  the  one  in  which  the  beds 
lie  at  a  higher  level  than  their  continuation  on  the  opposite  or 


FIG.   254.  —  Section  across  Yarrow  Colliery,  England,  illustrating  the  law  of  normal 
faults.     The  surface  is  restored.     (After  De  la  Beche.) 

downthrow  side  of  the  fault.  These  terms  are  used  without  refer- 
ence to  the  actual  direction  of  movement.  The  wall  of  the  down- 
throw side  is  called  the  hanging  wall,  and  the  opposite  wall,  the  foot- 
wall.  These  last  two  terms  originated  with  miners,  since  in  mining 
along  a  fault  the  overhanging  side  was  naturally  spoken  of  as  the 
"  hanging  wall,"  while  the  side  of  the  fault  upon  which  they  stood  was 
called  the  "  footwall."  The  fact  that  the  hanging  wall  in  normal 

faults  has  moved  down  relative  to  the 
footwall  is  utilized  in  coal  mining. 
For  example,  if  a  seam  of  coal  sud- 
denly disappears  at  a  fault,  the  fault 
plane  is  followed  down  or  up,,  depend- 
ing upon  the  inclination  or  hade  of 
the  fault  surface  (Fig.  254).  Normal 
faults  sometimes  shade  into  mono- 
clines (Fig.  248,  p.  256). 

Examples  of  Normal  Faults.  — 
The  earth's  crust  is  more  or  less 
broken  by  faults;  in  some  cases  the 
faulting  has  taken  place  so  widely 

and  the  various  systems  of  faults  ex- 

.  .          .  . 

tend  in  so  many  directions  that  a 

geological  map  of  the  region  has  the 

appearance  of  a  mosaic  (Fig.  255).  The  Colorado  Plateau  (Fig. 
256)  has  been  broken  by  normal  faults,  some  of  which  can  be  followed 
for  hundreds  of  miles  and  have  a  vertical  displacement  (throw)  of 
several  thousand  feet.  In  traveling  along  the  Mohawk  valley  be- 


f  IIGT  255'  7  Mosaic  A-  the  rM 

or  the  lonopah  mining  district,  Nevada, 
produced  by  faulting.    (After  Spurr.) 


THE   STRUCTURE  OF  THE  EARTH 


263 


tween  Albany  and  Little  Falls  one  passes  over  nine  easily  recognized 
faults  and  probably  over  many  smaller  ones. 

The  term  graben  (Fig.  257)  is  used  for  a  portion  of  the  earth's 
crust  bounded  by  faults,  which  is  depressed  relative  to  the  surround- 


HORST 


FIG.  256.  —  A  section  from  east  to  west  across  the  plateau  north  of  the  Grand 
Canyon,  with  a  bird's-eye  view  of  the  surface.  The  effect  on  the  topography  of  the 
great  faults,  folds,  and  monocline  are  well-shown.  (Powell.) 

ing  masses,  and  the  term  horst  for  a  section  which  is  elevated  relative 
to  the  surrounding  masses  and  separated  from  them  by  faults.  The 
Rhine  valley,  between  Basle  and  Mainz,  is  an  excellent  example  (Fig. 
81,  p.  100)  of  a  graben. 
Here  the  valley  is  limited 
by  parallel  faults,  the 
Black  Forest  lying  on  the 
east  and  the  Vosges  on  the 
west.  The  valley  occupied 

by  the  Jordan  River  and    ^  _.    , 

r  T>   i        •  rlG.  257. —  block  diagram  showing  the  appearance 

the  Dead  bea  ot  Palestine  and  origin  of  a  graben  and  horst> 

is  a  graben  in  which  a  fault 

block  has  sunk  2600  feet  below  the  level  of  the  plateau,  depressing  it 

in  places  below  the  level  of  the  sea. 

Reverse  or  Thrust  Faults.  —  In  reverse  faults  the  hanging  wall 
should  be  considered  as  having  moved  up,  and  we  thus  find  that 
instead  of  a  stratum  being  separated  as  a  result  of  the  faulting, 
the  two  ends  overlap  so  that  the  older  beds  are  pushed  over  the 


A  B  C 

FIG.  258.  —  A  fold  passing  into  a  thrust  fault.     (Heim.) 


264  PHYSICAL  GEOLOGY 

younger  ones  (Fig.  258  C).  The  result  of  thrust  faulting  in  the  Selkirks 
of  Canada  and  in  many  other  regions  has  been  to  move  strata  over  and 
upon  others  that  were  formerly  hundreds  of  feet  higher.  Occasion- 
ally, recumbent  folds  can  be  traced  into  thrust  faults  (Fig.  258  A,  B). 
This  is  not  surprising,  since  it  is  evident  that  both  are  due  to  lateral 
pressure ;  the  movement  being  so  great  in  the  latter  that  the  strain 
could  not  be  relieved  without  breaking.  Thrust  or  reverse  faults 
involve  a  shortening  of  the  earth's  crust,  as  has  been  said,  and  differ 
in  this  respect  from  normal  faults  which  are  the  result  of  a  stretching 
of  the  crust.  The  fault  plane  usually  approaches  the  horizontal  more 
nearly  in  thrust  faults  than  in  normal  faults;  that  is,  the  hade  of 
the  former  is  greater  than  that  of  the  latter. 

Examples  of  Thrust  Faults.  —  Many  examples  of  thrust  faults 
might  be  cited.  A  great  thrust  fault  (Bannock  overthrust) l  extends 
approximately  270  miles  from  northern  Utah  into  Idaho,  in  which 
the  older  strata  have  slid  over  the  younger  (horizontal  displacement) 


FIG.  259.  —  Thrust  faults  and  folds  in  the  southern  Appalachians. 

a  distance  of  at  least  12  miles.  In  the  same  region  other  faults  with 
a  throw  of  15,000  to  20,000  feet  have  been  described.  In  Massa- 
chusetts,2 a  thrust  fault  has  been  described  in  which  the  strata  have 
slid  along  a  fault  surface  for  15  miles.  The  southern  Appalachian 
Mountains  (Fig.  259)  are  broken  by  many  faults  that  run  parallel  to 
the  system.  In  Virginia  and  Georgia  15  or  more  parallel  thrust  faults 
occur,  running  from  northeast  to  southwest,  along  which  the  older 
strata  have  been  pushed  over  the  younger.  One  of  these  faults 
has  been  traced  for  375  miles,  and  its  greatest  horizontal  displacement 
is  at  least  n  miles. 

Vertical  and  Horizontal  Faults.  — *  If  faulting  has  taken  place 
along  a  vertical  (90°)  joint  or  other  fracture,  the  arrangement  of  the 
strata  may  give  the  appearance  at  the  surface  of  either  a  normal 
or  a  reverse  fault,  depending  upon  whether  the  right  or  left  side 
moved  down  (Fig.  260).  In  many  cases,  along  the  same  side  of  a 
given  fault  line  the  movement  may  have  been  upward  in  one  place, 
downward  in  another,  and  without  evident  movement  at  another. 
In  some  cases,  a  fault  occurs  along  bedding  planes  and  is  called  a 

1  Richards  and  Mansfield,  —  Jour.  Geol.,  Vol.  20, 1912,  pp.  681-709.  2  J.  Barrell. 


THE  STRUCTURE  OF  THE  EARTH 


265 


bedding  fault.  It  should  not  be  understood  from  the  foregoing  that 
the  movement  in  faults  is  always  merely  up  or  down.  Often  the 
movement  has  both  a  vertical  and  a  horizontal  component,  and  occa- 


FIG.  260.  —  Diagrams  showing  A,  horizontal  strata ;  B,  the  strata  displaced  by 
a  vertical  fault;  and  C,  the  fault  scarp  obliterated  by  erosion. 

sionally  the  vertical  movement  is  inconsiderable  and  the  horizontal 
important.  This  was  true  of  the  fault  that  caused  the  San  Francisco 
earthquake  (p.  275),  and  it  has  been  observed  in  mines,  where  the 


A  B  C 

FIG.  261.  —  Diagram  illustrating  a  dip  fault  and  the  effect  of  erosion  upon  the  outcrop. 

faulted  surfaces  can  be  studied,  that  evidences  of  horizontal  move- 
ment are  more  often  met  with  than  those  of  vertical. 

Influence  of  Faults  on  Topography.  —  If  faulting  did  not  take  place 
long  ago,  it  is  evident  that  a  cliff  or  fault  scarp  will  be  present,  the 


B 


FIG.  262.  —  Diagrams  showing  the  effect  of  an  oblique  fault  upon  dipping  beds, 
and  the  outcropping  of  the  stratum  on  the  surface  after  the  fault  scarp  had  been 
planed  by  erosion. 


266 


PHYSICAL   GEOLOGY 


height  and  prominence  of  which  will  depend  upon  the  amount  of 
the  faulting  and  its  recency  (Figs.  261,  262,  263).  Fault  scarps  are 
also  formed  when  weak  and  resistant  rocks  are  brought  into  contact 


FIG.  263.  —  The  effect  of  faulting  on  the  outcrops  of  anticlinal  and  synclinal 
beds  before  and  after  the  erosion  of  the  fault  scarp. 

by  faulting.  The  weak  beds  are  worn  away  more  rapidly  than  the 
hard,  so  that  after  prolonged  erosion  the  latter  may  form  cliffs,  even 
though  they  are  on  the  downthrow  side  (Fig.  264).  The  original 
fault  scarp  is  usually  soon  eroded  to  such  an  extent  that  the  con- 

A.__    B     figuration   of  the    land   gives 

little  or  no  indication  of  the 
existence  of  a  fault.  When, 
however,  a  resistant  bed  such 
as  a  sill  of  lava  (p.  326)  or 
a  quartzite  stratum  is  present 
—?''  ~  and  is  exposed  by  erosion,  the 

I  '  —  L     fault  will  again  be  the  indirect 

FIG.  264.  — Diagram  showing  a  cliff  BAG  cause  of  a  cliff  (Fig.  ^264).      In 

formed  by  faulting.    As  erosion  wore  away  the  Scotland    the     relatively    soft 

weaker  rock  a  cliff  resulted  from  the  presence  rQcks  of  the  central  lowlands 

of  the  strong  bed  CD.     Upon  further  erosion  .  .  .  . 

the  bed  GF  disappeared  and  the  cliff  F  was  have  been  brought  up  against 

formed.  the   relatively   hard   rocks  of 


THE  STRUCTURE  OF  THE  EARTH 


267 


the  highlands,  producing  a  strong  line  of  demarcation  (Fig.  265). 
Such  a  topographic  form  is  called  a  fault-line  scarp  or  cliff. 

Sometimes  the  topography  of  a  region  is  determined  largely  by 
faulting,  resulting  in  steplike  hills  or  mountains.  This  is  well  shown 
in  a  portion  of  Oregon  (Fig.  266),  in  the  Great  Basin  region  of 


FIG.  265.  —  Fault  in  the  Scottish  Highlands.     The  strata  on  the  right  are 
weaker  than  those  on  the  left. 

Utah  (p.  354),  the  Colorado  Plateau,  and  the  Connecticut  valley 
of  Massachusetts  and  Connecticut.  In  central  Sweden  the  criss- 
cross valleys  and  lakes  of  angular  and  zigzag  outline  are,  either 
directly  or  indirectly,  due  to  the  faulting  of  rhomboidal-shaped  blocks. 
The  greatest  example  of  faulting  now  apparent  in  the  topography  of 
the  continents  is  the  Great  Rift  valley  of  east  Africa,  which  consists 


FIG.  266.  —  Diagram  of  a  mountain  formed  by  faulting.     The  slope  of  the  surface  of 
the  original  block  is  shown  in  outline.     (Modified  after  Davis.) 

of  a  series  of  grabens  (p.  263)  in  the  bottoms  of  which  lakes  (Albert, 
Tanganyika,  etc.)  are  aligned.  The  magnitude  of  the  displacement 
is  indicated  by  the  depth  of  Lake  Tanganyika,  which  is  nearly  4190 
feet,  the  bottom  of  the  lake  lying  1600  feet  below  sea  level.  This 
great  rift  extends  from  Abyssinia  southward  for  some  1500  miles. 
In  the  Adirondacks  of  New  York  the  "  latticed  drainage  "  is  to  a 


268 


PHYSICAL  GEOLOGY 


great  extent  produced  by  faulting  which  caused  the  streams  to  flow 
in  fault  valleys. 

Minor  Features  of  a  Fault  Fracture.  —  When  a  fault  fracture  is 
visible,  it  is  often  found  that  it  is  represented  by  a  zone  of  angular 

rock  fragments,  often 
cemented  together  to 
form  a  fault  or  crush 
breccia  (Fig.  357), 
sometimes  several 
yards  wide.  Some 
important  gold  and 
silver  deposits  occur 
in  the  filling  of  such 
breccia  (p.  371). 
When  the  breccia  is 
very  resistant,  as  is 
the  case  when  the 
filling  is  quartz,  it 
may  stand  in  relief 
after  the  surrounding 
strata  are  denuded, 
resembling  a  dike. 
The  side  of  a  fault  surface  is  often  polished  and  striated  by  the  move- 
ment of  the  walls,  so  that  it  is  possible  to  tell  the  direction  of  the 
movement  from  the  striations.  Such  a  surface  is  called  a  slickenside 
(Fig.  267).  It  resembles  a  glaciated  surface  but  is  usually  more  glazed. 
Detection  of  Faults.  —  Topography,  as  has  been  seen,  is  not  al- 
ways a  safe  guide  for  the  detection  of  faults,  since  their  presence  is 
not  always  indicated  by  cliffs. 
The  most  satisfactory  evi- 
dence of  a  fault  is  to  be  ob- 
tained when  a  geological  map 
of  a  region  is  made,  and  it  is 
found  that  all  of  the  neigh- 
boring formations  end  upon  a 
more  or  less  straight  line.  A  sudden  change  in  the  soil  which  is  made 
apparent  in  the  degree  of  fertility  of  neighboring  fields,  and  the  pres- 
ence of  rapids  in  streams  which  cross  a  fault  are  also  indicative  of  the 
presence  of  a  dislocation.  Springs  often  occur  along  fault  lines,  and 
when  a  number  of  them  are  aligned  they  indicate,  but  do  not  prove 


FIG.  267.  —  A  slickenside  surface  formed  by  lateral 
movement  along  a  fault  plane.  The  rock  of  one  side  of 
the  fault  was  removed.  (U.  S.  Geol.  Surv.) 


FIG.  268.  —  Diagram  showing  "drag  dip"  near 
a  fault.     (Modified  after  W.  N.  Rice.) 


THE   STRUCTURE  OF  THE  EARTH 


269 


the  presence  of  a  fault.  Dikes  of  lava  also  sometimes  occur  in  fault 
fractures.  In  regions  of  horizontal  rocks  faults  can  often  be  traced 
along  the  surface  some  distance  from  the  fault  line  (p.  277)  by  the 
upturned  edges  of  the  strata  (drag  dip),  produced  by  the  friction 
along  the  fault  surface  of  the  edges  of  the  strata  during  the  faulting 
(Fig.  268). 

Origin  of  Faults.  —  In  seeking  an  explanation  for  normal  faults 
the  fractures  along  which  the  movements  occurred  must  first  be 
found.  These  are  often  joints  which,  as  has  been  seen,  have  probably 
been  formed  by  tortional  strains  (p.  259).  Normal  faults  indicate  a 
stretched  condition  of  the  crust,  which  permitted  the  unequal  settling 
of  blocks  bounded  by  joints  or  other  fractures.  In  the  formation  of 
grabens  and  horsts  (Fig.  257,  p.  263)  there  appears  to  have  been  first 
a  compression  which 


arched     the     strata, 
followed  by  a  relief  of 


the    pressure,   which      ^^§^^^ 


permitted     the     set- 
tling of  the  blocks  of 
the    arch.      Reverse 
faults    are    evidently     FIG.  269.  —  Diagram  showing  a  fault  shading  into  a  fold. 
the  result  of  lateral  (After  De  Martonne-) 

pressure  and  are  best  developed  in  highly  folded  and  distorted 
rocks.  Thrust  faults  often  pass  horizontally  into  folds,  and  verti- 
cally they  sometimes  shade  gradually  into  more  or  less  gentle 
folds  (Fig.  269).  The  origin  of  the  force  which  produced  folding 
will  be  taken  up  more  fully  under  the  discussion  of  the  formation 
of  mountains. 

Rapidity  of  Fault  Movements.  —  At  irregular  intervals  dislocations 
of  from  a  fraction  of  an  inch  to  20  or  more  feet  have  taken  place  in  a 
few  minutes  along  fault  planes.  In  Owen's  valley,  California,  in 
1872,  a  slipping  occurred  along  a  line  40  miles  long  which  resulted  in  a 
throw  of  from  5  to  20  feet.  Along  a  fault  50  miles  in  length  a  dis- 
placement of  as  much  as  30  feet  occurred  in  Japan  in  1891.  Such 
faulting  always  produces  earthquakes  (p.  281).  In  some  mines 
faulting  is  observed  to  be  taking  place  continually,  but  at  a  slow  rate. 
The  total  displacement  resulting  from  such  movements,  if  long-con- 
tinued, will  necessarily  be  great,  but  no  surface  features  may  mark  its 
presence,  since  erosion  may  cut  away  the  upthrow  side  as  fast  as  it  is 
formed. 


270 


PHYSICAL  GEOLOGY 


CONFORMITY  AND  UNCONFORMITY 

When  strata  are  deposited  one  upon  another  in  unbroken  succes- 
sion and  without  disturbance,  they  are  said  to  be  conformable  (Fig. 
I,  p.  23).  When,  however,  one  set  of  beds  is  deposited  on  another  which 
has  been  above  the  sea  and  there  eroded,  the  two  beds  are  said  to  be 
unconformable,  and  an  unconformity,  represented  by  the  erosion  sur- 
face which  separates  the  strata,  is  said  to  exist.  Unconformities  may 
exist  between  stratified  and  igneous  or  metamorphic  rocks  (Fig.  322, 


FIG.  270.  —  A  diagram  showing  an  unconformity  or  erosion  interval,  AB. 

p.  326).  Usually,  an  unconformity  is  marked  by  some  change 
in  the  relative  dip  of  the  beds,  one  set  resting  upon  the  upturned 
edges  of  an  older  series  (Fig.  270).  Such  an  unconformity  is  spoken 
of  as  an  angular  unconformity,  because  the  strata  below  the  uncon- 
formity meet  those  above  it  at  an  angle.  An  unconformity,  however, 
sometimes  occurs  between  strata  which  have  not  suffered  any  rela- 
tive change  in  dip  (Fig.  271) ;  i.e.,  both  the  younger  and  the  older 


FIG.  271.  —  Diagram  showing  unconformities.  The  stratum  C  is  separated  from 
the  strata  B  and  D  by  unconformities.  The  unconformity  between  B  and  C  is  not 
readily  noticeable,  because  the  strata  of  both  are  horizontal.  An  unconformity  exists 
also  between  the  glacial  drift  A  and  the  limestone  B.  (Near  Milwaukee,  Wisconsin.) 

strata  may  be  horizontal.  In  such  cases  the  unconformity  can 
usually  be  recognized  by  old  erosion  channels  or  by  basal  conglom- 
erates (p.  240),  but  occasionally  such  a  break  is  difficult  to  detect. 
Importance  of  Unconformities.  —  Unconformities  are  of  much 
importance  in  the  study  of  the  geology  of  a  region,  because  of  the 
geological  history  which  they  reveal.  From  the  unconformity 


THE  STRUCTURE  OF  THE  EARTH 


271 


in  Fig.  272  we  learn  (i)  that  there  was  a  long  period  of  quiet,  during 
which  the  lower  series  of  beds  were  deposited  continuously  on  the 
ocean  bottom.  This  was  followed  (2)  by  a  period  of  folding  or  tilting 
and  of  elevation,  so  that  these  beds  were  raised  above  the  level 
of  the  sea.  (3)  The  strata  were  then  subjected  to  erosion  for  such 
a  long  period  that  the  upturned  edges  were  worn  to  a  peneplain. 


FIG.  272.  —  An  unconformity  in  which  the  lower  beds  are  inclined,  while 
the  upper  are  horizontal.     Wyoming.     (U.  S.  Geol.  Surv.) 

How  long  this  erosion  interval  lasted  cannot  be  told  from  any  one 
geological  section.  (4)  The  land  was  later  depressed  and  became 
sea  bottom,  so  that  (5)  sediment  was  laid  down  on  the  old  land  sur- 
face. (6)  Reelevation  again  converted  the  sea  bottom  into  land, 
and  streams  are  now  at  work  carrying  the  rock  back  to  the  sci. 

Overlap  is  a  term  used  in  describing  an  unconformity  in  which  the 
younger  strata  cover  a  larger  area  than  the  older  ones  and  conse- 


FIG.  273.  —  Diagram  showing  the  overlapping  of  the  strata  AEDC  due  to  the  encroach- 
ment of  the  sea  upon  the  ancient  land  surface  AC.     (Modified  after  Grabau.) 

CLELAND    GEOL.  —  1 8 


272  PHYSICAL  GEOLOGY 

quently  overlap  the  older  strata  (Fig.  273).  They  were  deposited 
when  the  water  in  which  they  were  laid  down  had  a  greater  extent 
than  it  had  when  the  older  strata  were  deposited. 


CONSTITUTION  OF  THE  EARTH'S  INTERIOR 

The  ancient  Greeks  and  Romans  in  speculating  about  the  under- 
world came  to  the  conclusion  that  its  heat  and  other  igneous  phe- 
nomena were  due  to  the  work  of  imprisoned  giants.  Our  present 
theories  are  less  fanciful,  but  because  of  the  inaccessibility  of  the 
deeper  portions  of  the  earth's  interior  and  our  failure  to  reproduce 
the  conditions  there,  our  knowledge  of  its  constitution  is  far  from 
satisfactory. 

Zone  of  Variable  Temperature.  —  The  temperature  at  the  surface 
of  the  earth  is  variable  because  of  the  changes  in  daily  and  seasonal 
temperature,  but  at  a  depth  which  in  Java  and  India  is  about  12  feet 
and  in  New  York  about  50  feet  the  temperature  is  constant  through- 
out the  year. 

The  Interior  Heat  of  the  Earth.  —  Below  the  level  where  seasonal 
change  in  temperature  occurs,  the  temperature  increases  with  the 
depth.  This  fact  has  been  determined  by  well  borings,  by  tunnels, 
and  by  mines,  but  since  the  deeper  borings  are  only  a  little  more  than 
a  mile  in  length,  the  statements  in  regard  to  the  rate  of  increase  at 
great  depths  must  necessarily  be  theoretical.  The  rate  of  increase 
is  usually  estimated  at  i°  F.  for  60  to  75  feet,  but  it  varies  so  widely  at 
different  places  that  to  strike  an  average  is  difficult.  In  the  St. 
Gothard  tunnel  (Italy  to  Switzerland)  it  is  i  °  F.  for  82  feet ;  at  Calumet, 
Michigan,  the  average  for  4939  feet  is  i°  F.  for  103  feet ;  in  the  British 
Isles  it  varies  from  i°  F.  for  every  34  feet  to  i°  F.  for  every  130  feet. 
A  boring  in  West  Virginia  to  a  depth  of  5386  feet  showed  an  increase 
of  one  degree  for  80  or  90  feet  for  the  upper  half,  and  of  one  degree 
for  60  feet  in  the  lower  half.  Near  Leipzig,  Germany,  a  boring  5560 
feet  deep  showed  an  average  of  one  degree  for  56  feet.  In  South 
Dakota  the  artesian  wells  show  a  depth  of  from  17.5  to  45  feet  for 
each  degree.  The  rapid  increase  shown  in  South  Dakota,  however, 
is  probably  due  to  folding  in  the  water-bearing  stratum.  The 
presence  of  hot  igneous  bodies  would  also  increase  the  tempera- 
ture gradient. 

Since  the  temperature  gradient  follows  the  surface  configuration, 
the  level  of  the  Simplon  tunnel  which  connects  Switzerland  and 


THE  STRUCTURE  OF  THE  EARTH 


273 


Italy  was  made  higher  than  the  grade  of  the  railroad  required  in  order 
that  a  too  great  temperature  might  be  avoided,  but  even  with  this 
precaution  the  heat  in  some  portions  was  so  intense  as  almost  to 
stop  the  work  of  excavation. 

The  heat  of  the  interior  of  the  earth  is  known  also  from  lavas 
which  reach  the  surface  through  volcanoes  and  fissures  (p.  299). 
Copper  wire,  which  melts  at  about  2200°  F.,has  been  fused  when  thrust 
into  molten  lava.  In  one  case,  where  a  lava  stream  from  Vesuvius 
overflowed  a  village,  brass  was  decomposed  into  its  component 
metals.  It  is  estimated  that  the  initial  temperature  of  lava,  when  it 
issues  from  Vesuvius,  is  probably  more  than  2000°. 

THEORIES   OF   THE    PHYSICAL   STATE   OF   THE    EARTH'S   INTERIOR 

If  the  temperature  of  the  earth  increased  uniformly  from  the 
surface  downward  at  a  rate  of  one  degree  for  every  60  feet  of  descent, 
a  temperature  of  3000  degrees  would  be  reached  at  a  depth  of  about 
34  miles.  Such  a  temperature  would  be  sufficient  to  melt  all  but  the 
most  infusible  rocks  under  the  conditions  existing  at  the  surface  of  the 
earthy  but  since  the  rocks  at  this  depth  are  subjected  to  the  enormous 
pressure  of  the  overlying  rocks,  the  conditions  are  very  different. 

(i).  Internal  Fluidity  Theory.  —  Based  on  the  assumption  that  the  heat  increases 
regularly  from  the  surface  downward,  it  has  been  held  that  the  earth  is  a  molten  globe 
covered  with  a  thin  crust  25  to  30  miles  thick.  Although  the  belief  in  a  molten  in- 
terior has  a  wide  popular  acceptance,  there  are  a  number  of  serious  objections  to  it. 

(a)  Effect  of  Increasing  Density.  —  The  rate  of  increase  in  temperature  is  probably 
not  uniform,  but  diminishes  with  the  depth.     This  is  evident  from  the  fact  that  the 
average  specific  gravity  of  the  rocks  of  the  earth's  surface  is  about  2.8,  while  ^hat  of 
the  earth  as  a  whole  is  5.5,  so  that  the  specific  gravity  of  the  central  portion  must  be 
at  least  equal  to  iron,  which  is  7.7,  and  is  probably  higher.     This  increase  in  density 
is  due  to  some  extent  at  least  to  the  great  pressure  of  the  overlying  rocks,  but  may 
also  be  due  to  the  concentration  of  heavy  metals  within  the  center  of  the  earth.     It 
has  been  suggested  that  metallic  iron  is  to  be  found  in  greater  quantity  there  than  on 
the  surface.     But  whatever  the  cause,  the  effect  of  increasing  density  would  be  to 
increase  the  conductivity  of  the  rock,  so  that  instead  of  a  uniform  increase  of  one 
degree  for  each  50  or  60  feet  of  descent,  at  a  greater  depth  70  feet  might  be  required 
for  a  change  of  one  degree,  then  80  feet,  then  100  feet,  and  so  on.     The  increasing  con- 
ductivity of  the  rock  alone  would,  consequently,  carry  the  temperature  necessary  to 
fuse  rocks  much  deeper  than  30  miles.. 

(b)  Effect  of  Pressure.  —  Another  serious  objection  to  the  theory  of  a  molten  in- 
terior is  to  be  found  in  the  effect  of  pressure  on  the  melting  point  of  rocks.     Since  nearly 
all  substances  expand  upon  melting,  they  remain  solid  when  subjected  to  pressure 
which  prevents  expansion,  and  in  order  to  melt  them  it  is  necessary  to  raise  the  tem- 


274  PHYSICAL  GEOLOGY 

perature  so  high  as  to  overcome  the  effect  of  the  pressure.  It  is  evident  for  this  reason 
that  it  would  be  necessary  to  go  deeper  to  find  the  fluid  interior  than  if  the  pressure  of 
the  overlying  rocks  was  slight.  At  a  depth  of  50  miles  a  temperature  of  3500°  F., 
though  sufficient  to  melt  almost  any  rock  at  the  surface,  might  not  be  high  enough  to 
overcome  the  enormous  weight  of  the  overlying  rocks,  and  since  a  still  greater  pressure 
is  encountered  at  the  increased  depth  necessary  to  obtain  a  higher  temperature,  it  is 
evident  that  the  melting  point,  for  this  cause  alone,  might  never  be  reached.  t 

(c]  Rigidity  of  the  Earth.  —  Two  further  objections  to  this  theory  have  caused  its 
abandonment  by  scientists.  The  first  of  these  is  that  the  earth  is  not  pulled  out  of 
shape  by  the  attraction  of  the  moon  and  sun,  as  would  be  the  case  if  it  were  substantially 
a  molten  globe.  On  the  contrary  it  is  shown  to  be  more  rigid  than  glass  or  steel. 
The  second  objection  consists  in  the  fact  (Milne)  that  the  velocity  and  character  of 
earthquake  waves  (p.  283)  suffer  an  abrupt  change  at  a  depth  of  about  30  miles,  being 
transmitted  at  a  more  rapid  rate  below  this  level  than  on  the  crust,  showing  that  the 
nucleus  is  more  rigid  than  the  overlying  rocks. 

(2)  Solid  Interior.  —  The  second  theory,  as  has  already  been  indicated,  is  that 
the  earth  is  substantially  a  solid  because  of  the  increasing  conductivity  and  pressure. 

(3)  Gaseous  Center.  —  Upon  the  assumption  that  below  a  depth  of  190  miles 
the  temperature  of  the  earth  is  at  the  critical  temperature  of  all  substances  (the  tem- 
perature above  which  a  substance  can  exist  only  in  a  gaseous  state),  it  is  held  that  the 
solid  crust  passes  into  a  liquid  zone,  which  in  turn  passes  gradually  to  a  gaseous  magma. 
(Arrhenius.)     The  gaseous  magma  is  potentially  but  not  actually  a  fluid  with  a  tem- 
perature above  the  fusion  point  of  all  substances.     The  rigidity  of  the  earth,  according 
to  this  theory,  may  nevertheless  be  greater  rather  than  less  than  that  of  steel. 

(4)  Radioactivity  and  a  Solid  Center.  —  A  fourth  theory  is  in  direct  contradiction 
to  the  preceding  and  holds  that  the  temperature  of  the  interior  is  derived  from  the 
heat  given  off  by  the  radioactive  minerals  of  the  earth's  crust.     According  to  this  theory 
a  radioactive  crust,  30  to  45  miles  thick,  supplies  all  of  the  heat  for  the  interior  of  the 
earth,  and  below  a  depth  of  about  45  miles  the  earth  has  a  temperature  of  only  about 
1560°  C.     (Strutt.)     So  many  elements  of  doubt  enter  into  the  above  theory  that  it 
should   merely   be   considered  as   suggestive,   although   all   theories   must   take  into 
account  the  enormous  amount  of  heat  generated  in  this  way. 

(5)  Subcrust  Theory.  —  Another  theory   which    has    been  generally   abandoned 
holds  that  between  the  solid  crust  and  the  solid  center  is  a  fused  or  semifused  layer. 

Summary.  —  Any  hypothesis  of  the  constitution  of  the  earth's 
interior  which  is  in  accord  with  the  known  facts  must  hold  (i)  that 
the  earth  is  a  globe  which  increases  in  density  from  the  surface 
toward  the  center;  (2)  that  the  temperature  of  the  interior  is  in- 
tensely hot,  perhaps  20,000°  C.  at  the  center,  or  even  higher;  (3)  that 
the  rigidity  of  the  earth  as  a  whole  is  greater  than  that  of  steel. 

REFERENCES   FOR   STRUCTURE  OF  THE   EARTH 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  I,  2d  ed.,  pp.  486-589. 

GEIKIE,  J.,  —  Structural  and  Field  Geology,  3d  ed.,  1912. 

LEITH,  C.  K.,  —  Structural  Geology,  1913. 

REID,  H.  F.,  —  Bull.  Geol.  Soc.  America,  Vol.  24,  1913,  pp.  163,  186. 


CHAPTER  VIII 
EARTHQUAKES 

EARTHQUAKES  or  tremblings  of  the  earth's  surface,  when  severe, 
are  the  most  terrifying  phenomena  of  nature,  with  the  possible  ex- 
ception of  violent  volcanic  eruptions.  The  tremblings  of  the  earth 
vary  greatly  in  their  intensity,  from  those  which  cause  great  destruc- 
tion of  life  and  property  to  those  which  can  be  detected  only  by  deli- 
cate instruments.  Although  severe  earthquakes  occur  only  at 
irregular  intervals,  specially  constructed  instruments  called  seismo- 
graphs (Greek,  seismos,  earthquake,  and  graphein,  to  write)  show  that 
the  earth  is  never  free  from  minor  vibrations.  Such  minor  trem- 
blings are  produced  by  water  waves,  by  changes  in  atmospheric  pres- 
sure, by  readjustments  due  to  the  lightening  of  the  earth's  surface 
by  erosion  and  its  weighting  by  sedimentation,  by  the  strains  pro- 
duced by  the  attraction  of  the  moon  and  sun,  and  in  other  ways. 
Destructive  earthquakes  are,  however,  of  only  occasional  occur- 
rence and  arise  from  disturbances  within  the  earth's  crust  (p.  281). 

The  San  Francisco  Earthquake.  — The  earthquake  which  in  1906 
shook  California  and  wrought  such  havoc  in  San  Francisco  was  the 
most  disastrous  to  property  of  any  in  North  America  within  historic 
times,  although  the  loss  of  life  was  slight.  Much  of  the  destruction, 
however,  was  due  to  the  fires  which  were  started  as  a  result  of  the 
shocks,  and  to  the  breaking  of  the  water  mains  of  the  city,  which  made 
it  impossible  to  extinguish  the  flames. 

The  shock  came  without  warning,  as  is  usually  true  of  great  earth- 
quakes, and  lasted  less  than  one  minute.  It  was  followed  by  others 
of  less  intensity  during  the  day  and  for  several  weeks  afterwards. 
Where  the  shock  was  severe  trees  were  injured,  —  some  being  broken 
off,  some  overturned,  and  some  split  from  the  ground  upward  ;  build- 
ings were  shifted  horizontally  and  often  badly  broken;  animals  were 
thrown  from  their  feet  and  persons  from  their  beds.  It  was  found 
that  the  greatest  intensity  of  the  shock  was  along  a  fault  line  (p.  267), 
and  that  in  general  the  violence  diminished  with  distance  from  the 

275 


276 


PHYSICAL  GEOLOGY 


fault  on  either  side.  "  The  rate  of  diminution,  with  the  exceptions 
to  be  mentioned  presently,  may  be  expressed  by  saying  that  at  five 
miles  from  the  fault  only  a  few  men  and  animals  were  shaken  from 
their  feet,  only  a  few  wooden  houses  were  moved  from  their  founda- 
tions, about  half  the  brick  chimneys  remained  sound  and  in  condition 
for  use,  sound  trees  were  not  broken,  and  no  cracks  were  opened  which 
did  not  immediately  close.  At  a  distance  of  twenty  miles  only  an 
occasional  chimney  was  overturned,  the  walls  of  some  brick  build- 
ings were  cracked,  and  wooden  buildings  escaped  without  injury; 


FIG.  274.  —  The  slipping  of  alluvial  soil  toward  the  Salinas  River,  as  a  result 
of  the  San  Francisco  earthquake. 

the  ground  was  not  cracked,  landslides  were  rare,  and  not  all  sleepers 
were  wakened.  At  seventy-five  miles  the  shock  was  observed  by 
nearly  all  persons  awake  at  the  time,  but  there  were  no  destructive 
effects ;  and  at  two  hundred  miles  it  was  perceived  by  only  a  few 
persons."  l  The  exceptions  to  the  gradual  diminution  of  intensity 
occurred  on  the  artificially  filled  or  "  made  "  land  in  the  city,  and 
where  there  were  tracts  of  deep  alluvial  soil,  especially  where  ground 
of  this  character  was  saturated  with  water  (Fig.  274).  Such  ground 
behaved  during  the  earthquake  very  much  like  "  jelly  in  a  bowl," 
the  sudden  shock  causing  it  to  be  thrown  into  waves. 

1  G.  K.  Gilbert. 


EARTHQUAKES 


277 


This  earthquake  was  the  result  of  shocks  produced  by  renewed 
slipping  along  an  old  fault  line  which  had  been  known  to  geologists 
for  some  time.  The  "  earthquake  topography  "  of  such  a  fault  line 


FIG.  275.  —  Map  of  the  fault  line  formed  during  the  San  Francisco  earthquake  in 
1906.  It  was  the  vibrations  set  up  by  the  faulting  along  this  line  that  produced  the 
earthquake.  (U.  S.  Geol.  Surv.) 

is  described  on  page  265.  The  fault  line  along  which  the  slipping 
occurred  has  been  traced  about  three  hundred  miles  on  the  land 
(Fig.  275),  and  the  principal  movement  was  found  to  be  horizontal 


278 


PHYSICAL  GEOLOGY 


instead  of  vertical 
(Figs.  276,  277)  and 
to  measure  from  8 
to  20  feet.  Some 
vertical  movement 
also  occurred,  but  it 
was  inconsiderable. 

Distribution  of 
Earthquakes.  —  A 
study  of  the  distribu- 
tion of  earthquakes 
gives  a  clue  to  their 
cause.  They  occur 
(i)  in  volcanic  re- 
gions, where  the 
earth's  crust  is  sub- 
jected to  high  tem- 
perature and  to 
strains  produced  -by 
explosions ;  (2)  along 
belts  of  young  and 
growing  mountains, 
where  strains  are  being  relieved  from  time  to  time;  (3)  along  coasts, 
where  the  sea  bottom  descends  steeply  from  the  shores,  especially 
where  they  are  bordered  by  high  mountains.  The  conditions  last 
mentioned  are  well  illustrated  in  Japan,  where  the  earthquake 
records  since  the  beginning  of  the  seventeenth  century  show  that 


FIG.  276.  —  Fence  parted  by  an  earthquake  fault, 
1906.  The  fault  fracture  is  inconspicuous,  although  the 
horizontal  displacement  is  eight  and  a  half  feet.  Near 
San  Francisco.  (U.  S.  Geol.  Surv.) 


FIG.  277.  —  Diagrams  illustrating  the  nature  of  the  fault  which  produced  the 
San  Francisco  earthquake.     (After  Gilbert.) 

a  severe  shock  has  occurred  on  an  average  of  once  in  two  and 
a  half  years.  By  far  the  greater  number  of  these  have  been 
accompanied  by  a  movement  along  the  scarp  of  the  great  Tuscarora 
Deep,  which  lies  a  short  distance  off  the  coast.  (4)  Earthquakes 


EARTHQUAKES 


279 


also  occur  where  there  has  been  an  overloading  with  sediment.  The 
great  earthquake  of  the  Mississippi  Valley  in  1811  may  have  been 
caused  by  a  readjustment  of  the  crust  as  a  result  of  the  great  weight 
of  sediment  laid  down  there  in  recent  geologic  time.  Boundaries 
were  thrown  into  such  confusion  as  a  result  of  these  shocks  that  it  was 
necessary  for  the  government  to  make  a  resurvey  of  1,000,000  acres. 


FIG.  278.  —  Earthquake  regions  of  the  Eastern  Hemisphere,  shown  in  black. 
(After  Montessus  de  Ballore.) 

The  earthquake  "  danger  spots  "  of  the  United  States  are  situated 
on  the  Pacific  coast,  in  the  Great  Basin  (Utah),  and  in  the  lower 
Mississippi  Valley.  New  England  has  been  remarkably  free  from 
severe  shocks,  although  many  slight  earthquakes  have  been  recorded. 
In  the  last-named  region  the  danger  is  greatest  where  lines  of  fracture 
intersect.  Over  the  world  as  a  whole  two  zones  are  recognized  in 
which  earthquakes  are  most  frequent :  the  Mediterranean  belt,  which 


280 


PHYSICAL  GEOLOGY 


extends  from  Spain  through  the  Himalayas  to  eastern  China  and 
from  which  53  per  cent,  of  the  recorded  earthquakes  originated  ;  and 
the  Pacific  belt,  which  borders  the  Pacific  Ocean  and  from  which 
41  per  cent,  of  the  recorded  earthquakes  came.  Only  6  per  cent,  of 
the  recorded  earthquakes  have  occurred  outside  of  these  two  belts, 
showing  how  rare  severe  earthquakes  are  over  the  greater  portion 


FIG.  279.  —  Earthquake  regions  of  the  Western  Hemisphere,  shown  in  black. 
(After  Montessus  de  Ballore.) 

of  the  globe.  The  earthquake  zones  are  shown  in  Figs.  278  and  279. 
It  should  not  be  concluded  from  the  above  that  all  parts  of  the  earth- 
quake belts  are  equally  affected.  For  example,  along  the  line  of  the 
proposed  Nicaraguan  interoceanic  canal,  earthquake  shocks  are 
frequent  and  severe,  while  along  that  of  the  Panama  Canal  they  have 
been  few  and  slight,  as  is  shown  by  fragile  arches  which  have  re- 
mained standing  for  many  years. 


EARTHQUAKES  281 

Summary  of  the  Causes  of  Earthquakes.  —  The  earth  may  be 
caused  to  tremble  in  many  ways,  (i)  Severe  earthquakes  have  been 
produced  by  volcanic  eruptions,  but  the  disturbances  thus  caused 
are  confined  to  comparatively  small  areas,  as  they  are  the  result  of 
steam  explosions  and  of  the  fracturing  of  the  rock  as  lava  rises  in  the 
earth's  crust. 

(2)  The  falling  of  the  roof  of  a  cave  may  produce  a  jar  which  will 
cause  some  damage.     The  earthquake  shocks  at  Visp,  Switzerland, 
which  fissured  buildings  and  caused  landslides,  were  due  to  the  col- 
lapse of  cavern  and  tunnel  roofs,  and  the  earthquakes  which  are  of 
frequent  occurrence  in  the  Karst  region  (p.  72)  on  the  east  coast  of 
the  Adriatic  are  of  this  origin.     The  jar  produced  by  the  fall  of  an 
overhanging  rock  which  formed  the  brink  of  a  fall  has  been  sufficient 
to  break  windows  several  hundred  yards  distant. 

The  above  causes  (i)  and  (2)  are  unimportant,  and  their  effect 
is  small. 

(3)  The  great  earthquakes  of  the  world  are  a  result  of  the  fracturing 
of  the  rock  of  the  earth's  crust,  or  of  the  vibrations  produced  during 
faulting,     (a)  They  may  be  due  to  the  jolting  of  earth  blocks  whose 
movement  begins  and  ends  suddenly;  and  also  when  thick  delta 
deposits  suddenly  slump  an  earthquake  may  be  produced,     (b)  They 
are  also  due  to  the  vibrations  produced  during  faulting  by  the  friction 
of  one  block  as  it  rubs  against  another.     This  method  may  be  il- 
lustrated by  rubbing  the  closed  fist  on  a  table,  or  by  rubbing    two 
blocks  of  wood  together,     (c)  They  may  be  produced  by  a  simple 
breaking  of  the  rock.     It  has  been  suggested  that  some  at  least  of 
such  fracturing  "  may  have  relation  to  sudden  deformation  by  rock 
flowage."     (Leith.) l 

It  is  evident  from  the  above  that  great  earthquakes  are  most 
likely  to  occur  in  growing  regions ;  for  example  in  young  mountains, 
where  the  strains  have  not  yet  been  relieved. 

Displacements.  —  The  amount  of  the  movement  of  the  earth  along  faults  in  the 
production  of  earthquakes  varies  greatly.  After  the  California  earthquake  (1906)  it 
was  found,  as  already  stated,  that  the  movement  was  horizontal  and  varied  from  8  to  20 
feet.  The  movements  of  the  crust  in  the  Sumatra  (East  Indies)  earthquake  of  1892 
were  also  horizontal,  the  total  slip  of  the  fault  amounting  to  from  n  to  13  feet.  No 
trace  of  the  fault  was  visible  at  the  surface,  the  proof  of  the  movement  being  fur- 
nished by  geodetic  measurements.  Vertical  movements  are  perhaps  more  common 
than  horizontal,  although  they  are  usually  accompanied  by  some  horizontal  movement. 
The  Japanese  earthquake  of  1891,  for  example,  was  produced  by  a  fault  which  has  been 

1  Leith,  —  Structural  Geology,  1913,  p.  69. 


282 


PHYSICAL  GEOLOGY 


traced  40  miles,  on  one  side  of  which  the  ground  sank  from  2  to  20  feet,  while  on  the 
other  side  (the  east)  the  wall  of  the  fissure  was  moved  13  feet  northward  at  the  same 
time.  The  portion  of  the  Alaskan  coast  affected  by  the  earthquake  of  1899  was  found 
to  have  been  displaced  vertically  in  amounts  varying  from  zero  to  47  feet,  the  average 

being  between  5  and  12 
feet.  The  great  earth- 
quake of  Owen's  valley, 
California,  in  1872,  was 
produced  by  a  fault  which 
has  been  traced  40  miles, 
whose  vertical  displace- 
ment at  the  surface 
(throw)  was  from  5  to  20 
feet.  If  a  fault  caused 
the  Charleston  earthquake 
(Fig.  280),  no  evidence  of 
such  movement  appears 
at  the  surface. 


1       I        '       I         '     I         V:V- ;;;X;V.-:  ;;.-.;••>  :;-V;V-^  ;;;.•;! -;.V; 


FIG.  280.  —  Diagram  showing  the  absence  of  surface 
evidence  of  a  fault,  because  of  the  presence  of  a  thick, 
unconsolidated  bed  of  sand  above  the  solid  rock. 


It  is  sometimes 
found  that  the  dis- 
placement along  a 
fault  not  only  varies 

in  amount  at  different  places,  but  also  that  along  the  same  fault 
the  downthrow  side  in  one  portion  is  on  the  right,  for  example, 
and  at  another  on  the  left.  Such  a  fault  is  called  a  hinge  fault. 

Many  earthquakes  originate  beneath  the  sea,  some  of  which  have 
been  very  destructive.  Off  the  coast  of  Greece  the  telegraphic 
cable  broke  at  the  moment  of  an  earthquake  in  1873,  and  upon  sub- 
sequent examination  it  was  found  that  the  break  was  seven  miles 
from  land,  and  that  the  water  which  formerly  had  been  1400  feet  deep 
at  this  spot  was  2000  feet  in  depth  after  the  shock.  Submerged  preci- 
pices 3000  to  5000  feet  high  occur  in  this  region  and  are  doubtless 
fault  scarps  whose  formation  caused  many  earthquakes.  Many 
records  are  extant  of  vessels  which  were  made  to  vibrate  by  submarine 
earthquakes,  to  such  a  degree  that  the  crew  thought  that  they  had 
struck  a  reef;  loose  objects  rattled  about,  and  in  some  cases  men 
were  thrown  to  the  deck  by  the  violence  of  the  shock. 

Depth  of  the  Plane  or  Point  of  Origin.  —  Wherever  it  has  been 
possible  to  determine  the  direction  of  the  emergence  of  the  waves  of 
great  earthquakes,  it  has  been  found  that  they  converge  at  a  depth 
of  less  than  12  miles  and  usually  less  than  5  miles;  that  is  within 
the  zone  of  fracture.  The  point  or  place  of  origin  is  called  the  focus. 


EARTHQUAKES 


283 


Earthquake  Waves.  —  When  the  earth  is  shaken  by  an  earthquake 
two  sets  of  vibrations  are  started,  one  which  follows  the  surface  of 
the  earth  and  another  which  passes  through  it,  the  former  traveling 
more  slowly  than  the  latter,  which  passes  through  the  8000  miles 
of  the  diameter  of  the  earth  in  from  20  to  22  minutes.  Earthquake 
instruments  (seismographs,  p.  287)  situated  on  the  side  of  the  earth 
opposite  an  earthquake  shock  show  three  series  of  vibrations  :  (i)  pre- 
liminary tremblings,  20  to  22  minutes  after  the  shock,  followed  by 
(2)  the  strong  vibrations  of  the  principal  shock,  and  finally  by  (3)  a 


Vibrations  not 
far  distant 


Vibrations  at 
epicentrum 


FIG.  281.  —  Diagram  showing  the  path  of  earthquake  waves  and  the  vibrations 
which  they  produce.     (After  Sieberg.) 

series  of  feeble  vibrations  (Fig.  281).  Some  of  the  waves  are  (i) 
compressive  or  longitudinal  and  have  the  same  nature  as  the  vibra- 
tions which  travel  through  a  liquid,  and  some  are  (2)  transverse  and 
vibrate  at  right  angles  to  the  direction  of  the  transmission  of  the 
shock.  Such  waves  as  the  latter  can  be  propagated  only  in  a  solid. 
The  velocity  of  the  earthquake  waves  which  pass  through  the  earth 
is  uniformly  about  375  miles  a  minute,  on  the  assumption  that  this 
movement  is  along  a  straight  line.  This  indicates  a  rigidity  of  the 
earth's  interior  of  one  and  a  half  times  that  of  steel.  The  velocity  of 
the  surface  waves  varies  with  the  rock  through  which  they  pass,  and 
other  conditions  ;  that  of  the  earthquake  at  Naples  in  1857  being  nine 
or  10  miles  a  minute,  while  in  Germany  in  1874  the  rate  was  28  miles 


284  PHYSICAL  GEOLOGY 

a  minute.  The  velocity  depends  to  a  large  degree  upon  the  density 
and  elasticity  of  the  rock,  being  much  slower  in  sand  and  loose  sand- 
stone than  in  slate,  schist,  or  granite.  This  has  been  shown  experi- 
mentally by  noting  the  velocity  of  shocks  produced  by  explosions  of 
gunpowder,  and  it  has  been  found  that  the  velocity  is  825  feet  a 
second  in  sand,  and  1088  feet  a  second  in  slate  and  schist.  In  all  such 
experiments,  however,  account  must  be  taken  of  the  presence  of  fis- 
sures and  whether  or  not  the  fissures  are  filled  with  water. 

Amplitude  of  Vibration.  —  By  the  amplitude  of  vibration  is  meant 
the  distance  each  rock  particle  is  moved  from  its  position  of  rest 
during  an  earthquake  (Fig.  282).  It  is  a  common  notion  that  the 
amplitude  is  very  great,  but  measurements  show  that  they  are  minute, 


FIG.  282.  —  Wire  model  showing  the  motion  of  an  earth  particle  during  an 

earthquake. 

an  amplitude  of  20  millimeters  (three  fourths  of  an  inch)  being  suffi- 
cient to  destroy  a  city ;  one  of  10  millimeters  (three  eighths  of  an  inch) 
constituting  a  severe  earthquake ;  and  one  of  5  or  6  millimeters  being 
adequate  to  shatter  a  chimney.  Amplitudes  much  greater  than  the 
above  have  been  recorded.  It  should  be  remembered  in  this  con- 
nection that  it  is  the  suddenness  of  the  shock  that  makes  it  effective. 
This  can  best  be  illustrated  by  a  simple  experiment.  If  a  stone  or 
metal  slab  upon  which  a  marble  rests  is  struck  a  sharp  blow,  the 
marble  will  be  thrown  into  the  air,  but  it  is  evident  that  the  actual 
movement  of  the  particles  composing  the  slab,  and  through  which 
the  vibrations  were  transmitted  to  the  marble,  was  a  very  small  frac- 
tion of  an  inch,  the  projection  of  the  marble  being  due  to  the  great 
suddenness  of  a  small  movement.  This  phenomenon  is  well  illus- 


EARTHQUAKES 


285 


FIG.  283.  —  Pier  driven  into 
the  ground  by  an  earthquake 
shock. 


trated  in  some  earthquakes.  In  one  case  (Calabria,  Italy),  the  stone- 
work of  a  well  was  thrown  out  of  the  ground,  and  in  its  new  position 
resembled  a  small  tower.  In  an  Icelandic  earthquake  in  1896  persons 
lying  on  the  ground  near  a  cliff  were  thrown  over  the  edge.  More 
commonly  stones  are  thrown  into  the  air  and  overturned  (Assam, 
India).  Sometimes  heavy  objects  such  as  gravestones  are  embedded 
more  deeply  in  the  ground  (Fig.  283).  The  reason  for  this  can  be 
shown  by  another  simple  experiment.  If 
a  ball  of  soft  clay  upon  which  a  pebble 
rests  is  subjected  to  a  sudden  upward 
movement,  the  pebble  will  be  embedded, 
to  some  extent,  in  the  clay. 

The  amplitude  of  the  vibration  of  a 
rock  particle  should  be  distinguished  from 
the  earth  waves  which  are  produced  in 
loose  alluvium.  For  example,  during  the 

earthquake  which  shook  the  Mississippi  Valley  in  1811,  and  which 
was  probably  the  most  violent  that  has  taken  place  in  North 
America  since  its  settlement  by  Europeans  (although  not  the  most 
destructive  because  of  the  sparseness  of  the  population  and  the 
character  of  the  buildings),  the  ground  is  described  as  having  been 
thrown  into  great  waves,  so  that  the  branches  of  the  trees  inter- 
locked as  the  waves  passed  under  them.  Ip  this  case,  the  ampli- 
tude of  the  vibrations  of  the  rock  upon  which  the  thick  alluvial  soil 
rested  probably  did  not  exceed  a  few  centimeters. 

Vorticose  and  Twisting  Movements.  —  After  earthquakes,  pictures 
are  often  found  with  their  faces  toward  the  wall,  furniture  has  been 

turned  partly  or  completely 
around,  statues  have  been 
twisted  on  their  pedestals 
and  chimneys  have  been 
partly  turned  about.  No 
one  cause  can  be  assigned  to 
such  movements.  In  many 
cases  the  turning  was  due  to 

a  simple  motion  backward  and  forward ;  in  others  the  rotation  prob- 
ably resulted  from  "  a  combination  of  shocks  from  separate  faults." 
(Hobbs.)  The  latter  is  given  as  the  cause  of  the  turning  of  a  bronze 
angel  in  Belluno,  Italy,  through  an  angle  of  20°,  and  the  rotation  of 
the  statue  of  Queen  Victoria  in  Kingston,  Jamaica. 


FIG.  284.  —  Diagram  showing  a  railroad  track 
bent  during  the  Charleston  earthquake. 


286  PHYSICAL  GEOLOGY 

The  bending  of  railroad  tracks  (Fig.  284)  and  the  zigzag  position 
of  rows  of  trees  which  were  straight  before  the  earthquake  were  pro- 
duced by  the  lateral  shifting  of  earth  blocks. 

Duration. — The  duration  of  a  severe  earthquake  is  very  short. 
As  has  been  stated,  the  shock  which  destroyed  San  Francisco  lasted 
about  one  minute,  and  the  movement  along  the  300  to  400  miles  of 
fault  rift  probably  did  not  consume  two  minutes.  The  great  Assam 
(India)  earthquake  lasted  only  two  and  a  half  minutes,  and  the 
destruction  was  accomplished  during  the  first  15  seconds.  The  de- 
structive shocks  of  the  Charleston  earthquake  lasted  a  little  more 
than  half  a  minute.  Between  December  16,  1811,  and  March  16, 
1812,  at  least  1874  shocks  were  felt  in  the  Mississippi  Valley,  of  which 
eight  were  severe.  No  individual  shock,  however,  was  of  long 
duration. 

Frequency.  — Delicate  instruments  (seismographs)  show  that  the 
earth  is  continually  trembling  in  all  parts,  and  it  is  probable  that 
quakes  severe  enough  to  be  felt  are  shaking  the  earth  in  some  regions 
at  all  times.  Certain  portions  of  the  world  as,  for  example,  parts  of 
Japan  and  southern  Italy  are  subject  to  frequent  shocks.  In  the 
former,  a  severe  earthquake  occurs  on  an  average  of  every  two  and 
a  half  years,  and  minor  shocks  four  times  a  day.  A  careful  record  of 
the  aftershocks  of  the  earthquake  at  Messina,  Sicily,  in  1908,  shows 
that  87  shocks  were  felt  during  the  first  four  days  and  862  during  the 
following  year,  four  of  which  were  severe. 

All  definite  predictions  as  to  the  time  and  place  of  earthquakes  are 
of  little  value.  This  is  illustrated  in  the  case  of  San  Francisco.  The 
earthquake  rift  or  fault  line  was  known  before  the  earthquake.  It 
was*  believed  that  renewed  faulting  might  occur  at  any  time,  but 
whether  within  one  year  or  many  years  could  not  be  foretold.  Since 
earthquakes  are  the  result  of  a  relief  of  strain,  it  is  evident  that  a 
region  is  likely  to  be  immune  from  severe  shocks  for  some  years  after 
it  has  been  shaken,  since  the  strains  which  produced  the  shock  have 
been  partly  or  entirely  relieved,  and  a  shock  will  not  occur  until  strains 
have  again  accumulated. 

Areas  Affected  by  Certain  Earthquakes.  —  The  areas  affected  by 
earthquakes  vary  greatly  in  size.  A  region  four  times  the  size  of 
Europe  is  said  to  have  been  affected  by  the  Lisbon  (Portugal)  earth- 
quake of  1755  ;  that  shaken  by  the  great  Assam  (India)  earthquake 
of  1897  was  1,750,000  square  miles,  of  which  150,000  were  laid  in  ruins. 
An  earthquake  in  1891  shook  three  fifths  of  the  entire  area  of  Japan. 


EARTHQUAKES 


287 


The  Charleston  (South  Carolina)  earthquake  affected  an  area  1000 
miles  in  diameter. 

Instruments  for  Determining  and  Measuring  Earthquakes.  — 
Earthquake  instruments  or  seismographs  have  been  established  in 
many  parts  of  the  world,  and  from  them  the  location  and  intensity 
of  earthquakes  are  known.  For  example,  seismographic  records  will 
be  made  in  Germany,  the  United  States,  and  elsewhere  of  an  earth- 
quake in  Java  or  the  West  Indies.  Seismographs  vary  widely  in 
construction,  but  since  they  all  endeavor  to  show  the  direction  of  the 
vibrations,  the  essential  feature  consists  of  three  pendulums  arranged 
so  as  to  vibrate  in  mutually  perpendicular  directions,  the  record  being 
made  on  a  sheet  of  paper  which  moves  at  a  uniform  rate  (Fig.  28 1).1 


EFFECTS   OF   EARTHQUAKES 

Faults  and  Fissures.  —  We  have  seen  that  earthquakes  are  usually 
the  result  of  faulting.  Sometimes  the  fault  rift  extends  to  the  surface 
as  an  open  fissure,  but  more  often  the  fissure  is  closed.  When  deep 
alluvial  soil  is  shaken,  many 
cracks  are  often  formed,  as 
a  result  of  the  compacting 
of  the  loose  material  and  of 
its  slumping.  Such  fissures 
are  especially  likely  to  form 
in  stream  valleys  parallel  to 
their  course  (Fig.  285),  since 
the  alluvium  is  unsupported 
on  the  stream  side  and 
moves  in  that  direction,  if 
at  all.  As  a  result  of  such 
slumping  cracks  are  formed 
and  valleys  are  narrowed. 
Fissures  formed  during  the 
Mississippi  Valley  earth- 
quake of  1811-1812  are  still 
visible.  One  such  fissure 


diverted  the  course  of  the 
Mississippi  River  so  that  an 
oxbow  (p.  121)  was  cut  off. 


FIG.  285.  —  Diagrams  showing  the  effect  of 
earthquake  shocks  upon  loose  material.  The 
bridge  girder  has  remained  in  place,  but  the 
piers  have  moved  inward  at  the  bottom. 


1  For  a  more  complete  description  of  seismographs 
CLELAND   GEOL.  —  1 9 


: :  Hobbs,  Earthquakes,  pp.  257-275. 


288 


PHYSICAL  GEOLOGY 


In  Arizona  the  waters  of  several  streams  now  flow  into  a  fissure 
formed   during    an   earthquake    (Fig.   286).     In    some    earthquakes 

fissures  have  opened 
and  then  closed 
again,  entrapping 
people  and  animals, 
and  engulfing  houses. 
Changes  in  Level. 
—  It  is  usual  to  find 
that  the  level  of  the 
land  has  changed 
during  earthquakes. 
As  a  result  of  the 
earthquake  of  1811- 
1812  in  the  Missis- 
sippi Valley,  Reelfoot 

Lalfe' 2*  mil.ef long 

and  5  miles  wide,  was 
formed,  the  trees  still 
being  visible  on  its 
bottom.  In  the  same 
earthquake  Lake 

tofc1"    *i  *•  -    ^      Eulalie  was  drained. 

%;  ill      In  the  Indian  earth- 

quake of  1819,  a  lake 
about  the  same  size 
as  Reelfoot  Lake  was 
formed.  There  is 
also  often  a  lateral 
shifting  of  the  ground 
during  an  earth- 
quake, as  in  that  at  San  Francisco,  which  moves  the  plains  or 
mountains  in  one  direction  on  one  side  and  those  on  the  opposite  side 
in  the  opposite  direction  (p.  278).  The  changes  of  level  on  opposite 
sides  of  faults  which  produce  falls  and  lakes  have  already  been  dis- 
cussed (p.  262)  (Fig.  287) ;  elevation  is  as  frequent  an  accompaniment 
of  earthquakes  as  depression.  During  the  earthquake  which  shook 
lower  India  in  1819,  an  area  50  miles  long  and  10  miles  broad  was 
elevated  10  feet  and  is  called  the  Mount  of  God.  Over  an  area  of 
600,000  square  miles  the  coast  line  of  Chile  and  Patagonia  is  said  to 


FIG.  286.  —  Fissure  produced  at  the  time  of  an 
earthquake.     Arizona.     (U.  S.  Geol.  Surv.) 


EARTHQUAKES 


289 


have  been  elevated  dur- 
ing    an    earthquake    in 

1835. 

Landslides.  —  One  of 
the  most  obvious  effects 
of  a  severe  shaking  of 
the  earth  is  the  produc- 
tion of  landslides  and 
the  slumping  of  thick 
soil  which  rested  on  a 
slope.  The  hills  about 
Kingston,  Jamaica,  for 
example,  are  scarred  by 
landslides  formed  during 
the  earthquake  of  1907. 
As  a  result  of  an  earth- 
quake in  India  in  1897, 
the  hills  were  stripped 
of  their  forests  by  land- 
slides. This  permitted 
erosion  to  proceed  so 
rapidly  as  to  overload 
the  streams,  with  the 
result  that  the  rivers, 
instead  of  flowing  from 
deep  pools  over  rapids, 
flowed  in  broad,  shallow 
channels  over  a  sandy 
floor.  An  earthquake  in 
Greece  in  1870  caused 
great  landslides  which 
dammed  up  some  of  the 
valleys  and  formed  lakes, 
some  of  which  are  still 
in  existence. 

Earthquake  Topog- 


Scale  of  /1//e 


FIG.  287.  —  Map  of  the  Chedrang  fault,  India, 
showing  the  effect  of  faulting  on  drainage.  The 
figures  show  the  amount  of  vertical  elevation  in  feet. 
The  river  in  places  flows  along  the  downthrow  side 
of  the  fault,  and  is  ponded  back  in  others.  The 
tributary  streams  also  are  dammed,  forming  pools. 
Waterfalls  are  formed  where  the  river  crosses  the 
fault.  In  one  place  the  fault  runs  along  the  old  and 
now  dry  bed  of  the  river,  while  the  stream  itself  flows 
in  a  depression  on  the  downthrow  side.  The  large 
pools  are  not  formed  by  the  fault  scarp,  but  by  the 
reversal  of  the  original  slope  of  the  river  bed  by  the 
unequal  elevation  of  the  land,  there  being  no  eleva- 
tion at  the  pools,  but  an  elevation  of  more  than  30 
feet  above  each  pool,  and  a  lesser  elevation  below. 
(After  Oldham.) 


raphy.  —  A  description 
of  the  fault  rift  along  which  occurred  the  movement  which  produced 
the  San  Francisco  earthquake  will  serve,  in  a  general  way,  for  all 
such  earthquake  faults  or  earthquake  topography.  This  line  is  well 


290 


PHYSICAL  GEOLOGY 


marked  for  a  distance  of  43  miles  and  follows  a  system  of  long, 
narrow  valleys,  except  where  it  traverses  wide  valleys  for  short  dis- 
tances. In  some  places  it  passes  over  mountain  ridges,  sometimes 
a  pass,  but  in  some  cases  over  the  shoulder  of  a  mountain. 

Along  the  fault  line  low,  precipitous 
cliffs  or  scarps  occur.  Small  basins  or 
ponds,  many  having  no  outlet,  are  of 
fairly  frequent  occurrence  and  usually 

^e  at  t^e  kase  °^  ^e  scarPs-  Trough- 
like  depressions  bounded  on  both  sides 
by  scarps  also  occur,  and  are  due  to 
the  subsidence  of  the  ground  or  to  an 
uplift  on  one  or  both  sides.  In  the 
Japanese  earthquake  of  1891  the  fault 
FIG.  288.  —  Diagram  showing  a  line  showed  itself  in  some  places  as  a 

ridge'  as  if  made  by  a  gigantic  mole 
just  beneath  the  surface  (Fig.  288). 
Sounds. — Accompanying  or  slightly  preceding  earthquakes, 
sounds,  described  as  a  hollow  rumbling  or  grinding  and  sometimes  as 
a  roar,  have  often  been  noticed.  These  are  produced  by  the  breaking 
and  grinding  of  the  rock  as  it  is  thrown  into  vibrations,  and  by  the 
falling  and  breaking  of  objects  on  the  earth. 

Loss  of  Life.  —  The  destruction  of  life  is  more  impressive  than  any  other  effect  of 
an  earthquake.  In  1812  Caracas,  Venezuela,  was  so  severely  shaken  that  10,000 
people  were  killed,  while  the  loss  of  life  in  Lisbon  in  1755  amounted  to  30,000.  In 
1905  an  earthquake  in  India  (Kangra)  destroyed  20,000  people,  and  it  is  estimated 
that  in  526  A.D.  between  100,000  and  200,000  were  killed  by  the  shocks  which  dev- 
astated the  shores  of  the  Mediterranean.  In  1908  the  Messina  earthquake,  described 
as  the  world's  most  cruel  earthquake,  destroyed  77,283  people;  and  more  than  30,000 
were  killed  in  the  Italian  earthquake  of  1915. 

Fish  in  great  numbers  are  sometimes  killed  by  earthquake  shocks  which  affect  the 
sea,  lakes,  or  rivers. 

Effect  on  Underground  Water.  —  After  severe  earthquakes  it  is  not 
unusual  to  find  that  some  springs  have  become  dry,  that  some  have 
had  their  volumes  increased  or  decreased,  and  that  some  have  burst 
forth  where  none  formerly  existed.  Along  a  fault  rift  which  extends 
for  1 20  miles  in  Afghanistan  and  Beluchistan  over  mountain  and 
valley,  springs  are  found  in  abundance,  the  volumes  of  which  are  said 
to  be  augmented  after  an  earthquake  disturbance.  So  marked  is  this 
rift  that  it  has  long  been  utilized  as  a  thoroughfare.  The  composi- 
tion and  temperature  of  the  water  of  springs  is  also  sometimes  changed 


EARTHQUAKES 


291 


as  a  result  of  an  earthquake  shock.  It  is  evident  that  the  cause  of 
this  disarrangement  of  the  underground  water  is  the  opening  and 
closing  of  fissures 
leading  to  water- 
bearing strata  or 
joints,  and  to  fault- 
ing which  may  open 
a  water-bearing 
stratum  to  a  fissure. 

It  is  not  unusual 
to  find  sand  or  mud 
cones  and  "  crater- 
lets  "  after  earth- 
quake shocks  (Fig. 
289).  These  are 
formed  by  jets  of 
water  which  were 
forced  through  fis- 
sures during  the 
disturbance.  The 
water  forming  these 
jets  originated  in  a 
water-bearing  stra- 
tum or  in  water-bearing  strata,  or  in  fissures  and  caverns. 

Gases.  —  Gases,  usually  containing  large  amounts  of  sulphureted 
hydrogen  (H2S),  are  also  sometimes  discharged,  with  or  without 
water.  These  gases  were  imprisoned  in  the  soil  and  escaped  either 
as  a  result  of  the  fissuring  of  the  ground  or  by  being  forced  out  by  the 
shaking  together  of  the  loose  material.  The  sulphureted  hydrogen 
was  doubtless  formed,  for  the  most  part,  by  the  decomposition  of 
animal  and  vegetable  matter  in  the  soil,  just  as  is  that  which  rises 
from  the  mud  on  the  bottom  of  ponds  when  it  is  stirred  with  a  stick. 
The  escape  of  the  sulphureted  hydrogen  was  especially  noticeable  in 
the  Mississippi  Valley  and  Charleston  earthquakes. 

Construction  of  Buildings  in  Earthquake  Regions.  —  A  study  of 
the  effects  of  earthquakes  on  buildings  has  led  to  certain  recommenda- 
tions concerning  the  location  and  construction  of  houses  in  earth- 
quake regions,  (i)  Artificially  filled  ground  and  deep  alluvial  soils 
should  be  avoided,  since  these  are  likely  to  be  badly  fissured  and  are, 
moreover,  thrown  into  large  waves  by  a  shock.  (2)  A  firm  and 


FIG.  289.  —  Craterlet  formed  during  the  Charleston 
earthquake.     (U.  S.  Geol.  Surv.) 


292  PHYSICAL  GEOLOGY 

stable  foundation  is  of  paramount  importance,  and  particularly  on 
soft  and  "  made  "  ground.  (3)  Low  structures,  especially  when  well 
braced,  with  the  beams  and  rafters  attached  firmly  to  the  walls,  are 
most  desirable,  because  if  a  building  does  not  vibrate  as  a  whole,  the 
parts  act  as  battering-rams  to  throw  over  or  break  the  walls.  (4) 
Since  the  fires  which  almost  invariably  accompany  earthquakes  are 
often  more  destructive  than  the  earthquakes  themselves,  it  is  im- 
portant that  there  should  be  ample  fire  protection.  It  is  estimated 
that  had  the  buildings  at  Messina  been  properly  constructed  at  the 
time  of  the  earthquake  in  1908,  998  deaths  out  of  every  thousand 
would  have  been  prevented. 

Effect  of  Earthquakes  on  the  Sea.  —  One  of  the  most  disastrous 
effects  of  earthquakes  on  low  coasts  is  produced  by  the  great  sea  waves 
(tsunamis)  which  sometimes  follow  the  shocks.  After  the  first  severe 
trembling  which  shook  Lisbon  in  1755,  the  sea  retreated  from  the 
shore,  laying  bare  the  bottom  of  the  harbor,  and  then  returned  in  a 
wave  60  feet  high  which  completed  the  devastation  of  the  city.  This 
wave  was  destructive  along  the  coasts  of  Portugal  and  Spain  and  was 
felt  on  the  coasts  of  countries  far  distant.  A  great  sea  wave  cost 
the  lives  of  27,000  people  in  Japan  in  1896. 

The  velocity  of  great  sea  waves  and  the  distance  to  which  they  are  propagated  is 
well-known.  In  the  Japanese  earthquake  of  1896  the  wave  which  reached  Honolulu, 
3500  miles  away,  was  8  feet  high  at  that  place,  and  its  mean  velocity  between  these 
points  was  68 1  feet  a  second.  It  was  also  recorded  at  San  Francisco,  to  which  point 
its  mean  velocity  was  664  feet  a  second.  The  great  sea  wave  from  an  earthquake  in 
Peru,  South  America,  in  1868,  reached  Honolulu,  5500  miles  away,  in  12  hours,  and 
Japan,  over  10,000  miles  away,  the  next  day.  Because  of  their  great  wave  length 
(sometimes  200  miles),  great  sea  waves  may  not  be  sensible  to  vessels  in  mid-ocean  and 
are  never  destructive  until  they  reach  a  shallowing  shore. 

Great  sea  waves  are  apparently  not  all  due  to  the  same  cause. 
Some  are  probably  produced  by  a  sudden  depression  of  a  portion  of 
the  ocean  bottom  by  faulting  and  a  consequent  drawing  in  of  the  ocean 
water.  This  causes  the  withdrawal  of  the  water  from  the  land,  and 
the  wave  set  in  motion  by  the  meeting  of  the  water  then  spreads  in  all 
directions,  devastating  low-lying  coasts.  A  sudden  shock  on  the  sea 
bottom  is  probably  also  competent  to  give  rise  to  a  great  sea  wave. 
Explosions  of  submarine  volcanoes  set  waves  in  motion  which  may 
work  great  havoc  on  low  coasts. 

Evidence  that  a  Region  has  been  Free  from  Severe  Earthquakes. 
—  It  is  not  always  possible  to  tell  whether  or  not  a  region  has  been 


EARTHQUAKES  293 

subjected  to  earthquakes ;  but  some  features  such  as  pinnacled  rocks 
with  insecure  bases  and  steep  hillsides  covered  with  soil  give  evi- 
dence that  a  region  has  been  free  from  violent  shocks  for  many  cen- 
turies. For  example,  New  England  has  probably  been  free  from  dev- 
astating earthquakes  since  glacial  times,  as  the  almost  precipitous, 
soil-covered  slopes  of  many  hills  and  mountains  show.  This  is  also 
borne  out  by  the  occurrence  of  perched  bowlders  in  very  insecure 
positions. 

REFERENCES   FOR   EARTHQUAKES 

DAVISON,  CHAS.,  —  A  Study  of  Recent  Earthquakes,  1905. 

DUTTON,  C.  E.,  —  Earthquakes  in  the  Light  of  the  New  Seismology,  1904. 

FULLER,  M.  L., —  Our  Greatest  Earthquake:   Pop.  Sci.  Monthly,  Vol.  69,  1906,  pp. 

76-86. 

GEIKIE,  A., —  Textbook  of  Geology,  Vol.  I,  4th  ed.,  pp.  358-377. 
GILBERT,    G.  K., —  The   Investigation  of  the   San  Francisco   Earthquake:   Pop.    Sci. 

Monthly,  Vol.  69,  pp.  97-115. 

HOBBS,  W.  H.,  —  Earthquakes,  an  Introduction  to  Seismic  Geology,  1907. 
LAWSON,  A.  C.,  et  al, —  The  California  Earthquake  of  April  18,  1906:  Report  of  the 

State  Earthquake  Investigation  Commission,  Carnegie  Institution  of  Washington. 
TARR,  R.  S.,  and  MARTIN,  L.,  —  Recent  Changes  of  Level  in  the  Yakutat  Bay  Region, 

Alaska:  Bull.  Geol.  Soc.  America,  Vol.  17,  1906,  pp.  29-64. 
WRIGHT,  C.  W.,—  The  World's  Most  Cruel  Earthquake:  Nat.  Geog.  Mag.,  Vol.  20, 

1909,  PP-  373-396. 
CARNEGIE  INSTITUTION  PUBLICATION  87,  Vol.  i,  Plate  15  and  Atlas. 


CHAPTER  IX 

VOLCANOES  AND  IGNEOUS  INTRUSIONS 

A  VOLCANO  may  be  regarded  as  an  opening  in  the  earth's  surface 
through  which  various  gases  and  solid  or  molten  rocks  are  ejected. 
The  materials  brought  to  the  surface  accumulate  around  the  opening, 
forming  a  conical  hill  or  mountain.  The  rapidity  with  which  volcanic 
cones  are  built  up  is  in  contrast  to  the  slowness  with  which  other 
elevations  are  formed  (p.  362),  and  they  are  able,  consequently,  to 
defy  the  agents  of  erosion  during  the  period  of  rapid  growth.  Vol- 
canoes are,  moreover,  capable  of  destroying  in  a  very  short  time  re- 
liefs which  erosion  would  be  able  to  wear  down  only  after  centuries 
of  work  ;  as,  for  example,  when  one  destroys  a  large  part  of  its  cone  in 
a  few  hours.  However,  although  conspicuous,  the  work  of  volcanism, 
because  of  its  limited  extent  is  unimportant  as  compared,  on  the  one 
hand  with  the  movements  which  are  raising  the  earth's  surface,  and 
on  the  other  with  erosion,  which  is  lowering  it  and  is  universal  in  its 
effects. 

How  Volcanoes  Begin.  — The  first  step  in  the  development  of  a 
volcano  is  the  opening  of  a  passage  to  the  surface.  This  opening 
may  be  caused  by  the  "  blowing  out  "  of  a  portion  of  the  earth's 
crust  with  the  resulting  formation  of  a  funnel.  More  usually,  how- 
ever, it  is  a  fissure  through  which  gases,  ash,  and  lava  are  ejected. 
The  opening  through  which  the  material  is  ejected  is  usually  en- 
larged by  explosive  action  or  by  melting,  and  the  lava  and  other 
ejectamenta  tend  to  form  a  ring  as  they  fall  back  to  earth,  until  a 
hill  is  built  up  with  a  depression  or  crater  in  the  summit.  It  is  ap- 
parent, therefore,  that  the  cone  is  not  an  essential  part  of  a  volcano, 
but  is  secondary  to  the  vent,  being  merely  a  result  of  its  action,  not 
a  cause. 

New  Volcanoes.  —  It  is  not  often  that  man  has  seen  the  birth  of 
a  volcano,  but  a  few  well-known  instances  may  be  mentioned.  On 
the  shore  of  the  Bay  of  Naples,  amidst  gardens  and  cottages,  a  vol- 
cano called  Monte  Nuovo  had  its  birth  in  1538,  and  in  the  course  of 

294 


VOLCANOES  AND  IGNEOUS  INTRUSIONS  295 

two  days  built  a  cone  to  a  height  of  about  500  feet.  The  eruption 
lasted  only  a  week  and  has  not  been  renewed  since.  An  examination 
of  the  material  of  the  cone  showed  that  most  of  it  was  of  volcanic 
rock,  but  that  pieces  of  Roman  pottery,  fragments  of  the  surface 
rock,  and  marine  shells  were  also  present. 

On  a  plain  in  Mexico  between  2000  and  3000  feet  above  the  sea,  covered  with  fields 
of  sugar  and  indigo,  a  fissure  opened  in  1759,  from  which  rocks  were  thrown  to  great 
heights  and  about  which  several  cones  were  built  up,  the  smallest  to  a  height  of  300 
feet  and  the  largest,  Jorullo,  to  that  of  1300  feet  above  the  plain.  The  eruption,  which 
began  in  June,  1759,  ceased  in  February  of  the  following  year. 

Volcanic  cones  have  also  been  built  up  from  the  ocean  bottom  within  recent  times. 
In  1811  one  such  (Sabrina)  was  formed  ofF  the  Azores,  rising  to  a  height  of  300  feet 
above  the  sea.  As  it  was  composed  of  ash,  it  was  soon  washed  away  by  the  waves. 
Many  of  the  great  volcanoes  of  the  world,  such  as  Vesuvius,  Etna,  and  Mauna  Loa,  began 
as  submarine  volcanoes  many  thousands  of  years  ago  and  built  up  cones  from  abyssal 
depths. 

Classification  of  Volcanoes.  —  Volcanoes  are  usually  classified  as 
active,  dormant,  and  extinct.  This  classification  is  unsatisfactory, 
since  a  volcano  which  has  long  been  considered  to  be  extinct  may 
become  suddenly  active,  and  volcanoes  classed  as  dormant  may 
never  again  be  in  eruption.  For  example,  Vesuvius  must  have  been 
regarded  as  extinct  at  the  beginning  of  the  Christian  era,  since  it  had 
been  inactive  so  long  that  its  crater  was  covered  with  vegetation,  yet 
in  a  few  days  in  the  year  79  one  half  of  its  crater  was  blown  off  by  a 
series  of  powerful  explosions,  and  it  has  been  intermittently  active 
ever  since.  Volcanoes  which  have  not  been  in  eruption  during  his- 
toric times  are  said  to  be  extinct;  those  which  have  been  active  in 
modern  times,  but  are  now  inactive,  are  said  to  be  dormant.  All 
volcanoes  may  become  active  after  a  period  of  quiet,  or  may  become 
extinct  after  a  single  paroxysm ;  such,  for  example,  as  that  of  Monte 
Nuovo. 


MATERIALS   ERUPTED 

The  materials  brought  to  the  surface  by  volcanoes  may  be  classi- 
fied as  gases,  solid  matter,  and  lava  flows. 

Gases.  —  The  difficulty  in  collecting  gases  from  the  crater  of  a  vol- 
cano during  eruptions  renders  our  knowledge  of  them  rather  incom- 
plete. In  fact,  whatever  information  we  have  has  been  largely 
obtained  from  fumaroles  or  openings  on  the  flanks  of  the  volcano, 
and  from  the  crater  after  eruptions  have  ceased. 


296  PHYSICAL  GEOLOGY 

The  principal  gases  given  off  during  volcanic  eruptions  are  sul- 
phureted  hydrogen  (H2S),  sulphur  dioxide  (SO2),  carbon  dioxide 
(CO2),  carbon  monoxide  (CO),  hydrochloric  acid  (HC1),  hydrogen 
(H),  oxygen  (O),  nitrogen  (N),  argon  (A),  and  water.  It  is  stated  that 
the  gases  emitted  from  Vesuvius  in  1906  contained  so  much  ammonia 
and  hydrochloric  acid  that  the  glowing  lavas  were  shrouded  in  a  veil 
of  ammonium  chloride  (NH4C1)  vapor,  and  that  the  "  pine  tree  " 
cloud  of  yellowish  "  smoke  "  which  hangs  over  that  volcano  during 
eruptions  consists  chiefly  of  ammonia  compounds.  The  glare  of  the 
red-hot  lava  in  the  crater  is  reflected  from  this  cloud  and  gives  the 
appearance  of  a  burning  mountain. 

The  composition  of  the  vapors  depends  upon  the  state  of  activity 
of  the  volcano ;  chlorine  is  more  abundant  in  the  energetic  phases, 
while  sulphurous  and  carbonic  gases  characterize  the  dying  out  of 
activity. 

According  to  recent  investigations,  steam  seems  to  be  in  smaller 
quantities  than  formerly  thought.  This  contention  is  supported  by 
the  facts  (i)  that  the  amount  of  steam  in  craters  decreases  as  the 
center  of  the  crater  is  approached ;  (2)  that  the  white  cloud  which 
hangs  over  volcanoes  during  eruptions  is  a  mixture  of  solids  and 
gases,  and  not  steam  as  it  appears ;  (3)  that  volcanic  ash  is  invariably 
white  and  consequently  has  the  appearance  of  steam  when  in  sus- 
pension in  the  air;  (4)  that  the  volcanic  cloud  never  produces  rain- 
bows or  aureoles. 

The  great  quantity  of  steam  rising  from  some  parasitic  cones  and 
from  some  lavas,  however,  is  enormous.  For  example,  it  has  been 
estimated  that  from  one  of  the  many  parasitic  cones  of  Etna  suffi- 
cient steam  was  ejected  during  one  period  of  one  hundred  days  to 
form,  if  condensed,  462,000,000  gallons  of  water.  The  steam  of 
fumaroles  is,  however,  apparently  largely  of  surface  origin,  as  is 
shown  by  the  increase  in  quantity  after  rains. 

Fragmental  Materials.  — All  of  the  substances  thrown  into  the  air 
by  volcanic  explosions,  which  fall  to  the  ground  in  a  solid  state,  are 
included  under  the  term  fragmental  materials,  and  are  classified  as  (i) 
dust,  (2)  ash,  (3)  cinders,  (4)  bombs,  and  (5)  blocks  of  rock.  These  solid 
ejections  are  either  portions  of  the  rock  which  has  been  broken  into  pieces 
by  the  force  of  the  explosions,  or  lava  which  was  hurled  into  the  air 
in  a  liquid  condition  but  which  solidified  before  reaching  the  ground. 
The  size  of  the  fragments  varies  from  rocks  weighing  many  tons  to 
the  finest  dust,  which  may  remain  in  the  air  many  months.  The  term 


VOLCANOES  AND  IGNEOUS  INTRUSIONS 


297 


ash  applied  to  this  fine  material  is  misleading,  since  dust  and  ash  are 
not  the  result  of  combustion,  as  the  name  seems  to  imply,  but  of  the 
shattering  of  the  rock  or  lava  by  explosions,  the  pulverization  of 
lava  by  sudden  cooling  after  it  is  hurled  into  the  air,  and  the  col- 
lisions between  stones  as  they  are  hurled  from  the  crater  or  as  they 
fall  back  to  the  ground.  No  part  of  the  work  of  volcanoes  has  a 
greater  geological  importance  than  the  production  of  dust.  Some 
of  it  is  so  fine  that  no  watchcase  is  so  closely  fitted  as  to  prevent  its 
entrance.  Near  the  vents  it  is  sometimes  scores  of  feet  thick,  and 
in  regions  several  hundred  miles  away  it  is 
sometimes  deposited  to  a  depth  of  several 
inches.  For  example,  in  1783  the  dust  from  an 
Icelandic  volcano  was  carried  to  Scotland,  a 
distance  of  600  miles,  in  sufficient  quantity  to 
destroy  the  crops. 

The  larger  particles  are  termed  cinders  and 
often  -constitute  the  conspicuous  deposits  of  the 
volcanic  cone,  the  fine  dust  having  been  carried 
away  by  the  wind. 

When  a  mass  of  molten  lava  is  thrown  into 
the  air,  it  takes  a  more  or  less  globular  form 
and  is  called  a  bomb.     Two  kinds  of  bombs  are 
common  :   one  spindle  or  almond-shaped   (Fig. 
290),  with  an  exterior  only  slightly  cracked  ;    bomb,  Aukland. 
the  other  with  a  surface  cracked  and  broken,    ft^ 
like  that  of  the  crust  of  a  loaf  of  bread.     The    rotating  liquid  lava  are 
cause  of  the  difference  is  to  be   found  in  the    cooled    as   they   pass 

air<    (After 


FIG.    290.  —  Volcanic 
The 


degree  of  liquidity  of  the  lava.     The  spindle- 

shaped   bombs  were  formed   from  very  liquid 

lava,  and  their  shape  was  produced  by  their  gyratory  motion  in  the 

air,  while  the  "  bread-crust  "  bomb  was  formed  from  viscous  lava 

which  was  little  affected    by  the   rotation,  and   which  cracked    in 

cooling,   forming   a  glassy  surface  and    a    porous  interior.     Bombs 

vary  in  size  from  a  few  inches  to  several  feet  in  diameter. 

When  the  ejected  lava  is  blown  full  of  holes  by  the  expansion  of 
the  gas  which  it  contains,  it  becomes  so  cellular  that  it  is  practically 
rock  froth,  the  air  cavities  being  sometimes  eight  or  nine  times 
greater  than  the  inclosing  glass,  so  that  it  is  light  enough  to  float  upon 
the  water.  When  it  is  in  this  condition,  it  is  called  pumice.  After 
the  eruptions  of  certain  volcanoes  situated  on  shores,  great  quanti- 


298  PHYSICAL  GEOLOGY 

ties  of  floating  pumice  have  covered  the  neighboring  waters  so  thickly 
as  to  be  a  menace  to  navigation.  During  the  eruption  of  a  volcano 
in  Japan  so  much  pumiceous  material  was  thrown  out  that  it  was 
possible  to  walk  a  distance  of  twenty-three  miles  upon  the  debris 
floating  on  the  sea. 

The  size  of  the  blocks  of  rock  thrown  out  during  eruptions  varies  greatly  with  differ- 
ent volcanoes.  A  2OO-ton  block  is  said  to  have  been  hurled  a  distance  of  nine  miles  from 
the  volcano  Cotopaxi  in  South  America,  and  it  is  reported  that  a  rock  fragment  100  or 
more  feet  in  diameter  was  ejected  from  the  Japanese  volcano  Asama.  Some  volcanoes, 
however,  throw  out  no  rock  fragments. 

The  quantity  of  fragmental  material  ejected  by  volcanoes  can 
best  be  shown  by  a  few  examples.  It  has  been  estimated  that  4.3 
cubic  miles  of  material  were  ejected  from  Krakatao  (p.  304)  in  1883, 
and  28.6  cubic  miles  from  Timboro  in  1815.  During  the  eruption 
of  Sumbawa  in  the  same  year  an  area  of  nearly  1,000,000  square 
miles  was  covered  by  an  amount  of  fragmental  material  estimated  to 
be  sufficient  to  make  185  mountains  of  the  size  of  Vesuvius. 

Lava.  —  All  molten  rocks  which  issue  from  the  earth  and  also  the 
solid  rock  which  results  when  they  cool  are  included  in  the  term  lava. 

Lava  streams  issue 
-  either  from  the  crater 
of  a  volcano  by  over- 
flowing or  breaking 
through  its  rim ; 

from  fissures  or  open- 
FIG.  291. —  Small  craters,  c.  c,  c,  along  a  fissure,  through     .  n      , 

which  lava  has  been  extruded.  ings  on  lts  flanks  ?   or 

through    fissures    in 

the  earth's  surface  (Fig.  291),  where  there  are  no  volcanic  cones. 
When  they  issue  from  a  volcano  they  flow  down  the  steepest  slope ; 
when  they  reach  a  gentle  slope  they  spread  out ;  when  some  obstacle, 
such  as  a  stone  wall,  is  encountered,  their  progress  is  at  first  stopped, 
then  they  either  overflow  or  overthrow  it,  or  pass  around  its  ends. 

Lava  Streams.  —  The  surface  of  a  lava  stream,  which  at  first  glows 
like  red-hot  metal,  cools  quickly  and  blackens,  but  since  the  heat  of 
the  interior  is  kept  in  by  the  porous  crust  thus  formed  the  deeper 
parts  of  the  stream  remain  in  a  molten  condition  for  a  long  time,  oc- 
casionally foe  several  years.  One  can  often  walk  across  a  lava  flow 
a  few  days  after  it  ceases  to  move,  and  while  the  deeper  portions  are 
still  molten,  without  suffering  any  inconvenience.  After  the  crust 
has  hardened,  the  still  molten  lava  of  the  interior  may  continue  to 


VOLCANOES  AND   IGNEOUS   INTRUSIONS 


299 


flow  until  it  drains  out,  leaving  a  tunnel  which  may  be  several  miles 
long  (p.  309).  A  lava  tunnel  on  Mt.  Shasta,  California,  which  is  60 
to  80  feet  high  and  20  to  70  feet  broad,  has  been  explored  nearly  a  mile 
without  its  end  being  reached. 

Effect  of  Composition  on  Fluidity.  —  Lava  varies  greatly  in  com- 
position and  fluidity.  Some  lava  streams  have  flowed  20  to  30  miles 
or  more,  while  others  have  solidified  as  soon  as  they  issued  from  their 
craters ;  some  have  flowed  several  miles,  while  others,  with  an  equally 
high  temperature  and  even  greater  volume,  have  moved  a  much 
shorter  distance  on  an  equal  slope.  This  difference  in  the  fluidity  of 
lavas  is  due  largely  to  their  chemical  composition  and  to  their  tem- 
perature. The  basic,  dark-colored  lavas  (p.  329)  fuse  at  a  lower 
temperature  and  are  consequently  more  likely  to  flow  long  distances. 
The  acid,  usually  light-colored  lavas  (p.  329)  melt  at  a  higher  tem- 
perature and  consequently  become  solid  while  still  hot.  They  are 
therefore  likely  to  solidify  quickly.  It  is  evident,  however,  that  if  a 
basic  lava  has  a  temperature  which  is  but  slightly  above  the  melting 
point,  it  will  be  as  stiff  (viscous)  as  an  acid  lava  at  a  high  temperature. 

Temperature.  —  The  temperature  of  lava  when  it  issues  from  the 
vent  of  a  volcano  is  probably  often  greater  than  2000°  F.  This 
is  shown  by  the  fact  that  copper  wire,  whose  melting  point  is  2200°, 
was  fused  in  a  Vesuvian  lava  stream  which  had  already  lost  some  of 
its  heat  (p.  273).  The  temperature  of  the  lavas  in  Kilauea  in  July, 
1911,  was  1260°  C. 
(2300°  F.) ;  that  of 
Stromboli  in  March, 
1901,  was  1150°  to 
1176°  C.  (2102°  to 
2149°  F.). 

Surface  of  Lava 
Flows.  —  The  sur- 
faces of  lava  flows 
vary  greatly,  some 
being  so  rough  as  to 
make  walking  danger- 
ous and  difficult, 
while  others  are  com- 
paratively smooth. 
A  fluid  lava  will  con-  FlG>  292>  _  pahoehoe  type  of  lava  surface  in  the  crater 
solidate  with  smooth  of  Kilauea,  Hawaii.  (U.  S.  Geol.  Surv.) 


300 


PHYSICAL  GEOLOGY 


FIG.  293.  — The  rough  aa  surface  of  a  lava  flow  on  the 
volcano  Colima.     Mexico. 


and  ropy  surfaces,  while  a  viscous  one  will  become  very  scoriaceous. 
This  latter  condition  is  partially  due  to  the  gas  in  the  lava,  which 

instead  of  escaping 
freely  to  the  air  forms 
bubbles  in  the  sur- 
face of  the  lava,  just 
as  air  blown  into 
soapy  water  forms  a 
frothy  surface ;  the 
crust  may  also  be 
broken  to  some  ex- 
tent by  the  continued 
movement  of  the 
more  liquid  mass 
below,  causing  an  ex- 
tremely rough  sur- 
face when  the  mass 
hardens.  The  Ha- 
waiian word  pahoehoe  is  used  to  designate  the  smooth  type  of 
lava,  with  the  gently  rounded,  ropy  surface  which  is  characteristic 
of  fluid  lavas  (Fig.  292) ;  while  another  Hawaiian  term  aa  is  used 
for  the  rough,  cindery  surface  (Fig.  293). 

Velocity  of  Lava  Flows.  — The  rate  of  flow  of  lava  depends  upon 
its  fluidity  and  upon  the  slope  over  which  it  moves.  In  Iceland  lava 
streams  have  flowed  over  surfaces  which  appear  flat  to  the  naked  eye, 
while  elsewhere  they  have  consolidated  on  slopes  which  were  almost 
vertical.  A  lava  stream  on  Mauna  Loa  flowed  fifteen  miles  in  two 
hours,  and  the  main  stream  from  Vesuvius  in  1906  descended  the 
first  steep  slopes  with  a  velocity  of  about  two  miles  an  hour.  Such 
rates  as  the  above  are,  however,  rather  unusual.  The  rate  of  flow  is 
gradually  reduced  as  the  stream  cools  and  as  the  slope  diminishes. 
Lava  often  continues  moving  for  a  long  time  after  the  eruption  ceases. 
A  lava  stream  which  began  to  move  on  Vesuvius  in  1895  was  found 
to  be  still  in  motion  four  years  afterward. 

Nature  of  Lavas.  —  The  slag  formed  in  an  iron  furnace  is  really  an 
artificial  lava,  and  from  it  much  concerning  the  nature  and  behavior 
of  lavas  can  be  learned.  When  lava  is  spoken  of  as  a  molten  rock  it 
should  be  understood  that,  since  rocks  are  composed  of  minerals 
varying  in  fusibility  and  solubility,  it  is  really  a  liquid  rock  in  which 
some  mineral  matter  is  dissolved  in  other  mineral  matter;  i.e.,  it  is  a 


VOLCANOES  AND  IGNEOUS   INTRUSIONS 


301 


mutual  solution  of  mineral  matter  in  mineral  matter.     Gases  as  well 
as  mineral  matter  enter  into  the  solution. 

This  can  best  be  illustrated  by  a  well-known  experiment.  If 
crystals  of  snow,  salt,  and  sugar  are  mixed  together  and  compacted 
at  a  low  temperature,  an  artificial  rock  will  be  formed  in  which  the 
constituents  can  be  recognized.  If  the  temperature  of  this  solid  is 
now  raised  to  about  32°  F.  the  mass  will  become  a  liquid,  even  though 
the  melting  points  of  salt  and  sugar  are  very  much  higher.  In  this 
case,  a  rise  in  temperature  sufficient  to  melt  but  one  of  the  constituents 
is  necessary,  since  this  one  is 
then  capable  of  dissolving  the 
others.  If  the  temperature 
of  such  a  solution  is  again 
lowered,  the  salt  and  sugar 
will  not  crystallize  out  until 
they  are  forced  to  take  the 
solid  form  by  the  crystalliza- 
tion (freezing)  of  the  water. 
It  is  evident  that  both  in  the 
process  of  solution  and  in  that 
of  crystallization  the  important 
factor  is  solubility,  and  that  a 
temperature  merely  sufficient 
to  melt  one  of  the  constituents 
is  necessary. 

That  lava  should  be  con- 
sidered as  a  solution  of  vari- 
ous minerals  is  evident  when 
cooled  lavas  are  examined.  If 
lavas  were  simply  molten  rocks  in  which  the  minerals  had  melted 
according  to  their  fusibility,  we  should  find  that  upon  cooling  the 
least  fusible  mineral  would  crystallize  out  first,  then  the  others  in 
the  order  of  their  fusibility.  Such,  however,  is  not  always  the  case ; 
often  the  least  fusible  mineral  is  the  last  to  take  the  solid  form.  This 
is  due  to  the  fact  that  the  liquid  mass  is  a  solution  in  which  the 
various  minerals  assume  the  liquid  state,  and  upon  cooling,  the  solid 
state,  depending  upon  their  solubility  more  than  upon  their  fusibility, 
the  least  soluble  rather  than  the  most  infusible  crystallizing  first. 

When  a  lava  cools  very  slowly,  as  is  usually  the  case  when  it  is 
intruded  beneath  the  surface,  the  molecules  of  which  it  is  composed 


FIG.  294.  —  Scoriaceous  lava. 
National  Museum.) 


(U.  S. 


302  PHYSICAL  GEOLOGY 

tend  to  collect  into  crystals.  When  the  process  is  long  continued 
the  point  of  saturation  of  the  other  minerals  is  reached,  crystals  are 
formed,  and  a  rock  composed  entirely  of  crystals  results.  Granite 
and  coarse-grained  traps  (p.  330)  are  such  rocks.  If  the  cooling  is 
more  rapid,  rocks  composed  of  fine  crystals  such  as  rhyolites  and 
basalts  (p.  331)  may  be  formed.  When,  however,  a  lava  flow  cools 
so  rapidly  that  no  crystals  or  only  a  few  can  form,  volcanic  glass,  or 
obsidian,  is  produced.  Often  a  lava  flow  passes  from  a  glassy  to  a 
crystalline  state  from  the  surface  downward.  When  such  cooling 
lava  is  under  little  pressure,  the  gases  in  the  surface  portions  are  able 
to  expand,  and  often  produce  a  surface  which  is  called  scoriaceous 
(Fig.  294)  if  cindery,  or  pumiceous  if  the  pores  are  very  numerous 
and  small. 

TYPES  OF  VOLCANOES 

Because  of  their  destructiveness  volcanoes  probably  inspire  greater 
interest  than  any  other  natural  phenomenon,  and  it  will  consequently 
be  well  to  discuss  briefly  the  various  types  of  volcanoes.  It  should, 
however,  be  remembered  that  the  aggregate  work  of  volcanoes  is 
inconsiderable  as  compared  with  that  of  streams,  the  ocean,  and  other 
less  conspicuous  forces. 

The  chemical  composition  of  lavas,  as  will  be  seen,  has  a  con- 
siderable influence  upon  the  character  of  eruptions,  but  the  principal 
factor  is  the  physical  state  of  the  lava ;  i.e.,  whether  it  is  fluid  or 
viscous  and  stiff.  If  the  molten  rock  is  so  liquid  that  the  gases  can 
escape  rapidly,  they  do  not  accumulate  into  great  bubbles  which 
throw  the  lava  high  into  the  air  when  they  break.  If  on  the  other 
hand  the  lava  is  stiff,  the  gases  gather  into  great  bubbles  which  upon 
bursting  throw  out  the  lava  as  dust,  cinders,  and  bombs. 

I.    The  Explosive  or  Vesuvian  Type 

(i)  Vesuvius.  —  Vesuvius  has  been  more  carefully  studied  than  any 
other  volcano  in  the  world  and  is  yearly  ascended  by  so  many  travelers 
that  its  value  as  an  illustration  is  unsurpassed. 

Previous  to  79  A.D.  Vesuvius  seemed  to  be  extinct,  but  before  the 
close  of  that  year  a  great  eruption  occurred  which  destroyed  the  cities 
of  Herculaneum  and  Pompeii,  and  laid  waste  a  great  extent  of  coun- 
try. During  this  eruption  no  lava  was  poured  out,  but  a  large  part 
of  the  crater  was  blown  off  and  the  outline  of  the  mountain  greatly 
changed.  Ash  and  dust  were  thrown  to  great  heights  and  were  carried 


VOLCANOES  AND  IGNEOUS  INTRUSIONS 


303 


63    A.D 


A.D.78-I63I 


1822 


long  distances  by  the  wind.  Pompeii,  at  the  foot  of  the  mountain, 
was  buried  beneath  25  or  30  feet  of  ash,  and  Herculaneum  beneath 
60  feet  of  mud  and  ash,  the  latter  being  covered  by  a  layer  of  lava 
during  a  later  eruption.  So  completely  were  these  cities  hidden  that 
their  sites  were  unknown  for  more  than  1600  years.  After  this  first 
historical  eruption  (79  A.D.),  which  is  well  described  by  the  younger 
Pliny  in  a  letter  to  Tacitus,1  the  volcano  was  occasionally  eruptive 
until  1139.  Then,  for  a  period  of  almost  500  years,  with  the  exception 
of  one  feeble  eruption,  the  volcano  seemed  again  to  have  become 
extinct,  and  the  crater  was  choked  with  rubbish  and  covered  with 
trees.  In  1631  another  violent  eruption 
occurred  (Fig.  295) ;  fissures  opened  in  the 
side  of  the  mountain,  through  some  of 
which  steam  and  ash  were  thrown,  and 
four  streams  of  lava  poured  from  the 
crater,  three  of  which  reached  the  sea. 
During  this  eruption  the  cone  was  reduced 
about  525  feet  in  height.  During  the 
eruption  in  1906,  the  main  lava  stream 
flowed  at  a  rate  of  a  little  less  than  two 
miles  an  hour  where  the  slope  was  steep, 
but  more  slowly  when  passing  over  a  lower 

t  -rT71  ii'  i  rIG.  2QC.  —  Section  through 

grade.  When  wooden  objects,  such  as  Vesuvius,  showing  the  changes 
trees,  were  encountered  by  this  lava  in  the  shape  of  the  volcano 
stream,  they  were  charred  but  not  burned  ;  b£tween  63  A-D-  and  l868- 
some  were  broken  off  by  the  weight  of  the 

lava  and  carried  on  the  surface  of  the  stream.  When  a  large  object 
was  reached  the  lava  piled  up  behind  it  until  it  was  moved  aside, 
overflowed,  or  the  stream  moved  around  it. 

A  summary  of  the  sequence  of  events  in  a  recent  eruption  is  as  follows :  In  1904 
Vesuvius  was  almost  quiet,  but  soon  explosions  occurred  of  sufficient  force  to  throw 
fragments  short  distances  above  the  crater's  rim.  In  1905  a  narrow  stream  of  lava 
flowed  from  a  fissure  in  the  cone  throughout  the  year.  On  April  4  of  the  following  year 
a  great  cauliflower-shaped  cloud  of  dust  and  gas  rose  from  the  crater,  and  lava  streamed 
in  small  quantities  from  successively  lower  openings  in  the  side  of  the  cone.  On  April 
7  an  explosion  occurred  which  sent  a  column  of  dust-laden  gas  four  miles  vertically 
into  the  air,  and  new  and  larger  fissures  opened  through  which  lava  flowed.  The 
dust  so  weighted  the  roofs  of  the  houses  as  to  cause  them  to  collapse  with  loss  of  life. 
One  lava  stream  destroyed  the  town  of  Boscotrecase. 

1  Translation  in  Shaler's  Aspects  of  the  Earth,  pp.  50-56,  or  in  Lyell's  Principles  of  Geology, 
p.  603. 

CLELAND   GEOL.  —  2O 


304 


PHYSICAL  GEOLOGY 


N.W 


-/v: 


FIG.  296.  —  A,  Krakatao  as  seen  from  the  north  after  the  eruption  of  1883. 
B,  an  outline  of  the  crater  of  Krakatao.     The  dotted  lines  show  the  contour  of  the 
island  before  the  eruption,  the  continuous  lines  as  it  is  now. 

(2)  Krakatao.  —  The  most  stupendous  volcanic  explosion  of  mod- 
ern times  occurred  in  the  East  Indies  when  the  island  of  Krakatao, 
lying  between  Java  and  Sumatra,  suddenly  became  eruptive  in  1883. 


FIG.  297.  —  Map  of  a  portion  of  Alaska  showing  the  thickness  in  inches  of  the  deposit 
of  ash  from  an  eruption  of  the  volcano  Katmai  in  1912. 


VOLCANOES  AND  IGNEOUS   INTRUSIONS 


305 


This  island  was  indeed  not  known  to  be  a  volcano,  until  in  August  of  the  above- 
mentioned  year  it  became  violently  explosive  (Fig.  296)  and  in  two  days  blew  away 
about  one  half  of  its  surface,  so  that  now  the  sea  is  1000  feet  deep  where  the  central 
part  of  the  mountain  formerly  stood.  The  amount  of  ash  ejected  was  so  great  that 
the  neighboring  seas  and  land  were  in  total  darkness  during  the  eruption.  The  ash 
was  thrown  to  a  height  of  17  miles,  and  remained  in  the  air  many  months,  causing 
brilliant  sunrises  and  sunsets  throughout  the  world  (p.  54).  Ships  1600  miles  away 
were  covered  with  dust  three  days  after  the  eruption;  stretches  of  water  with  an  aver- 
age depth  of  117  feet  were  so  filled  with  the  debris  as  to  be  no  longer  navigable.  The 


FIG.  298.  — A  roof  collapsed  by  the  weight  of  ash  from  Katmai,  Alaska,  one  hun- 
dred miles  distant.  The  drift  in  front  of  the  porch  is  volcanic  ash.  (National 
Geographic  Magazine.) 

noise  of  the  explosions  was  heard  2000  miles  away,  and  the  shock  produced  waves  50 
to  80  feet  high,  which  swept  the  adjacent  shores,  deluging  1295  villages  and  drowning 
about  35,000  people.  The  height  and  strength  of  the  waves  is  well  shown  in  the  fact 
that  a  large  vessel  was  carried  one  and  one  half  miles  inland  and  left  stranded  on  land 
30  feet  above  sea  level,  and  that  blocks  of  rock  weighing  30  to  50  tons  were  carried 
inland  two  or  three  miles. 


(3)  Katmai.  —  The  eruption  of  Katmai,  a  volcano  in  the  Alaskan  peninsula,  in  June, 
1912,  was  one  of  considerable  violence,  but  one  which  did  little  damage  because  of  its 
situation  in  an  almost  uninhabited  region.  As  will  be  seen  from  the  map  (Fig.  297), 
the  fall  of  ash  was  50  inches  deep  30  miles  from  the  volcano,  and  6  inches  deep  160 
miles  to  the  east  of  the  mountain.  So  great  was  the  amount  of  dust  in  the  air  that 


306 


PHYSICAL  GEOLOGY 


100  miles  away  total  darkness  prevailed  for  60  hours  (Fig.  298).     The  sound  of  the 
explosions  was  carried  along  the  coast  for  750  miles. 

(4)  Mt.  Pelee.  —  The  eruption  of  Mt.  Pelee  (1902)  on  the  island  of  Martinique  in  the 
West  Indies  was  remarkable  because  of  two  unusual  features,  (i)  A  great  blast  of 
highly  heated  air  mingled  with  incandescent  dust  swept  down  one  side  of  the  moun- 
tain and  overwhelmed  the  town  of  St.  Pierre,  killing  all  but  two  of  its  30,000  inhabitants, 
one  of  these  being  a  prisoner  in  an  underground  cell  to  which  the  air  had  access  only 
through  a  small  opening.  The  cause  of  this  mortality  was  due  almost  entirely  to  the 
fine,  hot  dust  which  penetrated  into  all  of  the  houses  and,  when  breathed,  resulted  in 
almost  instant  death.  The  reason  for  the  descent  of  the  blast  on  one  side  only  of  the 


Cap  StMartii 


U  Perle- 


FIG.  299. — Map  of  Mt.  Pelee  and  environs,  showing  the  portion  of  the  island  of 
Martinique  devastated  by  the  volcanic  eruption  of  1902.  The  breach  in  the  crater 
wall  is  also  indicated.  (Hill,  National  Geographic  Magazine.} 


mountain  is  readily  seen  when  the  form  of  the  crater  is  studied  (Fig.  299).  With  the 
exception  of  one  place  (opposite  St.  Pierre),  where  the  Riviere  Blanche  had  cut  a  deep 
gorge,  the  rim  of  the  crater  was  several  hundred  feet  high.  When  the  explosion  oc- 
curred its  force,  instead  of  being  expended  entirely  upward,  was  partly  directed  through 
the  gash  in  the  side  of  the  crater,  and  a  great  cloud  of  intensely  hot  gas,  dust,  and 
bombs  moved  down  upon  St.  Pierre.  (2)  The  second  unusual  feature  was  noticed 
after  the  principal  eruption  was  over.  It  consisted  in  a  spine  of  solid  rock  rising  from 
the  crater  (Fig.  300),  which  began  to  grow  in  October,  1902,  and  reached  an  elevation 
of  1000  feet  at  the  end  of  seven  months.  Much  discussion  has  arisen  as  to  the  origin 
of  this  spine,  but  it  is  generally  believed  that  it  was  formed  by  very  stiff  lava  which 
solidified  into  a  steep-sided  column  as  rapidly  as  it  was  forced  to  the  surface.  Other 


VOLCANOES  AND  IGNEOUS   INTRUSIONS 


307 


volcanoes  are  known  in  which  the  lava 
forced  out  of  craters  near  the  close  of 
eruptions  assumed  the  form  of  steep- 
sided  cones. 

(5)  Bandai-san.  —  An  eruption  in 
which,  so  far  as  known,  no  lava  was 
discharged  took  place  in  1888  in  Japan. 
For  1000  years  Bandai-san,  a  volcanic 
cone  2000  feet  high,  had  been  dormant, 
when  suddenly  a  terrific  explosion  blew 
away  the  greater  part  of  the  mountain 
(Fig.  301).  Since  this  one  explosion, 
the  volcano  has  shown  no  signs  of 
activity.  The  catastrophe  was  due  to 
the  heating  of  water  which  had  perco- 
lated from  the  surface,  and  was  in  fact 
a  steam  explosion.  A  priest  living  on 
the  mountain  reported  that  the  gases 
surrounding  him  were  respirable. 

T        ,,      r     ,  •  r     i  FIG.    300. — The   spine   of   Mt.   Pelee, 

In  all  of  the  eruptions  of  the    Martiniqjue>   French  West  Indies>  I902/ 

explosive    type,    dust,    cinders,     (After  E.  O.  Hovey.) 
and   usually   bombs   are  thrown 

out;    earthquakes  are  prevalent  previous  to  and  accompanying  the 

eruptions;    sometimes  lava  is  poured  out  from  the  crater  or  from 

......  fissures    in    the    mountain    side. 

The  greatest  eruptions  often  occur 
after  long  periods  of  inactivity. 


II.    The  Quiet  or  Hawaiian  Type 


FIG.  301. -Volcano  Bandai-san.  The  .  The  Hawaiian  type  of  volcano 
portion  enclosed  by  the  dotted  line  was  IS  in  marked  contrast  to  the  CX- 
blown  off  during  an  eruption  lasting  less  plosive  Or  Vesuvian  type,  since  in 
than  two  hours.  The  height  of  the  cliff  ,  r 

is  about  1500  feet.  the  former  eruptions  are  not  ac- 

companied  by  severe  explosions, 

but  consist  largely  in  the  gentle  welling-out  of  lava  from  the  crater 
or  from  mouths  in  the  sides  of  the  mountain.  In  general,  the 
features  most  characteristic  of  volcanoes  of  this  type  are:  (i)  their 
gentle  slopes  which  do  not  average  more  than  7  degrees,  (2)  the 
large  size  of  their  craters  or  calderas?  (3)  the  quietness  of  the  erup- 
tions, (4)  the  fusibility  of  the  (basic)  lava  which  they  discharge,  and 
(5)  the  absence  of  severe  earthquakes  during  and  preceding  eruptions. 


1  See  footnote  on  p.  309  for  restricted  use  of  term  caldera. 


308 


PHYSICAL  GEOLOGY 


The  summit  of  Mauna  Loa  on  the  island  of  Hawaii  is  13,675  feet 
high,  while  the  volcano  Kilauea  on  its  flanks  20  miles  distant  is 
only  about  4000  feet  above  the  sea.  Though  forming  one  mountain 
the  two  volcanoes  are  entirely  independent,  having  been  joined  by 
the  gradual  growth  of  the  two  cones.  The  surface  near  the  summit  of 
Mauna  Loa  is  nearly  flat  for  several  square  miles,  and  the  crater  can- 
not be  seen  until  one  is  close  upon  it,  the  mean  slope  within  a  circle 
of  five  miles  around  the  crater  being  about  three  degrees.  If  one 
conceives  of  the  ocean  as  removed,  this  volcano  (Mauna  Loa)  would 
tower  above  the  floor  of  the  sea  as  a  broad-topped  mountain,  to  a 
height  of  more  than  30,000  feet,  with  a  base  many  miles  in  diameter. 
Every  island  of  the  Hawaiian  group  is  of  the  same  nature  and  is 
usually  built  up  by  lava  from  several  cones.  With  the  exception  of 
Iceland,  the  island  of  Hawaii  is  the  largest  pile  of  lava  in  the  world. 

Crater  of  Kilauea.  —  The  caldera  of  Kilauea  will  be  taken  as  a  type  of  volcanoes  of 
this  class.  On  the  top  of  the  mountain  is  a  great  pit,  three  miles  long  and  two  miles 


FIG.  302. — Map  of  the  Kilauea  caldera,  Hawaii,  in  1886. 

wide,  surrounded  by  vertical,  terraced  walls  (Fig.  302).  The  floor  of  the  caldera  is 
composed  of  a  plain  of  black  lava  in  which  lies  a  lake  of  liquid  lava  of  a  bright  orange 
color.  The  surface  of  the  lake,  except  near  the  center,  is  covered  by  a  scum  of  frothy 
lava.  During  eruptions,  great  volumes  of  this  fiery  liquid  are  thrown  many  feet  into 
the  air.  From  time  to  time  the  surface  of  the  molten  lake  cools  sufficiently  to  permit 
it  to  harden.  The  lava  crust  thus  formed  then  cracks,  and  through  the  cracks  jets 
and  fountains  of  lava  are  ejected.  The  level  of  the  lava  floor  does  not  remain  station- 
ary, but  gradually  rises  previous  to  an  eruption,  sometimes  as  much  as  100  feet  a  year, 


VOLCANOES  AND  IGNEOUS   INTRUSIONS  309 

until  the  lava  may  reach  to  within  300  feet  of  the  rim  of  the  crater,  but  never  (in  modern 
times)  overflowing  it.  After  the  eruption  the  floor  may  be  1000  feet  below  the  edge 
of  the  crater. 

Eruptions.  —  When  the  lava  rises  in  the  crater,  it  is  evident  that  the  pressure  on  its 
walls  is  greatly  increased,  since  a  column  of  liquid  lava  50  feet  high  exerts  a  pressure 
of  about  625  pounds  to  the  square  inch.  The  result  of  this  increased  pressure  is  either 
actually  to  fracture  the  mountain  and  thus  to  afford  an  avenue  of  escape  for  the  lava, 
or  to  aid  it  to  break  and  fuse  its  way  through  the  porous  lava  of  which  the  side  of  the 
mountain  is  built.  During  the  eruption  of  Kilauea  in  1840  lava  first  made  its  appear- 
ance five  miles  from  the  main  crater;  later  it  sank  in  this  new  crater  and  reappeared 
at  other  smaller  openings  farther  down  the  mountain  side ;  finally,  it  was  poured  out 
on  the  surface  still  lower  down  and  flowed  in  a  molten  stream  to  the  sea.  During  the 
eruption  of  Mauna  Loa  in  1853,  a  fountain  of  lava  200  to  700  feet  in  height  and  1000 
feet  broad  burst  out  at  the  base  of  the  cone  as  a  result  of  hydrostatic  pressure. 

Lava  Streams.  —  The  flow  of  lava  from  Kilauea  on  one  occasion  "  swept  away  forests 
in  its  course,  at  times  parting  and  inclosing  islets  of  earth  and  shrubbery,  and  at  other 
times  undermining  and  bearing  along  masses  of  rock  and  vegetation  on  its  surface.  It 
plunged  into  the  sea  with  loud  detonations.  The  burning  lava,  on  meeting  the  waters, 
was  shivered  like  melted  glass  into  millions  of  particles,  which  were  thrown  up  in  clouds 
that  darkened  the  sky  and  fell  like  a  storm  of  hail  over  the  surrounding  country.  The 
light  was  visible  for  over  a  hundred  miles  at  sea,  and  at  the  distance  of  forty  miles 
fine  print  could  be  read  at  midnight.  "  (J.  D.  Dana.)  Such  explosive  action,  however, 
does  not  always  take  place  when  lava  reaches  water,  probably  because  of  the  cooler 
and  more  stony  character  of  the  lava.  This  was  true  of  a  lava  stream  from  Vesuvius 
in  1794,  which  entered  the  sea  so  quietly  that  it  was  possible  to  watch  its  progress  from 
a  boat  close  to  its  front. 

The  tunnels  and  caves  on  the  Hawaiian  volcanoes,  caused  by  the  draining  out  of 
the  lava  from  below  the  hardened  crust,  are  hung  with  lava  stalactites  20  to  30  inches 
long,  and  stalagmites  formed  by  lava  dripping  from  above  project  from  the  floor. 
Such  tunnels  are  sometimes  buried  beneath  later  flows  and  may  later  be  utilized  as 
outlets  for  lava,  such  as  occurred  during  the  Kilauea  eruption  just  described,  when 
the  lava  burst  out  near  the  foot  of  the  mountain. 

An  interesting  form  of  lava  found  on  Kilauea,  called  Pelee's  hair,  is  composed  of 
hair-like  threads  of  lava  glass,  and  in  masses  resembles  tow.  It  is  formed  when  the  wind 
catches  particles  of  molten  lava,  either  from  the  lava  froth  or  from  the  jets  thrown  up 
from  the  crater,  and  draws  them  out  into  glassy  threads. 

Origin  of  Calderas.  —  The  craters  of  the  Hawaiian  volcanoes  have  been  enlarged  by 
the  sinking  in  of  their  sides  and,  as  has  been  said,  are  called  calderas.  Calderas  l  are 
also  formed  as  a  result  of  violent  explosions  which  blow  off  the  top  of  a  cone,  as  was  true 
of  Vesuvius  during  the  first  historic  eruption  (p.  302).  Calderas  are  craters  of  unusual 
size,  varying  from  one  to  five  or  more  miles  in  diameter.  One  of  the  most  remarkable 
calderas  in  the  world  is  that  of  Crater  Lake,  Oregon  (Fig.  303),  which  is  five  to  six  miles 
in  diameter  and  2000  feet  deep,  the  walls  standing  900  to  2200  feet  above  the  water. 
A  small  cone,  called  Wizard  Island,  rises  a  few  hundred  feet  above  the  lake.  The 

1  Daly  restricts  the  term  caldera  to  great  craters  formed  by  explosions,  such  as  that  of  Kra- 
katao.  The  word  sink  is  suggested  for  the  Hawaiian  and  Crater  Lake  (Oregon)  craters, 
formed  by  the  sinking  in  of  the  top  of  the  mountain. 


PHYSICAL  GEOLOGY 


presence  of  glacial  striae  on  the  crater  rim  proves  that  the  sum- 
mit of  the  mountain  was  at  one  time  much  higher  than  now, 
and  that  a  glacier  moved  down  it  over  what  is  now  the  rim. 
It  is  believed  that  the  crater  was  formed  by  the  sinking  in  of 
the  top  of  the  mountain,  the  absence  of  volcanic  ejectamenta 
about  the  mountain  being  proof  that  the  mountain  was  lowered 
in  this  way  rather  than  by  an  explosion.  The  craters  of  the 
moon  are  two  to  twenty  times  larger  in  diameter  than  those 
of  the  earth  and  may  have  been  formed  in  the  same  manner 
as  those  of  the  Hawaiian  type. 

Steep  Lava  Cones:  Volcanoes  of  the  Chim- 
borazo  Type.  —  It  should  not  be  concluded  from 
a  study  of  the  Hawaiian  volcanoes  that  all  lava 
cones  have  a  gentle  slope.  When  the  lava  is 
viscous,  steep-sided  cones  are  formed.  The  great 
Ecuador  volcano,  Chimborazo  (20,498  feet),  is 
composed  of  lava  and  is,  moreover,  craterless.  As 
the  lava  welled  up  from  the  vent,  it  left  upon 
cooling  no  depression  in  the  summit  of  the 
mountain. 


6 

2 
U 

•S 


When  lava  is  very  fluid  and  in  great  quantity, 
it  may  flow  long  distances  and  form  compara- 
tively level  plains  many  square  miles  in  extent. 
The  most  remarkable  lava  plateau  of  this  kind  in 
North  America  (Fig.  304)  covers  an  area  of  200,000 
to  250,000  square  miles  in  Washington,  Oregon, 
Idaho,  and  California  (p.  583).  It  is  a  vast  plain 
of  black  basic  lava  over  which  one  may  ride  for 
many  hours  on  a  level  surface.  The  lava  which 
overspread  this  region  poured  out  from  fissures 
instead  of  from  volcanoes,  with  little  or  no  explo- 
sive action,  and  since  it  was  very  fluid,  flowed  for 
long  distances,  filling  the  valleys  and  covering  the 
smaller  hills.  Some  portions  of  the  region  were 
buried  hundreds  of  feet  deep,  the  greatest  depth 
being  estimated  at  3700  feet.  This  great  plain 
was  not  built  up  by  a  single  great  outpouring  of 
lava,  but  by  a  number  of  flows,  some  of  which 
followed  each  other  in  rapid  succession.  On  the 


VOLCANOES  AND  IGNEOUS  INTRUSIONS 


other  hand,  the  surfaces  of  some  of  the  flows  were  exposed  to  the 

action    of   the   weather   many   years    before    the   next   outpouring 

occurred,  as  is  shown  by  the 

thick  layers  of  soil  between 

the  lava  flows.     Previous  to 

the  extrusion  of  the  lava  the 

region  was  a  deeply  dissected 

one,  but  the  lava  filled  the 

valleys,  buried  the  lower  hills, 

and  surrounded  some  of  the 

mountains,  leaving  them   as 

islands  in  a  molten  sea.     The 

border  of  the  lava  plateau  is 

very   irregular,    since    ridges 

and    spurs    extend    into    it         FlG.  304.  _  Lava  fields  in  Washington, 

from  the  higher  land,  and  it  Oregon,  Idaho,  and  California. 

in  turn  protrudes  long  fingers 

between  the  mountain  masses.     The  edge  of  the  sheet  can  best  be 

compared  to  the  shore  line  of  a  submerged  coast  (p.  226). 

Recent  Icelandic  Lava  Sheets.  —  Much  of  the  nature  of  such  lava  plains  as  those 
described  can  be  learned  from  a  study  of  recent  eruptions  in  Iceland,  a  region  which 
exhibits  marks  of  igneous  activity  in  greater  variety  and  magnitude  than  any  other 
spot  in  the  world.  In  1783  lava  welled  out  for  several  months  from  the  great  Laki 
fissure.  This  fissure  is  20  miles  long,  and  on  it  were  formed  more  than  one  hundred 
low  craters,  from  which  sheets  of  lava  were  spread  out  on  either  side  (Fig.  291,  p.  298). 
From  the  place  of  eruption  the  lava  stream  flowed  47  miles  on  one  side  and  28  miles  on 
the  other,  covering  an  area  of  220  square  miles  to  an  average  depth  of  100  feet.  The 
longest  flow  on  record  in  Iceland  is  90  miles,  the  slope  of  which  is  so  gentle  as  to  be 
almost  imperceptible,  the  angle  being  only  a  little  more  than  one  half  of  a  degree.  In 
some  cases  lava  has  welled  up  from  fissures  in  Iceland  without  the  formation  of  cones; 
the  longest  flow  of  this  class  is  19  miles.  In  other  parts  the  lava  has  built  up  great 
domes  similar  to  those  in  Hawaii ;  one  of  these  is  4600  feet  high,  with  an  elliptical 
crater  about  three  quarters  of  a  mile  across  at  its  widest  point. 

In  1913  a  fissure  three  miles  long  was  formed  in  Iceland  from  craters  on  which  lava 
poured  forth  and  covered  the  plains.  In  some  cases,  the  lava  shot  up  in  a  jet  like  a 
geyser ;  in  others,  it  flowed  out  like  a  fiery  waterfall. 


CHARACTERISTICS  OF  VOLCANIC  CONES 

Profiles  of  Volcanoes.  —  The  slope  of  a  volcanic  cone,  as  has  been 
seen,  depends  upon  the  character  of  the  material  of  which  it  is  made. 
If  it  is  composed  entirely  of  cinders  and  ash,  the  slope  will  be  at  the 


312 


PHYSICAL  GEOLOGY 


angle  of  repose,  which  may  be  as  great  as  30°  or  40°  for  coarse  ash  or 
cinders  (Fig.  305  A).  The  slope  is  more  gentle,  however,  at  the  base 
of  the  cone,  since  the  dust  is  carried  farther  from  the  summit  than  the 

coarser  material  and 
is  washed  farther  still 
by  rain  and  rills.     If 
the  cone  is  of  lava, 
its  slope  will  depend 
upon  the  fluidity  of 
FIG.  305.  —  Angle  of  slope  of  volcanoes.     A  cone  com-    ^g    Java        Volcanic 
posed  of  ash  will  be  steep,  as  much  as  30°,  while  that  of  ,     *  r   , 

lava  may  not  be  more  than  9°,  as  in  the  Hawaiian  Islands.    cones    built   ot    basic 

lava     usually     have 

broad,  flattened  domes  (Fig.  305  B),  since  such  lava  cools  at  a  low 
temperature  and  consequently  may  remain  liquid  for  a  considerable 
time  and  flow  long  distances  before  solidifying.  The  Hawaiian  and 
Icelandic  volcanoes  are  examples.  If  stiff,  viscous  lava  is  dis- 
charged, the  slope  may  be  very  steep  (Fig.  307).  Cones  made  of  a 
combination  of  lava  and  ash  are  more  common  than  any  others  and 
are  usually  steep-sided.  The  vol- 
cano Fuji,  so  often  pictured  by 
the  Japanese  artists,  is  of  this 
type,  as  are  many  of  the  highest 
volcanoes  of  the  world. 

A  volcanic  cone  is  seldom  sym- 
metrical, since  if  it  has  been  long 
in  existence  it  has  suffered  many 
changes  (Fig.  306).  The  irregular 
outline  of  Vesuvius,  as  has  been 
seen,  is  the  result  of  the  blowing 
off  of  the  greater  part  of  an 
earlier  crater,  so  that  the  present 
cone  is  partially  surrounded  by 
Mt.  Somma,  a  remnant  of  the 
ancient  crater.  The  slopes  of 
Etna  are  roughened  by  scores  of 
parasitic  cones.  The  volcano 
Colima,  in  Mexico  (Fig.  308), 
would  be  beautifully  symmetrical  were  it  not  for  a  cone  formed  on 
the  flanks  of  the  mountain  in  1869.  This  secondary  cone  is  crater- 
less,  showing  that  near  the  close  of  the  eruption  the  lava  was  so 


FIG.  306.  — Outlines  of  volcanic  cones  : 
Ay  a  cone  formed  of  ash;  B,  a  cone  from 
which  the  top  was  blown  by  a  great  explo- 
sion; C,  a  caldera  formed  by  faulting. 


VOLCANOES  AND  IGNEOUS   INTRUSIONS 


313 


PHYSICAL  GEOLOGY 


FIG.  308.  —  Volcano  Colima  and  a  secondary  cone 
on  the  left. 


stiff  that  it  solidified  as  soon  as  it  reached  the  surface.  The  profile 
of  a  cone  depends  also,  to  a  greater  or  less  degree,  upon  the  force 
and  direction  of  the  wind  during  eruptions,  upon  the  position  of  the 

crater,  and  upon  the 
amount  of  erosion 
which  it  has  suffered. 
Shape  of  Craters. 
—  The  shape  of  the 
crater  of  a  volcano 
depends  both  upon 

(1)  the    violence    of 
the    explosions,    the 
diameter     of    the 
crater  of  an  explosive 
volcano      being,     in 
general,  proportional 
to  the  violence  of  the 
eruption;    and  upon 

(2)  the  character  of 
the  materials.     A  crater  has  steep,  rugged  inner  walls  when  lava 
and  coarse  cinders  are  ejected  (Fig.  309  A),  but  a  much  less  steep 
slope  when  dust  and  fine  ash  are  thrown  out  and  fall  back  into  it 
(Fig.  309  B). 

Erosion  of  Volcanic  Cones.  —  Up  to  this  point  we  have  discussed 
the  phenomena  of  an  eruption,  the  shape  of  cones  and  craters,  and 
other  features  connected  with 
recent  volcanoes,  but  aside  from 
these  observations,  we  have 
learned  little  of  the  internal 
structure  of  volcanic  cones.  The 
structure  is,  however,  revealed  to 
us  by  an  examination  of  ancient 
volcanoes  which  have  been  deeply 
eroded  by  atmospheric  agencies 
or  by  the  sea  (Fig.  310).  Great  FlG-  3Q9-  —  A,  a  cone  formed  of  coarse 
i  •  i  i  fragments;  B,  a  cone  formed  of  ash. 

explosions  also,  as  we  have  seen   (After  Haug ) 

in  the  case  of  Krakatao,  expose 

the  internal  structure  to  some  extent,  and  it  is  also  brought  to  light 
when  the  top  of  the  volcano  sinks  in,  as  in  the  case  of  Crater  Lake, 
Oregon  (p.  309). 


VOLCANOES  AND   IGNEOUS   INTRUSIONS 


315 


As  long  as  a  volcano  remains  active,  the  ravages  of  rain  and 
torrents  are  repaired  by  the  material  ejected,  but  when  it  becomes 
extinct  the  work  of 
denudation  contin- 
ues uninterruptedly. 
The  rate  of  erosion 
varies  greatly,  de- 
pending upon  the 
nature  and  structure 
of  the  materials  and 
upon  the  climate. 
Cones  composed  of 
coarse  cinders  are 
likely  to  endure  a 
time  than 


longer 


FIG.    310.  —  Rocks,  St.  Paul  Island;  a  volcanic  cone 
dissected  by  the  waves  until  the  crater  has  been  reached, 
forming  a  harbor, 
those  of  dust.     They 

are  more  porous  and  therefore  absorb  the  rain  falling  on  them  to  so 
large  a  degree  that  little  water  is  left  for  erosion.  Even  before  a 
volcano  becomes  extinct,  deep  V-shaped  valleys  are  cut  into  its  sides. 
We  find  also  that  the  dust  and  ash  are  in  layers,  and  that  sometimes 

black  beds  (Fig.  311) 
composed  of  disin- 
tegrated ash  and 
humus,  varying  from 
a  few  inches  to  several 
feet  in  thickness,  are 
interbedded  with  the 
ash.  These  black 
beds  are  ancient  soils 
and  prove  that  in  the 
past  the  volcano  ex- 
perienced many  years 
of  inactivity,  which 
were  followed  by 
eruptions. 

After       prolonged 
erosion  it  often  hap- 


FIG.  311.  —  A  ravine  (baranca)  in  the  side  of  the  vol- 
cano Toluca,  Mexico.  The  light-colored  deposit  is  vol- 
canic ash ;  the  dark  bands  are  ancient  soils  which  prove 
long  periods  of  quiet  after  periods  of  activity. 


pens  that  long,  wall-like  bodies  of  hardened  lava,  called  dikes  (p. 
324),  are  exposed.  These  dikes  were  formed  during  eruptions,  when 
the  force  of  the  explosions  or  the  pressure  of  the  column  of  lava  in 


316 


PHYSICAL  GEOLOGY 


the  vent  was  so  great  as  actually  to  rend  the  cone.  Into  these  cracks 
the  lava  was  forced  and  cooled.  It  will  readily  be  seen  that  a  cone 
buttressed  by  dikes  will  be  greatly  strengthened,  and  that  such  a 
cone  will  be  better  able  to  withstand  erosion  than  one  composed 
entirely  of  fragmental  materials. 

Necks  and  Plugs.  —  After  the  upper  portion  of  a  cone  has  dis- 
appeared, the  neck\¥ig.  312),  as  the  compact  lava  or  debris  filling 


FIG.  312.  —  Diagram  illustrating  the  destruction  of  volcanoes.     (After  A.  Geikie.) 

the  vent  is  called,  is  exposed.  The  neck  is  composed  either  of  lava 
or  of  the  rocks  or  other  fragmental  materials  which  fell  back  into  the 
crater  and  were  consolidated  to  form  a  volcanic  breccia.  They  vary 
in  diameter  from  a  few  yards  to  two  miles.  Volcanic  necks  or  plugs, 

when  exposed  by  erosion,  are 
often  conspicuous  features  of 
the  landscape.  Many  ex- 
amples are  to  be  found  in 
North  America.  From  Mon- 
treal one  can  see  several  hills 
of  this  origin.  In  New 
Mexico,  Arizona,  California, 
and  other  western  states  of 
the  United  States  volcanic 
necks  are  to  be  seen.  They 
are  not  uncommon  in  por- 
tions of  Europe,  where  they 
are  frequently  the  sites  of 
castles  or  churches  (Figs.  313, 
314).  When  erosion  has 
succeeded  in  entirely  tearing 
down  a  volcanic  cone,  it  is 
often  found  that  the  neck 
pierced  the  surrounding  rock 
without  the  aid  of  a  fissure  or  fault,  and  that  it  is  independent  of  the 
folds  of  the  rocks.  The  great  diamond  mines  of  South  Africa  are 


FIG.  313.  —  Volcanic  neck  upon  which  a  chapel 
has  been  built.      Le  Puy,  France. 


VOLCANOES  AND  IGNEOUS  INTRUSIONS 


317 


FlG'  3'4'        °lcanic  "eck 


u        « 

Mexico.     (Photo.  D.  W.  Johnson.) 


regi°n'  New 


located  in  the  necks 

of     volcanoes,      the 

brecciated     rock    of 

which  is  called  "blue 

ground  "    and     con- 

tains    the    gems. 

These    latter    necks 

have  a  diameter   of 

300  to  1000  feet. 
Age  of  Volcanoes 

in  the  United  States. 

—  In  regions  of  ex- 
tinct volcanoes  every 

stage  in  the  process 

of  demolition  may  be  studied  (Fig.  315  A,  B),  from  the  perfect  cone, 

whose  slopes  have 
as  yet  barely  been 
touched  by  erosion, 
to  that  in  which  the 
only  evidence  that  a 
volcano  formerly  ex- 
isted is  to  be  found 
in  a  spot  of  igneous 
rock,  a  few  feet  or  a 
few  hundred  feet 
in  diameter,  sur- 
rounded by  sedimen- 
tary or  other  rock. 
The  various  stages 
in  the  erosion  of  vol- 
canic cones  ,are  well 
shown  in  the  western 
United  States,  where 
every  gradation  may 

FIG.  315.  —  Diagram  A  shows  an  active  or  recently  extinct  volcano  with  widespread 
lava  flows  at  its  base.  Diagram  B  is  the  same  region  after  prolonged  erosion.  The  ash 
of  which  the  cone  was  composed  has  been  eroded  away,  leaving  the  volcanic  neck  pro- 
truding. The  lava  flows  have  been  cut  by  erosion  into  flat-topped  hills  or  mesas. 
In  the  section  on  the  front  of  A  the  former  successive  positions  of  the  streams  are 
shown,  their  courses  having  been  diverted  as  they  were  filled  with  lava  from  the 
volcano.  (Modified  after  Davis.) 


PHYSICAL  GEOLOGY 


be  seen  from  young  cones,  such  as  Lassen  Peak,  California,  which  was 
active  in  1914-1915,  to  those  which  have  been  worn  down  to  their 
roots.  Mt.  Shasta,  California,  14,350  feet  high  (Fig.  316),  is  a  good 


FIG.  316. — Mt.  Shasta,  California,  a  partly  denuded  volcanic  cone. 

example  of  a  volcano  which  has  suffered  much  erosion,  but  Mt.  Hood, 
Oregon,  is  still  more  worn,  the  sides  being  deeply  trenched  by  ravines 
and  only  a  part  of  the  wall  of  the  crater  being  left. 


DISTRIBUTION  AND  NUMBER  OF  VOLCANOES 

Number  of  Volcanoes.  —  It  is  impossible  to  determine  accurately 
the  number  of  active  volcanoes,  since  some  that  appear  to  be  extinct 
may  be  merely  dormant,  and  others  that  have  recently  been  active 
and  from  which  steam  is  still  rising,  may  have  been  in  eruption  for 
the  last  time.  It  is,  moreover,  sometimes  difficult  to  distinguish 
between  independent  and  subsidiary  vents.  It  is  safe  to  say  that 
there  are  approximately  325  active  volcanoes,  of  which  one  third  are 
on  the  continents. 

Distribution.  —  A  glance  at  a  map  of  the  world  in  which  the  vol- 
canoes are  conspicuously  indicated  (Fig.  317)  shows  some  striking 
features  of  their  distribution.  It  is  seen  that  they  are  not  scattered 


VOLCANOES  AND  IGNEOUS  INTRUSIONS 


319 


haphazard  over  the  world,  but  are  for  the  most  part  concentrated 
along  lines  or  belts  near  the  edges  of  the  continents,  and  dot  limited 
areas  of  the  oceans.  The  volcanic  belts  are  not  continuous,  how- 


/ft 


' 


i 


r 


e 


3 
-Q 

.i 

-3 

u 


s 
I 

K. 


CLELAND   GEOL.  —  21 


320  PHYSICAL  GEOLOGY 

ever,  but  are  interrupted  in  many  places  by  areas  in  which  no 
volcanoes  occur. 

Although  volcanoes  are  usually  situated  along  the  borders  of  con- 
tinents, this  is  not  always  the  case ;  some  volcanoes  in  Ecuador,  for 
example,  are  150  miles  inland,  and  in  East  Africa  the  volcano  Kirunga 
is  600  miles  from  the  coast. 

The  most  important  of  the  volcanic  belts  almost  encircles  the 
Pacific  Ocean,  extending  from  the  southern  tip  of  South  America 
northward  along  the  Andes  on  the  western  coast  of  that  continent, 
through  Mexico,  and  along  the  western  coast  of  North  America  to 
Alaska.  From  Alaska  it  curves  westward  and  southward  through 
the  Japanese  and  Philippine  archipelagoes  to  New  Zealand  and  to  the 
Antarctic  volcanoes.  The  borders  of  the  Atlantic,  in  contrast  to  those 
of  the  Pacific,  are  almost  free  from  volcanoes.  Two  important  belts, 
however,  occur  in  this  ocean ;  one  stretches  from  Iceland  south  to  St. 
Helena  and  includes  the  Azores  and  other  volcanic  islands ;  the  other 
includes  the  West  Indies  and  the  shores  of  the  Mediterranean  Sea. 

Cause  of  Distribution.  —  A  study  of  regions  of  volcanic  activity 
brings  out  the  fact  that  they  have  recently  undergone  severe  move- 
ments, or  are  actually  being  deformed  at  the  present  time.  In  other 
words,  volcanoes  are  situated  where  mountain-making  forces  (p.  358) 
are  active,  and  where,  consequently,  the  earth  is  much  fissured  and 
fractured  (p.  360).  The  fact  that  belts  of  active  volcanoes  are  usu- 
ally found  where  mountain  ranges  are  near  or  parallel  to  great  deeps 
in  the  neighboring  oceans  has  given  rise  to  the  belief  that  the  eleva- 
tion of  the  strata  of  which  mountain  ranges  or  islands  are  composed 
is  compensated  by  a  sinking  of  the  ocean  bottom,  and  that  as  a  result 
of  these  movements  lava  and  ash  are  ejected  to  form  volcanoes.  It 
is  to  be  noted  in  this  connection  that  volcanic  activity  tends  to  die 
out  in  the  older  rocks  and  to  appear  in  those  of  later  date. 

It  is  evident  from  the  above  that  the  problem  of  the  distribution 
of  volcanoes  is  an  important  one,  since  on  its  solution  must  depend  in 
a  large  measure  the  much  more  general  one  of  the  cause  of  volcanism. 

Ancient  Volcanoes.  —  The  volcanoes  of  the  past  had  as  a  rule  a 
different  distribution  from  those  of  the  present.  For  example,  Great 
Britain  and  central  France  were  the  scenes  of  intense  volcanic  activity ; 
the  Connecticut  valley,  northern  New  Jersey,  and  many  of  the  western 
states  (Wyoming,  Colorado,  New  Mexico,  Idaho,  and  others)  have  ex- 
perienced great  lava  flows,  or  many  and  great  volcanic  eruptions.  At 
a  much  earlier  period  in  the  earth's  history  (Pre-Cambrian)  volcan- 


VOLCANOES  AND  IGNEOUS  INTRUSIONS  321 

ism  appears  to  have  been  widespread  in  eastern  and  central  Canada, 
and  large  areas  in  Wisconsin  and  Minnesota  are  underlain  chiefly 
with  volcanic  rock.  Throughout  geologic  history  periods  of  unusual 
volcanism  have  been  followed  by  others  of  comparative  quiet.  The 
last  important  period  of  volcanism  preceded  the  advent  of  the  Great 
Ice  Age  (p.  643),  and  it  is  possible  that  we  are  now  living  in  the  de- 
clining phases  of  the  activity  of  that  time. 


IMPORTANCE  OF  VOLCANISM  TO  MAN 

(1)  Beneficial  Effects.     Volcanic  regions  are  interesting  not  only 
because  of  the  striking  character  of  their  phenomena  and  scenery,  but 
also  because  of  their  economic  value.     Abundant  springs  are  usually 
found  in  the  neighborhood  of  volcanoes.     The  ashes  from  recent  erup- 
tions often  form  a  fertile  and  easily  worked  soil.     When  the  surfaces 
of  flows  composed  of  dark-colored  lavas  (basic,  p.  329)  are  decomposed, 
they  furnish  a  soil  which  contains  all  of  the  elements  needful  for 
plant  life,  many  of  which  are  lacking  in  granite  and  other  soils ;  in 
Central  America,  certain  regions  are  benefited  far  more  than  they 
are  injured  by  the  showers  of  volcanic  ash,  because  of  the  increased 
fertility  resulting  from  the  minerals  which  these  contain.     Vesuvius 
is  surrounded  by  a  ring  of  villages  in  spite  of  the  danger  of  eruptions, 
and  the  flanks  of  Etna  support  an  extremely  dense  population. 

Of  benefit  to  man,  also,  are  the  many  lakes  that  rest  in  the  craters 
of  inactive  volcanoes.  The  lakes  of  the  Alban  Hills  near  Rome,  as 
well  as  Lake  Bracciano  and  other  lakes  of  Italy,  are  crater  lakes. 
Crater  Lake  in  Oregon  (p.  309)  is  also  a  famous  example.  Lakes 
have  also  been  formed  by  the  damming  of  river  valleys  by  lava 
streams.  At  the  foot  of  Mt.  Shasta,  California,  are  rich  tracts  of 
alluvium,  the  sites  of  lakes  formed  in  this  way  and  later  filled  by  de- 
posits which  now  constitute  rich  agricultural  land. 

Many  important  ore  deposits  (p.  371)  have  resulted  from  the 
intrusion  of  molten  rock. 

(2)  Harmful   Effects.     Although    volcanoes   are    sometimes   indi- 
rectly beneficial  to  man,  this  does  not  compensate  for  the  destruc- 
tion of  life  and  property  which  result  from  an  eruption.     In  addition 
to  the  destruction  wrought  by  the  fall  of  ash  and  the  outpouring  of 
lava,  great  disaster  has  been  caused  in  other  ways.     Many  times  in 
the  past,  great  floods  have  been  brought  about  by  the  discharge  of 
the  water  from  lakes  which  rested  in  craters,  and  by  the  melting  of 


322  PHYSICAL  GEOLOGY 

the  snow  and  ice  on  and  near  the  summits  of  the  volcanoes.  During 
an  eruption  of  Cotopaxi  in  1877  enormous  torrents  of  water  and  mud 
produced  by  the  melting  of  the  snow  and  ice  on  the  cone,  together 
with  great  blocks  of  ice  from  the  glaciers,  rushed  down  the  mountain, 
burying  fields  and  villages  beneath  mud,  lava,  and  ice  for  a  distance 
of  10  miles.  In  contrast  to  the  above  is  the  existence  of  a  great  sheet 
of  ice  on  Mt.  Etna,  which  for  nearly  one  hundred  years  has  been 
protected  from  evaporation  and  thaw  by  a  sheet  of  lava  which  over- 
flowed it  without  the  heat  being  sufficient  to  melt  it. 

As  torrents  of  water  rush  down  the  side  of  a  volcano,  they  not  only 
erode  it  deeply  but  are  also  soon  converted  into  streams  of  mud, 
when  dust  and  ash  are  abundant  on  the  cone.  Herculaneum  (p.  303) 
was  buried  in  this  manner,  and  in  Java  in  1881  torrents  of  mud  and 
water  from  Galoon-goon  flooded  the  rivers  to  such  an  extent  that 
every  village  and  plantation  in  this  populous  region  was  entirely  de- 
stroyed for  a  distance  of  24  miles. 

During  a  comparatively  recent  eruption  of  Vesuvius  so  much  hy- 
drochloric acid  was  dissolved  in  the  rain  water  which  fell  through  the 
clouds  of  volcanic  gases  that  the  vegetation  for  miles  around  was 
injured  by  it. 

A  subsidence  of  the  land  sometimes  follows  an  eruption,  as  has  been 
noted  in  the  case  of  the  Temple  of  Jupiter  near  Naples  (p.  229).  The 
sinking  is  probably  brought  about  either  by  the  withdrawal  of  molten 
rock  from  beneath  the  affected  areas  or  by  the  weighting  of  the  ad- 
jacent land  by  the  ejected  material,  or  by  a  combination  of  both. 

Volcanoes  and  Climate.1  —  It  has  been  shown  that  volcanic  dust  in  the  high  atmos- 
phere decreases  the  intensity  of  solar  radiation  in  the  lower  atmosphere.  Therefore 
the  average  temperature  of  the  earth  is  decreased  when  dust  is  present.  From  these 
observations  some  investigators  have  concluded  that  volcanic  dust  must  have  been  a 
factor,  possibly  an  important  one,  in  the  production  of  many  climatic  changes  of  the 
past.  It  has  not,  however,  been  shown  that  the  periods  of  glaciation  coincided  with 
prolonged  volcanic  outbursts. 


SUBORDINATE  VOLCANIC  PHENOMENA 

There  are  a  number  of  phenomena  which  are  the  direct  result  of 
heat  and  are  usually  connected  with  present  or  comparatively  recent 
volcanism. 

1  Bull.  Mt.  Weather  Observatory,  Vol.  6,  Pt.  I,  1913 ;  Smithsonian  Misc.  Coll.,  Vol.  59, 
No.  29,  1913- 


VOLCANOES  AND  IGNEOUS   INTRUSIONS  323 

Mud  Volcanoes.  —  Cones  built  of  mud  with  small  craters  in  their  summits  are  called 
mud  volcanoes.  They  vary  in  height  from  a  foot  or  two  to  more  than  a  hundred  feet ; 
some  are  continuously  active  and  some  are  intermittent;  some  are  quiet  and  a  few 
are  violently  eruptive.  In  order  that  mud  volcanoes  may  be  formed  it  is  necessary 
that  (i)  steam  be  present  and  that  (2)  it  rise  through  a  surface  layer  of  clay  which  will 
make  mud  when  wet.  As  the  steam  rises  through  the  mud,  it  carries  some  up  with  it 
and  so  builds  a  cone.  As  such  cones  are  composed  of  soft  material,  they  have  a  short 
life,  since  they  are  readily  destroyed  by  rains.  The  heat  and  steam  necessary  for  the 
formation  of  mud  volcanoes  come  from  lavas  which  are  present  at  a  comparatively 
short  depth,  or  may  be  produced  by  chemical  action,  such  as  occurs  when  sulphur  is 


FIG.  318.  —  Mud  volcanoes,  Lower  California.     (Photo.  D.  T.  MacDougal.) 

oxidized.  Mud  volcanoes  are  found  in  the  Colorado  desert,  in  Lower  California 
(Fig.  318),  and  in  other  parts  of  the  world.  The  "  paint  pots  "  of  the  Yellowstone 
National  Park,  so-called  because  of  their  shape  and  varied  colors,  are  miniature  mud 
volcanoes.  The  eruptions,  produced  by  the  bursting  of  bubbles  of  steam,  occur  fre- 
quently, and  can  be  safely  and  easily  studied. 

Solfataras.  —  Lava  streams  sometimes  retain  their  heat  hundreds  of  years  after 
they  have  been  poured  out  in  sufficient  amount  to  convert  the  water  which  percolates 
to  them  into  steam.  This  is  also  true  of  the  lava  in  the  craters  of  volcanoes.  Although 
meteoric  waters  probably  furnish  the  greater  amount  of  water  which  is  returned  as 
steam,  yet  the  quantity  of  steam  exhaled  directly  from  lavas  appears  to  be  considerable 
in  some  cases.  The  term  solfatara  is  used  for  a  volcanic  vent  or  area  in  which  only 
gases  and  steam  are  discharged,  the  name  being  derived  from  the  volcano  Solfatara 
near  Naples,  which  has  been  giving  off  only  steam  and  gases  since  its  last  eruption  (in 
1198).  ,-,. 

Geysers  (p.  67)  are  found  only  in  regions  in  which  acidic  lava  is  still  hot,  and  hot, 
carbonated  springs  (p.  66)  occur  in  similar  situations,  although  their  heat  does  not 
always  have  this  origin. 


PHYSICAL  GEOLOGY 


INTRUSIVE  OR  PLUTONIC  ROCKS 

Igneous  rocks  have  either  been  extruded  on  the  surface  in  the  form 
of  volcanic  products  and  lava  flows,  or  they  have  failed  to  reach  the 
surface  and  have  consolidated  beneath  it.  The  latter  are  called 
plutonic  (after  Pluto,  the  Greek  god  of  the  lower  world)  or  intrusive 
rocks.  The  quantity  of  lava  which  failed  to  reach  the  surface  is 
probably  many  times  greater  than  that  which  was  poured  upon  it. 
The  deep-seated  intrusive  rocks  are  never  vesicular  (full  of  gas  blebs), 
since  their  contained  gases  were  prevented  from  expanding  by  the  over- 
lying pressure.  They  are  coarsely  crystalline  because  they  cooled  so 
slowly,  owing  to  the  fact  that  they  were  deeply  buried,  so  that  the 
crystals  had  time  to  grow.  Such  rocks  are  exposed  at  the  surface 
only  by  the  erosion  of  the  rock  strata  which  formerly  covered  them. 
Doubtless  many  such  masses,  now  exposed  at  the  surface,  were  at 
one  time  the  deep-seated  reservoirs  from  which  the  lava  of  volcanoes 

came.  There  are  some  rocks 
which  link  the  extrusive  and 
the  plutonic  rocks  and  may 
be  classed  simply  as  inter- 
mediate. 

The  mechanics  of  igneous 
intrusions  is  discussed  on 
page  334.  It  will  be  shown 
that  intrusions  probably  work 
their  way  toward  the  surface 
largely  by  sloping  (p.  337). 
When  they  have  reached 
within  a  few  thousand  feet 
of  the  earth's  surface,  they 
take  advantage  of  any  planes 
of  weakness,  such  as  joints 
and  faults,  and  continue  their 
journey  through  fissures. 

/.    Injected  Masses 

Dikes.  —  Dikes  are  masses 
of  igneous  rock  which  have 

FIG.  3 19-  -  A  vertical,  branching  dike.          hardened  in  more  or  less  verti- 
(Photo.  F.  B.  Sayre.)  calcracks  or  fissures  (Fig.  3 19). 


VOLCANOES  AND   IGNEOUS  INTRUSIONS 


325 


They  vary  in  width  from  a  fraction  of  an  inch  to  several  hundred  feet. 
Their  length  may  be  considerable ;  one  in  the  north  of  England  runs 
from  the  coast  inland  for 
about  100  miles,  and  a  length 
of  5  to  20  miles  is  not  un- 
common. In  Scotland  a 
series  of  lava  dikes  run  par- 
allel to  each  other  for  a  dis- 
tance of  from  20  to  30  miles, 
while  on  the  coast  of  New 
England  and  in  many  other 
parts  of  North  America  they 
are  very  common.  When 
the  surrounding  rocks  decay 
more  easily  than  the  dike 
rocks,  the  latter  project 
above  the  surface  of  the 
ground  like  walls  (Fig.  320) 
and  are  sometimes  used  in 
Scotland  as  inclosures.  Near 
Spanish  Peaks,  Colorado,  a 


FIG.  320.  —  Dikes,  the  Devil's  slide,  Weber's 
Canyon,  Utah.     (U.  S.  Geol.  Surv.) 


dike  stands  as  a  great  wall 
100  feet  high.  Occasionally  the  dike  rock  weathers  more  readily 
than  that  which  it  cuts  (Fig.  321),  in  which  case  the  position  of 
the  dike  may  be  indicated  by  a  trench-like  hollow.  When  the 
dike  and  the  surrounding  rocks  are  about  equally  resistant,  no 

topographic  features 
result. 

The  texture  of  the 
rocks  of  dikes  de- 
pends upon  a  num- 
ber of  conditions : 
(i)  if  the  fissure 
through  which  the 


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FIG.  321.  —  Diagram  showing  the  effect  of  weathering 
upon  two  dikes  (shown  by  horizontal  lines),  one  of  which 
is  more  resistant  than  the  surrounding  rock  and  the  other 
less  resistant. 


lava  was  forced  was 
narrow,  the  rock  of 
the  dike  is  either 
glassy  or  finely  crystalline;  (2)  if,  however,  the  fissure  was  wide, 
the  dike  rock  may  be  coarsely  crystalline,  with  narrow  margins  of 
less  crystalline  or  glassy  rock. 


326 


PHYSICAL  GEOLOGY 


Dikes  are  found  cutting  rocks  of  all  ages,  and  they  extend  across 
the  country  without  reference  to  topography. 

Sills.  —  Lavas  which  have  been  forced  between  sedimentary  strata 
and  have  formed  sheets  which  have  a  small  thickness  as  compared 


FIG.  322.  —  Diagram  illustrating  the  relation  of  the  Palisades  of  the  Hudson 
(vertical  lines)  to  the  strata  in  which  this  sill  was  intruded. 

with  their  extent  are  called  sills  (Fig.  322).  Sills  sometimes  extend 
long  distances  along  the  same  bedding  plane,  but  often  cut  across 
from  one  stratum  to  another.  They  vary  in  thickness  from  a  few 
feet  to  several  hundred  feet  and  sometimes  have  an  extent  of  many 
square  miles.  When  they  have  been  exposed  by  erosion,  they  can 
be  distinguished  from  extruded  lavas  by  the  absence  of  vesicular 


FIG.  323.  —  The  Palisades  of  the  Hudson.  The  sheer  face  of  the  upper  portion  is 
due  to  the  vertical  jointing  of  the  trap,  and  to  the  more  rapid  erosion  of  the  weaker, 
underlying  rock.  (Photo.  D.  W.  Johnson.) 

lava  on  the  upper  surface.  The  Palisades  of  the  Hudson,  which  ex- 
tend for  30  miles  along  the  west  bank  of  the  river  as  a  bold  cliff  several 
hundred  feet  high,  form  a  part  of  a  sheet  of  intrusive  lava  (sill)  which 
is  underlain  by  sandstone  and  was  formerly  overlain  by  other  sedi- 
mentary strata  (Fig.  323). 


VOLCANOES  AND  IGNEOUS   INTRUSIONS 


327 


Laccoliths  (Greek,  lakkos,  a  cistern,  and  lithos,  stone).  —  This  term 
has  been  given  to  mushroom-shaped  intrusions  of  lava  which  have 
been  forced  along  bedding  planes  and  have  domed  up  the  overlying 
strata.  They  are 
formed  when  molten 
rock  rising  through 
a  pipe  or  fissure  is 
unable  to  break 
through  the  overly- 
ing rock  and  spreads 
between  the  strata, 
lifting  them  and  thus 
producing  domelike 
elevations  (Figs.  324, 


FIG.   324. —  Diagram  illustrating  the  form  and  relations 
of  dikes,  A  and  D ;  sills,  C  and  E ;  and  a  laccolith,  B. 


325).  The  difference 
between  a  sill  and  a 
laccolith  is  conse- 
quently a  difference  in  the  degree  of  the  doming  of  the  overlying 
strata.  Laccoliths  may  be  a  mile  or  more  thick  and  a  number  of 
miles  in  diameter.  Mountains  of  considerable  height  have  been 
formed  in  this  way.  The  Henry  Mountains  of  southern  Utah, 
the  Elk  Mountains  of  Colorado,  and  many  other  elevations  in  the 

Rocky  Mountains 
are  laccoliths  (Fig. 
325).  Laccoliths  are 
composed  of  lava 
which  was  probably 


FIG.  325.  —  Diagram  of  a  laccolith,  showing  the  rela- 
tion of  the  igneous  intrusion  to  the  overlying  and  under- 
lying strata. 


stiff  and  viscous  and 
could  consequently 
more  easily  lift  the 

strata  than  force  its  way  between  them.  The  lavas  of  sills,  on  the 
other  hand,  were  probably  quite  fluid  and  therefore  could  spread 
long  distances. 

//.    Subjacent  Masses 

Stocks.  — The  name  stock  is  applied  to  large  bodies  of  igneous  rock 
lying  in  the  midst  of  other  formations.  Stocks  are  usually  circular  or 
elliptical  in  outline  and  vary  from  a  few  hundred  yards  to  many 
square  miles  in  extent,  usually  increasing  in  size  downward  (Fig. 
326  A,  B).  Since  they  are  composed  of  more  resistant  rock  than 


328 


PHYSICAL  GEOLOGY 


that  in  which  they  were  intruded,  they  often  form  knob-like  elevations 
and  are  consequently  often  called  bosses.     Stocks  resemble  volcanic 

necks  (p.  316),  but  are  usually 
larger;  the  term  neck,  more- 
over, is  employed  only  when 
there  is  evidence  that  it 
represents  the  chimney  of  a 
volcano. 

Batholiths  (Greek,  bathos, 
depth,  and  lithos,  stone)  are 
great  irregular  masses  of 
igneous  rock  which  stopped 
in  their  rise  many  feet  from 
the  surface  of  the  earth,  but 
have  since  been  exposed  by 
erosion.  They  are  often 
many  hundreds  of  square 
miles  in  area  and  may  be 


FIG.  326.  —  Section  A  and  map  B  of  a  stock 
or  boss.  The  granite  intrusion  being  more  re- 
sistant than  the  enclosing  rock  forms  a  hill. 


considered  as  merely  very 
large  and  irregular  stocks. 

In  the  aggregate  these  bodies  cover  many  thousands  of  square 
miles,  and  although  less  striking  are  much  more  important  than 
volcanoes. 

Some  Effects  of  Intrusions.  —  The  rock  with  which  a  molten 
magma  comes  in  contact  is  more  or  less  changed  ;  the  larger  and  hotter 
the  intrusions  the  greater  being  the  effect.  This  phenomenon  will 
be  discussed  under  metamorphism  (p.  341).  It  is  believed  that  some 
of  the  explosions  which  have  taken  place  on  or  near  volcanoes  were 
due  to  the  presence  of  molten  rock  at  a  short  distance  below  the 
surface.  Since  igneous  rocks  are  usually  harder  than  those  into  which 
they  are  .intruded,  they  are  often  left  in  relief  as  buttes  (p.  106)  and 
bosses,  as  the  land  is  reduced  by  erosion. 

Since  igneous  rocks  are  composed  of  minerals  which  differ  in  com- 
position and  often  in  color  and  therefore  expand  and  contract  differ- 
ently when  heated  and  cooled,  we  find  in  desert  and  tropical  regions 
that  granites  and  other  igneous  rocks  exfoliate  (p.  32)  under  the 
influence  of  diurnal  temperature  changes  (p.  31),  producing  sphe- 
roidal bowlders  which  are  often  poised  on  rounded  surfaces.  These 
rocks  resemble  glacial  bowlders,  and  the  smooth  surfaces  on  which 
they  rest,  roches  moutonnees  (Fig.  142,  p.  157). 


VOLCANOES  AND  IGNEOUS  INTRUSIONS 


IGNEOUS  ROCKS 


329 


Igneous  rocks,  as  we  have  seen,  have  consolidated  from  a  state  of 
fusion.  The  character  of  the  rocks  thus  formed  depends  principally 
(i)  upon  the  chemical  composition  of  the  molten  mass  and  (2)  upon 
the  rapidity  with  which  the  magma  cooled.  Other  conditions,  such 
as  fluidity  and  pressure,  are  likewise  important. 

Subdivisions  Depending  upon  Chemical  Composition.  —  Igneous 
rocks  which  contain  a  large  percentage  of  silica  (65  per  cent,  or  more) 
are  termed  acid  rocks,  silica  being  an  acid-forming  oxide.  Acid  rocks 
are  usually  light-colored  when  crystalline,  and  are  lighter  in  weight 
than  basic  rocks  which  contain  much  less  silica  (55  per  cent,  or  less) 
and  a  correspondingly  larger  amount  of  the  bases,  such  as  potash, 
soda,  lime,  and  magnesium.  Basic  rocks  are  usually  dark-colored 
and  fuse  at  a  lower  temperature  (p.  299)  than  acid  rocks.  They  are 
the  common  extrusive  rocks  and  sometimes  cover  tens  of  thousands 
of  square  miles  of  the  earth's  surface,  and  when  weathered  often 
produce  soil  rich  in  plant  food. 

Subdivisions  Depending  upon  Texture.  —  The  term  texture  as 
applied  to  igneous  rocks  refers  to  their  smaller  features.  When  a 
rock  is  described  as  being  granitoid,  or  having  a  granular  texture,  the 
reference  is  to  one  in  which  the  crystals  are  distinct  and  are  all  of  about 
the  same  size.  A  rock  with  afelsitic  texture  is  one  composed  of  a  mass 
of  very  fine  microscopic  crystals.  A  rock  is  described  as  glassy  when 
it  is  made  up  largely  or  in  part  of  glass  in  which  no  definite  crystals 
are  to  be  seen. 

The  rate  of  cooling  appears  to  be  the  really  important  factor  in  deter- 
mining the  texture  of  igneous  rocks,  although  other  conditions  have 
considerable  effect.  The  molten  magma  from  which  granitoid  rocks 
were  crystallized  was  so  deeply  buried  that  the  rate  of  cooling  was 
slow,  thus  giving  an  opportunity  for  the  molecules  of  the  same  chem- 
ical composition  to  gather  to  form  large  crystals  and,  consequently, 
granitoid  or  granular  rocks.  Felsitic  rocks1  are  the  result  of  somewhat 
more  rapid  cooling,  and  are  found  on  the  margins  of  great  masses  of 
granitoid  rocks  which  did  not  reach  the  surface,  or  in  offshoots  from 
them  in  the  form  of  dikes.  Glassy  rocks  are  those  which  cooled  so 
rapidly  that  the  minerals  had  little  opportunity  to  form.  Rocks 
with  a  glassy  texture,  consequently,  occur  chiefly  in  surface  flows 
and  on  the  margins  of  dikes. 

1  Felsite  is  also  often  the  product  of  the  devitrification  of  glassy  rocks. 


330  PHYSICAL  GEOLOGY 

CLASSIFICATION  OF  IGNEOUS  ROCKS 

I.    Coarse-grained  Igneous  Rocks 

The  rocks  included  in  this  group  are  those  whose  mineral  grains 
are  approximately  of  equal  size  and  are  large  enough  to  be  distinctly 
seen. 

Granite.  —  Granites  are  composed  of  quartz,  feldspar,  and  usu- 
ally of  smaller  amounts  of  either  mica  or  hornblende.  The  grains 
of  feldspar  are  usually  easily  distinguishable  because  of  their  shiny 
(cleavage)  surfaces  and  their  opaque  white,  gray,  or  red  color.  The 
quartz  grains  vary  in  tint  from  colorlessness  to  smoky  gray,  and 
can  usually  be  recognized  by  their  glassy  luster  and  irregular  frac- 
ture. Mica  may  be  either  muscovite  or  biotite,  and  may  be  told  by 
its  brilliant  cleavage  surface.  The  thin  leaves,  unless  too  small, 
can  be  easily  separated  with  the  point  of  a  penknife.  Hornblende 
occurs  in  green  to  black  opaque  grains  or  needles.  Other  minerals, 
such  as  pyrite  or  garnet  and  other  less  common  minerals,  may  also 
be  present. 

Numerous  names  are  given  to  granites,  some  of  them  (commer- 
cial) depending  upon  their  color  and  their  desirability  for  building  or 
monumental  purposes,  such  as  red,  gray,  yellow;  while  others  are 
locality  names.  The  color  of  the  stone  depends  largely  upon  that  of 
the  feldspar,  and  upon  the  relative  abundance  of  dark  minerals.  A 
red  granite  owes  its  color  to  its  red  feldspar;  a  gray  color  may  be  due 
either  to  the  color  of  the  feldspar  alone,  or  to  the  combination  of 
black  hornblende  or  mica,  and  white  feldspar. 

Syenite  is  a  rock  which  may  be  described  as  a  granite  without 
quartz.  It  very  closely  resembles  a  granite,  and  is  usually  sold 
under  the  latter  name. 

Diorite  is  a  granular  igneous  rock  composed  of  hornblende  and 
feldspar  of  any  kind,  in  which  the  amount  of  hornblende  usually 
exceeds  that  of  the  feldspar,  although  the  two  may  be  in  equal 
amounts.  Its  color  is  dark^  gray^r_greenish. 

Gabbro  is  made  up  of  pyroxene  (augite),  with  usually  smaller 
amounts  of  feldspar  of  any  kind.  Gabbro  and  diorite  are  often  dis- 
tinguished with  difficulty,  because  of  the  similarity  of  hornblende 
and  augite  to  the  naked  eye. 

Peridotite  is  a  dark  green  to  black  rock,  composed  chiefly  of  such 
minerals  as  hornblende,  olivine,  and  pyroxene  (augite). 


VOLCANOES  AND  IGNEOUS  INTRUSIONS  331 

//.    Compact  or  Fine-grained  Igneous  Rocks 

In  this  group  are  included  rocks  in  which  the  grains  are  so  fine  that 
the  individual  crystals  cannot  be  distinguished  by  the  naked  eye. 
They  are  intermediate  between  the  granitoid  rocks,  composed  of 
clearly  distinguishable  crystals,  and  the  glasses.  No  definite  line 
can  be  drawn  between  the  two  groups ;  in  some  dikes,  for  example, 
a  glass  shades  imperceptibly  into  a  microcrystalline  rock,  and  then 
into  a  coarsely  crystalline  or  granitoid  rock. 

This  group  is  divided  into  two  classes  on  the  basis  of  color :  (i)  the 
light  felsites  and  (2)  the  dark  basalts. 

(1)  Felsites  vary  greatly  in  color,  but  are  not  dark  gray,  dark  green, 
or  black.     To  the  naked  eye  the  rock  has  a  flinty  aspect,  but  with  a 
lens  it  is  often  seen  that  it  consists  of  mineral  grains,  too  small  for 
determination.     When  large  crystals  (phenocrysts)  occur  embedded 
in   the   fine-grained   "  ground   mass,"    the   rock   is   called    a  felsite 
porphyry.     Porphyries  contain  feldspar  phenocrysts  (Greek,  phain- 
esthai,  to  appear,  and  krystallos,  crystal).     If  quartz  is  also  present, 
they    are    known    as   quartz   porphyries,  and  if  hornblende  is   con- 
spicuous, they  are  called  hornblende  porphyries.     Felsites  occur  in 
dikes  and  sheets. 

(2)  Basalts  form  a  very  large  and  important  group  of  igneous  rocks. 
They  are  all  heavy  black,  gray,  brown,  or  greenish  rocks  of  fine  tex- 
ture, and  have  a  wide  distribution,  covering  many  thousands  of  square 
miles  of  the  earth's  surface.     The  name  trap  is  also  used  to  include 
basalts  and  any  dark-colored,  heavy  igneous  rocks  whose  mineral 
constituents  have  not  been  determined. 

When  the  air  cavities  of  vesicular  basalts  or  of  other  igneous  rocks 
are  filled  with  minerals,  the  rocks  are  called  amygdaloidal.  This  is 
one  mode  of  occurrence  of  copper  in  some  of  the  mines  of  northern 
Michigan  (p.  396). 

///.   Glassy  Rocks 

Rocks  which  are  composed  partly  or  wholly  of  glass  are  included 
in  this  group.  They  were  formed  as  stated  (p.  329),  when  molten 
rock  solidified  rapidly.  They  are  therefore  lavas  which  were  either 
poured  out  on  the  surface,  or  in  crevices  where  they  were  subjected  to 
rapid  cooling.  One  sometimes  finds  the  sides  of  dikes  glassy,  while 
the  interior  is  crystalline.  The  texture  of  glassy  rocks  is  sometimes 
vesicular  (Fig.  294)  and  sometimes  pumiceous. 


332 


PHYSICAL  GEOLOGY 


FIG.  327.  —  A  hand  specimen  of  obsid- 
ian, showing  the  characteristic  conchoi- 
dal  fracture.  (U.  S.  National  Museum.) 


Obsidian  or  Volcanic  Glass  (Fig. 
327)  is  pure,  natural  glass,  entirely 
or  nearly  devoid  of  crystal  grains. 
It  is  usually  jet  black  in  color,  but 
is  sometimes  gray,  green,  red,  or 
yellow.  Because  of  the  sharp  edges 
which  form  when  it  is  broken,  it  was 
highly  prized  by  the  Mexicans  and 
other  primitive  peoples  for  the 
manufacture  of  sharp  implements, 
such  as  knives  and  arrowheads. 

Pitchstone  is  a  variety  of  ob- 
sidian in  which  the  luster  is  resin- 
ous or  pitch-like.  The  chemical 
and  other  differences  between  this 
rock  and  obsidian  are  slight. 
Pitchstones  are  variable  in  color. 
When  conspicuous  crystals  are 
scattered  through  the  rock,  it  is 
called  pitchstone  porphyry. 


FRAGMENTAL  VOLCANIC  ROCKS 

Rocks  formed  from  the  material  thrown  out  by  volcanoes  are  in- 
cluded under  this  head  and  are  made  by  the  consolidation  of  dust, 
ashes  (material  the  size  of  a  shot),  lapilli  (the  size  of  a  nut),  and  bombs 
(pieces  the  size  of  an  apple,  or  larger). 

Tuff.  —  When  the  rock  is  composed  entirely  of  the  finer  kinds  of 
volcanic  detritus,  it  is  called  volcanic  tuff.  Rocks  of  this  type  are 
light  in  weight  and  usually  loose  in  texture,  although  some  are  almost 
as  compact  as  felsites.  Tuffs  contain  fossils  if  the  dust  and  ashes 
of  which  they  are  composed  fell  on  a  land  surface  covered  with  vege- 
tation, or  in  water  in  which  marine  organisms  were  living.  Some  of 
the  rock  through  which  the  Panama  Canal  was  cut  is  a  tuff  containing  1 
marine  shells.  Tuffs  are  widely  used  in  Mexico  for  building  stones. 

Volcanic  Breccia.  —  This  is  a  rock  compose^  of  angular  fragments 
of  volcanic  rock,  bombs,  etc.,  which  are  cemented  together  with  ash 
and  dust. 


1  For  a  more  detailed  study  of  igneous  rocks  the  student  is  referred  to  L.  V.  Pirsson,  Rocks 
and  Rock-Minerals ;  and  J.  F.  Kemp,  Handbook  of  Rocks. 


VOLCANOES  AND  IGNEOUS  INTRUSIONS 


333 


FIG.  328.  —  Basaltic  columns  in  a  lava  flow  near 
the  city  of  Mexico. 


Columnar  Struc- 
ture of  Lava.  —  A 
striking  feature  of 
many  ancient  lava 
flows  whose  lower 
portions  have  been 
exposed  to  observa- 
tion by  erosion  is 
their  columnar  struc- 
ture, the  lava  being 
broken  up  into  an- 
gular columns  which 
are  often  six-sided. 
If  the  lava  sheet  is 
horizontal,  the  col- 
umns are  vertical 
(Fig.  328) ;  if  it  has  been  intruded  into  a  fissure  (dikes),  the  columns 

are  horizontal  (Fig. 
329).  One  may  ob- 
serve similar  joints 
in  dried  mud  and 
starch,  but  in  these 
substances  the  sides 
are  much  less  regular. 
The  explanation  of 
columnar  jointing  is 
to  be  found  in  the 
contraction  of  the 
lava,  resulting  from 
cooling  and  loss  of 
gas,  and  the  conse- 
quent cracking  of  the 
rock.  Since  the  least 
expenditure  of  energy 
is  required  to  relieve 
the  strain  when  three 
cracks  radiate  from* 

equidistant  points  at 
329.  —  A  lava  dike  (depressed)  showing  the  basal-         •  0 

tic     jointing     at    right    angles     to    the    walls.      Maine.      Angles    of     I2O  ,     the 
(Photo.  F.  Bascom.)  formation  of  six-sided 


334  PHYSICAL  GEOLOGY 

columns  usually  results,  and  the  direction  of  the  columns  is  at  right 
angles  to  the  cooling  surface.     The  reason  for  the  horizontal  position 


B 


FIG.  330.  —  Diagrams  showing  the  origin  of  basaltic  jointing.  In  shrinking,  the 
least  number  of  cracks  that  will  relieve  the  tension  in  all  directions,  A,  is  three.  Similar 
radiating  cracks  from  other  centers  complete  the  six-sided  prism,  B.  When  cracks 
fail  to  develop  about  some  one  point,  a  five-sided  prism,  C,  results.  (Modified  after 
Chamberlin  and  Salisbury.) 

of  the  columns  of  vertical  dikes  and  the  vertical  position  of  those 
of  lava  flows  is  thus  explained  (Fig.  330  A-C). 

AGE  or  IGNEOUS  ROCKS 

The  exact  age  of  ancient  volcanoes  or  of  igneous  intrusions  can 
seldom  be  ascertained,  but  the  relative  age  is  often  known.  The 
relative  age  is  determined  as  follows  :  a  volcanic  neck  is  clearly  younger 
than  the  rocks  which  it  penetrates ;  a  laccolith  or  sill  is  of  later  age 
than  the  beds  in  which  it  was  intruded,  and  a  lava  flow  is  more  recent 
than  the  formations  over  which  it  spreads.  The  eruption  or  intru- 
sion in  each  case  could  not  have  taken  place  before  these  rocks  were 
laid  down.  If  on  the  other  hand  pebbles  of  igneous  rock  are  found 
in  sedimentary  rocks,  we  know  that  the  rocks  from  which  they  were 
derived  were  at  the  surface  before  the  sediments  were  deposited,  or 
while  the  deposition  was  taking  place.  For  example,  if  Devonian 
strata  are  cut  by  a  volcanic  neck,  we  know  that  the  neck  is  younger 
than  the  Devonian,  and  if  pebbles  from  this  same  neck  are  found  in 
Middle  Carboniferous  sediments,  it  is  evident  that  the  lava  was  prob- 
ably intruded  in  early  Carboniferous  times.  Sometimes  the  presence 
of  fossils  in  volcanic  tuff  shows  definitely  at  what  time  the  eruption 
occurred. 

THEORIES  OF  VOLCANISM 

So  many  theories  of  volcanism  have  been  offered  that  it  is  impossible 
in  an  introductory  volume  to  do  more  than  briefly  indicate  a  few  of 


VOLCANOES  AND   IGNEOUS   INTRUSIONS  335 

them.  The  theories  may  be  classed  under  three  heads :  (I)  those 
which  assume  a  molten  interior;  (II)  those  based  upon  the  assump- 
tion that  the  earth  is  solid  from  the  surface  to  the  center;  (III)  those 
holding  that  a  few  miles  below  the  surface  a  zone  of  rock  exists  which 
is  either  molten,  or  at  any  rate  in  a  non-crystalline  condition. 

/.    Theory  Based  upon  the  Assumption  that  the  Interior  is  Molten 

The  theory  of  a  molten  interior  is  now  held  by  few  geologists  because  of  the  many 
objections  to  it  (p.  273).  In  the  earlier  days  of  geology  when  this  theory  had  general 
acceptance,  the  difficulty  of  accounting  for  the  independence  of  volcanic  eruptions 
brought  forth  much  discussion,  and  a  number  of  modifications  to  the  theory  were  sug- 
gested. If  all  lavas  came  from  one  great  reservoir,  it  is  evident  that  according  to  the 
law  of  hydrostatics  eruptions  would  be  simultaneous,  or  in  two  adjacent  vents,  from 
the  lowest  one. 

II.    Theories  Based  upon  the  Assumption  that  the  Earth  is  Solid 

(a)  Heat  by  Friction.  —  This  theory  is  based  on  the  fact  that  heat  is  developed  by 
friction  when  rocks  grind   and   crush  each  other.     It  is  held  that  when  great  earth 
blocks  (segments)  move  past  each  other,  the  pressure  and  friction  along  the  lines  of 
movement  develop  heat  on  a  large  scale.     If  fluxes  (rocks  which  upon  uniting  with 
others  produce  a  substance  that  will  melt  readily)  are  present  to  lower  the  melting 
point  of  the  rock  silicates,  the  heat  may  be  sufficient  to  produce  molten  rocks  and 
volcanoes.     Since  all  rocks  contain  more  or  less  water,  steam  under  immense  pressure 
will  be  developed  upon  the  fusion  of  the  rock.     Explosions  of  the  steam  developed  in 
this  way  are  believed  to  be  competent  to  drill  channels  to  the  surface,  and  to  eject  the 
molten  rock  through  the  chimneys  thus  formed.     The  intermittent  action  and  extinc- 
tion of  volcanoes,  according  to  this  theory,  are  dependent  upon  the  movement  of  the 
earth's  segments.     It  should  be  noted  in  this  connection  that  no  observations  have 
been  made  of  faults  the  walls  of  which  are  fused  as  a  result  of  slipping.1 

(b)  Formation  of    Lava  Reservoirs  by  Relief  of  Pressure.  —  This  theory  rests  on 
the  assumption  that  at  moderate  depths  the  heat  of  the  earth  is  so  great  that  the 
solid  state  of  rocks  is  maintained  only  by  the  pressure  of  overlying  rocks.     If  this 
assumption  is  correct,  it  is  only  necessary  to  show  that  the  pressure  of  highly  heated 
rocks  can  be  relieved.     This  is  thought  by  the  advocates  of  the  theory  to  be  accom- 
plished  when   deeply  buried,   sedimentary   strata   are  folded.     If  a   stratum   strong 
enough  to  sustain  the  weight  of  the  overlying  rocks  is  arched,  and  the  underlying  rocks 
are  thus  relieved  of  some  of  the  pressure,  the  latter  may  melt.     A  volcano  or  fissure 
eruption  may  then  occur  if  a  crack  to  the  surface  is  present  through  which  the  lava  can 
force  its  way.     The  supply  of  lava  would  depend  upon  the  amount   of  molten  rock 
under  the  arch,  and  the  extinction  of  the  volcano  would  result  from  its  exhaustion. 

Two  strong  objections  to  the  theory  are :  (i)  the  difficulty  of  accounting  for  a  tem- 
perature in  sedimentary  rocks  high  enough  to  fuse  them,  and  (2)  the  difficulty  of  ex- 
plaining the  presence  of  sedimentary  rocks  of  basaltic  composition  (p.  331)  of  suffi- 
cient thickness  under  an  arch  to  be  a  source  of  the  lava  of  massive  plateaus, 

1  Schwartz,  E.  H.  L.,  —  Causal  Geology,  p.  241. 
CLELAND   GEOL.  —  22 


336 


PHYSICAL  GEOLOGY 


(c)  Liquid-thread  Theory.1  —  This  theory  assumes  that  the  earth  grew  by  the  slow 
accession  of  meteorites  (planetesimals),  varying  greatly  in  size  and  composition  (p. 
386),  and  that  the  interior,  though  solid,  has  become  very  hot  as  a  result  of  the  com- 
pression of  the  interior  masses  by  the  accumulation  of  the  outer  envelopes.  In  a  globe 

composed  of  masses  varying  greatly  in 
composition  and  fusibility,  it  is  evident 
that  some  particles  will  be  molten  while 
others  are  still  solid.  As  a  result  of  the 
strains  to  which  the  earth  is  subjected,  the 
liquid  portions  gradually  move  toward 
the  surface,  uniting  in  their  upward 
course  into  larger  and  larger  threads 
(Fig.  331).  Because  of  their  heat  these 
threads  finally  reach  the  zone  of  fracture 
by  fusing  and  fluxing.  When  such 
threads  of  lava  attain  the  zone  of  frac- 
ture, they  take  advantage  of  any  fissures 
or  fractures  which  exist,  and  are  poured 
out  on  the  surface  as  fissure  or  volcanic 
eruptions.  The  intermittency  of  volcanic 
action  is  due,  according  to  this  theory, 
to  temporary  deficiencies  in  the  supply, 
and  the  force  of  expulsion  is  produced 
especially  by  tidal  and  other  stresses  and 
by  the  slow  pressure  brought  to  bear  on 
the  threads  of  liquid  rock,  until  the  upper 
level  is  reached,  where  the  expansion  of 
the  occluded  gases  begins  to  operate. 

This  hypothesis,  as  has  been  pointed 
out,  is  based  upon  the  assumption  that 
the  earth  has  never  been  in  a  molten  con- 
dition, and  that  its  interior  is  composed 
of  a  heterogeneous  mass  varying  greatly 
in  composition.  If  the  observations  upon 


FIG.  331.  —  Diagram  illustrating  Cham- 
berlin's  theory  of  volcanism.  S  is  the 
surface  of  the  earth ;  aa',  the  zone  of  frac- 
ture ;  a/,  zone  of  flow ;  jfc,  interior  portion 
whose  temperature  rises  from  the  surface 
melting  point  at  ff  to  a  maximum  at  c  ;  w, 
threads  or  tongues  of  molten  rock  rising 
from  the  interior  to  various  levels,  many 
of  these  lodging  within  the  zone  of  frac- 
ture as  tongues,  batholiths,  etc.  PP  are 
explosive  pits  formed  by  volcanic  gases 
derived  from  tongues  of  lava  below.  (After 
Chamberlin  and  Salisbury.) 


which   the   following   hypothesis  (abyssal 

injection)  is  based  are  well-founded,  —  namely,  that  the  crust  of  the  earth  is  com- 
posed of  acid  (granitic)  rock  and  that  this  is  underlain  by  a  basic  (basaltic)  sub- 
stratum,—  the  hypothesis  cannot  stand,  since  the  latter  holds  that  the  earth  was 
formerly  molten  at  the  surface. 

///.    Abyssal  Injection  Hypothesis 2 

This  hypothesis  is  based  both  upon  laboratory  experiments  and  upon  many  obser- 
vations of  the  occurrence  and  relationships  of  igneous  rocks  in  various  parts  of  the  world. 
It  should  be  distinctly  borne  in  mind,  however,  that  the  hypothesis  is  as  yet  unproved. 

1  Chamberlin  and  Salisbury,  —  Geology,  2d  ed.,  Vol.  i,  p.  629. 

2  Daly,  R.  A., —  The  Nature  of  Volcanic  Action:  Am.  Academy  of  Arts  and  Sciences,  Vol. 
47,  June,  1911,  pp.  47-122;    and  Igneous  Rocks  and  their  Origin. 


VOLCANOES  AND   IGNEOUS   INTRUSIONS 


337 


It  assumes  that  there  exists,  at  a  depth  estimated  at  40  kilometers  (about  23  miles), 
a  basaltic  (basic)  substratum  which  underlies  an  almost  universal  shell  of  acid  rock 
(granite,  etc.).  Because  of  the  pressure  of  the  overlying  rocks,  this  substratum  of 
actually  or  potentially  fused  rock  is  so  rigid  as  to  act  as  a  solid.  It  holds  that  all  igneous 
action  is  the  result  of  the  mechanical  intrusion  of  the  substratum  basalt  into  the  over- 
lying crust.  The  intrusion  of  the  magma  is  accomplished  largely  by  "  stoping";  that 
is,  cracks  in  the  overlying  rocks  are  entered  by  the  molten  mass,  blocks  are  wedged  off 
and  are  ultimately  absorbed  in  the  magma.  At  the  contact,  solution  takes  place  to 
some  extent,  but  this  is  believed  to  be  of  secondary  importance  to  stoping.  The  vent 
of  the  volcano  or  fissure  for  the  last  few  hundred  or  thousand  feet  may  have  been 
opened  by  explosions  or  by  fissuring.  Once  the  movement  of  the  molten  magma  is 
started,  the  original  heat  of  the  intrusion  is  maintained  by  chemical  and  exothermic 
reactions  (the  heat  liberated  in  the  formation  of  chemical  compounds). 

The  explosiveness  of  volcanoes,  according  to  this  theory,  is  the  result  of  the  original 
gases  of  the  molten  rock,  as  well  as  of  the  water  which  the  magma  absorbs  from  the 
intruded  rocks  in  its  ascent. 
The  cause  of  the  extinction 
of  volcanoes  is  shown  in 
the  diagram  (Fig.  332). 
The  active  vent  is  situated 
at  the  highest  point  of  the 
injected  lava,  and  it  is  in 
this  place  that  the  gases  of 
the  lava  accumulate.  The 
temperature  of  the  lava  in 
such  situations  is  believed 
to  be  not  only  that  of  its 
primal  heat  but  also  to  be 
increased  by  chemical  re- 
actions and  by  other  means 
connected  with  the  pres- 
ence of  the  gas.  Vents 
become  extinct  when,  be- 
cause of  the  higher  position 

of  the  lava  in  other  locations,  the  gases  which  cause  the  fusion  accumulate  elsewhere. 
According  to  this  theory,  the  composition  of  the  lava  ejected  from  a  volcano  depends 
upon  whether  it  is  composed  entirely  of  the  basaltic  lava  of  the  substratum,  or  is  a 
mixture  produced  by  the  solution  of  the  rock  through  which  the  basaltic  lava  has 
passed. 

RESUME  OF  PRESENT  KNOWLEDGE  OF  VOLCANISM 

There  is  no  agreement  as  to  the  origin  of  lava ;  (i)  some  investiga- 
tors hold  that  a  portion  is  derived  from  deeply  buried  sedimentary 
rocks  which  have  a  high  temperature  as  a  result  of  the  rise  of  heat 
from  the  earth's  interior,  so  that  when  the  pressure  of  the  overlying 
rocks  is  relieved,  the  more  fusible  strata  liquefy;  (2)  some  hold  that 
it  is  formed  as  the  result  of  heat  produced  by  friction  between  great 


ABYSSAL 


INJECTION 


FIG.  332.  —  Ideal  section  illustrating  the  abyssal  in- 
jection theory.  The  middle  vent  is  active  because  it 
originates  at  the  highest  point  in  the  injected  body.  The 
other  vents  are  extinct  because  of  the  advantage  of  the 
middle  vent.  The  arrows  show  the  movement  of  the  gas ; 
the  solid  black,  the  crystallized  portion  of  the  injection. 


338  PHYSICAL  GEOLOGY 

blocks  of  the  earth,  when  the  earth's  crust  is  yielding  to  strains; 
(3)  some,  that  it  is  chiefly  or  entirely  primal,  i.e.,  derived  from  a  sub- 
stratum of  unknown  thickness.  Of  these,  the  last  (3)  seems  to  be 
more  in  accord  with  the  known  facts  (p.  337)  than  the  others. 

The  activity  of  a  given  volcano  is  usually  independent  of  all  others, 
as  is  shown  in  the  history  of  Mauna  Loa  and  Kilauea  (p.  308),  which, 
though  forming  one  great  mound  of  lava,  erupt  independently.  On 
the  other  hand,  eruptions  of  Pelee  and  Soufriere  on  the  West 
Indian  islands  of  Martinique  and  St.  Vincent  have  been  almost 
simultaneous. 

Origin  of  Volcanic  Gases.  —  The  problem  of  the  origin  of  gases 
and  water  vapor  is  to  a  large  extent  identical  with  that  of  the  origin 
of  lava.  It  has  been  proved  by  experiment  that  all  rocks,  even  the 
most  dense  and  most  crystalline,  contain  large  quantities  of  gas,  so 
that  a  comparatively  small  volume  of  rock  would  be  sufficient  to 
furnish  practically  all  of  the  gases  and  all  of  the  water  vapor  given  off 
during  an  eruption  even  of  the  first  magnitude.  It  has  been  held, 
however,  that  water  vapor,  which  constitutes  the  greater  part  of  the 
emanations  of  dormant  volcanoes,  as  has  been  stated,  is  derived,  to 
a  large  extent  at  least,  from  either  sea  water  or  from  meteoric  water 
which  has  percolated  down  to  the  molten  lava  and  been  absorbed 
by  it. 

Cause  of  the  Ascension  of  Lava.  —  Every  theory  of  volcanism 
must  account  for  the  force  which  raises  the  lava  to  the  surface  of 
the  earth  and  often  throws  it  as  fine  dust  thousands  of  feet  into  the 
air.  There  is  general  agreement  that  this  force  is  to  be  found  (i) 
in  the  tidal  and  other  strains  to  which  the  earth  is  subjected ;  (2)  in 
hydrostatic  pressure  resulting  from  the  weight  of  the  overlying  rock ; 

(3)  in  the  enormous  expansive  force  of  the  gases  dissolved  in  the 
molten  magma,  whether  original  or  derived  from  other  sources ;    and 

(4)  to    some   degree    in    the    expansional    energy    of   the    injected 
mass. 

Cause  of  Periodicity.  —  The  lava  which  cools  in  the  throat  of  a 
volcano  is  characteristically  tough.  Since  the  cones  of  explosive 
volcanoes  are  built  of  loose  ash  deposits  of  little  strength,  it  is  evi- 
dent that  if  renewed  activity  were  to  result  from  an  explosion  alone, 
an  opening  would  usually  be  made  through  the  side  of  the  mountain 
instead  of  through  the  crater.  The  latter  is,  however,  usually  the 
case.  There  must  therefore  be  some  preliminary  weakening  of 
the  plug,  and  apparently  the  only  cause  for  such  weakening  is  to  be 


VOLCANOES  AND   IGNEOUS   INTRUSIONS 


339 


Crater 


found  in  the  fluxing  (Fig.  333)  by  intensely  hot  gas1  from  deep  in 
the  earth.  When  the  plug  has  been  formed,  heat  is  developed  be- 
neath it  by  the  compression  of  the  gas,  by  chemical  reaction,  and  by 
gas  solution.  After  the  plug 
is  shortened  by  the  melting 
away  of  its  lower  end  in  this 
intense  heat,  the  gas  pressure 
may  become  great  enough  to 
blow  out  the  remaining  part. 
After  the  explosion,  the  lava 
in  the  throat  of  the  volcano 
again  cools  and  a  period  of 
inactivity  ensues.  The  cause 
of  extinction  is  discussed  on 


P-  337- 

In  individual  cases,  as  for 


FIG.  333.  —  Section  of  a  dormant  volcano, 
showing  how  the  lava  plug  may  be  weakened 
by  gas  fluxing.  The  broken  line  shows  the 


example  in  that  of  Stromboli,     original  ,dePth  °f  the  so^/luf  a,nd,  thenpr,°g; 

ress  made  by  the  gas.     (Modified  after  Daly.) 

eruptions  occur  when  gas  has 

accumulated  under  the  scum  of  lava  in  the  crater  in  sufficient  volume 
to  cause  an  eruption,  after  which  quiet  ensues,  the  surface  of  the 
lava  hardens,  and  the  gases  again  begin  to  accumulate. 

Influences  of  the  Atmosphere,  etc.  —  Volcanic  eruptions  seem  to  be 
somewhat  more  prevalent  when  (i)  the  atmospheric  pressure  is  high 
than  when  it  is  low,  (2)  after  heavy  rains  rather  than  before,  and 
(3)  when  tidal  strains  are  unusually  severe.  None  of  these  causes 
could  produce  an  eruption,  but  it  is  probable  that  the  increased  weight 
of  the  atmosphere  over  a  large  area  would  aid  in  forcing  out  the  lava, 
as .  would  also  the  weight  of  the  water  after  heavy  rains.  Tidal 
strains  would  have  a  similar  effect.  None  of  these  agencies  could  be 
effective  unless  the  eruption  was  imminent,  only  a  slight  additional 
force  being  necessary  to  start  it. 

It  has  long  been  noticed  that  the  volcano  Stromboli  (p.  299)  dis- 
charges a  greater  quantity  of  steam  and  bombs  in  stormy  than  in 
fine  weather,  and  the  fishermen  make  use  of  it  as  a  "  weatherglass  "  : 
the  increase  of  activity  indicating  a  falling  barometer  and  conse- 
quently stormy  weather;  and  a  diminution  in  activity  promising 
fair  weather, 

1  Such  gases,  called  primeval  gases,  are  believed  to  come  directly  from  great  depths  and  reach 
the  surface  for  the  first  time.  They  are  distinguished  from  resurgent  gases  which  have  a  second- 
ary origin,  that  is,  those  which  are  absorbed  from  the  intruded  rock. 


340  PHYSICAL  GEOLOGY 

REFERENCES  FOR  VOLCANOES 

BONNEY,  T.  G.,  —  Volcanoes,  their  Structure  and  Significance. 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  i. 

DALY,  R.  A.,  —  Igneous  Rocks  and  their  Origin. 

DANA,  J.  D.,  —  Manual  of  Geology. 

GEIKIE,  A.,  —  Textbook  of  Geology. 

GILBERT;  G.  K.,  —  Report  on  the  Geology  of  the  Henry  Mountains. 

HITCHCOCK,  C.  H.,  —  Hawaii  and  its  Volcanoes. 

HULL,  E.,  —  Volcanoes:  Past  and  Present. 

IDDINGS,  J.  P.,  —  The  Problems  of  Volcanism. 

JUDD,  J.  W.,  —  Volcanoes. 

RUSSELL,  I.  C.,  —  Volcanoes  of  North  America. 

SCHWARTZ,  E.  H.  L.,  —  Causal  Geology. 

SHALER,  N.  S.,  —  Aspects  of  the  Earth,  pp.  50-56. 

TOPOGRAPHIC  MAPS,  U.  S.  GEOLOGICAL  SURVEY,  ILLUSTRATING  IGNEOUS  ACTIVITY 

Volcanoes  Laccoliths 

Lassen  Peak,  California.  Henry  Mts.,  Utah. 

Crater  Lake  National  Park  Sturgis,  South  Dakota. 

(Special),  Oregon. 

Shasta,  California.  Laoa  Plains 

Marysville  Buttes,  California.  Bisuka,  Idaho. 

San  Francisco  Mt.,  Arizona.  Ellensburg,  Washington. 
Island  of  Kauai,  Hawaiian  Islands. 

Lava  Sills 

New  York  City  and  Vicinity*  (Folio). 
Holyoke  Folio,  Massachusetts. 
New  Haven,  Connecticut. 


CHAPTER  X 

METAMORPHISM 

WHEN  either  sedimentary  or  igneous  rocks  have  been  affected, 
either  in  their  mineral  composition  or  in  their  texture  or  in  both,  so 
that  their  original  character  is  altered  or  entirely  changed,  they  are 
called  metamorphic  rocks,  and  the  process  is  known  as  metamorphism 
(Greek,  metamorphoun,  to  transform  or  change).  The  term,  as  used 
here,  will  be  limited  to  those  changes  which  have  resulted  from  heat 
or  pressure,  or  both,  whether  produced  locally,  as  for  example  by 
batholiths ;  or  over  large  areas  by  pressure  and  heat. 

Contact  Metamorphism.  —  The  form  of  metamorphism  most 
easily  understood  is  that  produced  when  sedimentary  rocks  are  cut 
by  great  igneous  intrusions,  such  as  batholiths.  Under  such  condi- 


IMPURE 
^=1  SANDSTONE 
QUARTZ  SCHIST 
8,  GNEISS 

INTRUSIVE 
GRANITE 


FIG.  334.  —  Diagram  showing  the  metamorphism  produced  by  a  great  igneous 
intrusion  upon  the  surrounding  rock. 

341 


342 


PHYSICAL  GEOLOGY 


tions,  it  is  often  found  that  the  sedimentary  rocks  are  greatly  altered 
near  the  source  of  the  heat  (Figs.  334,  335).  This  is  shown  by  a 
change  in  color,  in  hardness,  and  in  texture,  and  in  some  cases  by 
the  development  of  new  minerals.  Bituminous  coal  is  changed  to 

anthracite    coal,    or 
the 


even     in 


most 


FIG.  335.  —  Map  showing  the  metamorphic  zone  (dotted) 
about  an  igneous  intrusion. 


extreme  stage  to 
graphite ;  limestone 
is  metamorphosed  to 
marble;  soft  sand- 
stone may  be  con- 
verted into  hard 
quartzite;  and  shale 
may  be  metamor- 
phosed to  dense, 
compact  rocks,  such 
as  schist  and  horn- 
fels,  a  compact  flint- 
like  rock.  The  amount  and  extent  of  contact  metamorphism  depends 
upon  the  amount  of  heat  and  to  an  important  degree  upon  the  gase- 
ous emanations  (mineralizers)  given  off  by  the  molten  rock.  For 
example,  if  molten  rock  is  intruded  into  a  narrow  fissure,  the  surround- 
ing rock  will  usually  be  little  affected  (Fig.  336),  since  the  magma, 
having  a  comparatively  small  amount  of  heat,  soon  loses  it  to  the 
neighboring  rocks.  Moreover, 
the  quantity  of  gas  present  is 
too  small  to  produce  a  marked 
change.  In  the  case  of  great 
intrusions,  however,  such  as 
stocks  or  batholiths,  the  country 
rock  may  be  greatly  altered 
thousands  of  feet  away.  The 
effect  of  an  intrusion  is  naturally 
greatest  when  the  supply  of  heat 
is  large  and  long-continued.  In 
some  cases,  where  fragments  of 

the  surrounding  rock  have  been  inclosed  in  the  magma,1  black  shale 

has  been  baked  to  a  hard,  red,  porcelain-like  rock ;   granite  has  been 

more  or  less  completely  fused   to   dark   green  or  black  glass ;    and 

1  Powers,  S.,  —  The  Origin  of  the  Inclusions  in  Dikes,  Jour.  Geol.,  Vol.  23,  1915,  pp.  i-io. 


FIG.  336.  —  Diagram  showing  the  greater 
metamorphic  effect  of  an  igneous  intrusion 
along  bedding  planes. 


METAMORPHISM  343 

occasionally  the  fragments  have  been  completely  absorbed.  The 
metamorphism  resulting  from  intrusions  is  more  extended  when  the 
intrusion  cuts  across  strata  than  when  it  follows  bedding  planes, 
since  under  the  former  conditions  the  effect  of  the  heat  is  felt  along 
the  several  bedding  planes  with  which  the  lava  comes  in  contact 
(Fig.  336). 

The  metamorphic  effect  of  an  intrusion  is  greater  than  that  of 
an  extrusion,  since  in  the  former  the  heat  of  the  magma  is  lost  more 
slowly,  and  the  neighboring  rocks  are  consequently  heated  to  a  higher 
temperature  and  for  a  longer  time.  Moreover  moisture,  which  is 
a  powerful  agent  in  metamorphism  and  in  the  production  of  crystal- 
line structure  in  rocks,  is  more  likely  to  be  present  under  the  former 
conditions.  It  frequently  happens  that  the  rock  underlying  a  lava  flow 
is  so  little  metamorphosed  that  no  change  is  visible  to  the  naked  eye. 

The  effect  of  great  intrusions  has  already  been  discussed  under 
Subjacent  Masses  (p.  327). 

Regional  Metamorphism.  —  Thousands  of  square  miles  of  the 
earth's  surface  are  underlain  by  metamorphic  rocks.  They  occur 
over  large  areas  in  Canada,  in  the  Adirondacks,  over  the  greater  part 
of  New  England,  in  the  Piedmont  region  east  of  the  Appalachian 
Mountains,  in  a  large  area  south  of  Lake  Superior,  and  in  the  Cordil- 
leras. 

Widespread  or  regional  metamorphism  may  be  brought  about  in 
one  of  two  ways,  (i)  It  may  result  from  great  igneous  intrusions, 
such  as  deep-seated  batholiths.  (The  metamorphism  of  the  older 
rocks  of  the  Laurentian  region  of  Canada  seems  to  have  been  pro- 
duced largely  in  this  way.)  (2)  Great  lateral  pressure  may  also  pro- 
duce sufficient  heat  to  recrystallize  the  rocks  affected.  In  regions 
where  igneous  intrusions  are  absent,  as  in  New  England,  the  meta- 
morphism appears  to  have  been  caused  by  lateral  pressure  alone. 
The  fact  that  the  rocks  of  some  metamorphic  regions  are  more  or 
less  highly  folded  and  that  the  intensity  of  the  metamorphism  is, 
to  some  degree,  in  direct  proportion  to  the  intensity  of  the  deforma- 
tion is  offered  as  proof  that  the  alteration  of  the  rocks  in  such  re- 
gions was  due,  either  directly  or  indirectly,  to  the  cause  or  causes 
which  produced  the  folding.  The  indirect  cause  is  believed  to  have 
been  the  pressure  which  produced  the  deformation ;  the  direct  causes, 
the  heat  resulting  from  the  rock  mashing  produced  by  pressure,  and 
the  presence  of  underground  water  which  aided  powerfully  in  bringing 
about  the  molecular  changes  which  resulted  in  the  crystalline  texture. 


344  PHYSICAL  GEOLOGY 

CLASSIFICATION  OF   METAMORPHIC   ROCKS 

Quartzite. — A  quartzite  is  a  metamorphic  sandstone.  It  is  a  com- 
pact rock  composed  of  grains  of  quartz  sand  cemented  by  material 
of  the  same  kind,  that  is,  by  silica.  It  can  usually  be  distinguished 
from  sandstone  by  its  appearance  when  broken.  The  broken  sur- 
face of  sandstone  usually  has  a  more  or  less  granular  feel  and  appear- 
ance, since  the  fracture  takes  place  in  the  weak  cement  leaving  the 
grains  outstanding.  In  quartzite,  on  the  other  hand,  since  the  grains 
and  cement  are  of  the  same  material,  the  fracture  takes  place  in 
cement  and  grains  alike.  It  is  often  difficult  to  state  whether  a 
quartzite  owes  its  character  to  heat  and  pressure  or  to  cementation 
by  underground  water.  A  quartz  schist  is  a  quartzite  in  which  a 
foliated  structure  has  been  developed,  the  planes  of  foliation  being 
covered  with  white  mica. 

Marble.  —  Commercially,  any  calcareous  rock  which  will  take 
a  polish  is  called  a  marble,  but  in  a  more  technical  sense  a  marble 
is  a  metamorphic  limestone.  It  is  distinguished  from  limestone  by 
its  granular  appearance  (texture)  and,  unlike  most  metamorphic 
rocks,  if  pure  is  not  schistose.  When  limestone  is  heated  where  the 
pressure  is  slight,  it  is  converted  into  quicklime  by  the  escape  of 
carbon  dioxide ;  but  when  heated  under  pressure,  which  prevents 
the  escape  of  the  gas,  it  crystallizes  into  marble.  The  clouded 
shadings  and  "  veins  "  of  marble  are  produced  by  the  crystallization 
of  impurities,  with  the  resulting  formation  of  colored  minerals. 

Slate.  —  This  rock  may  be  considered  as  a  hardened  shale  or  mud 
in  which  a  tendency  to  break  along  parallel  planes  —  not  bedding 

planes  —  is  developed.  This  con- 
dition is  called  slaty  cleavage  and 
by  its  means  the  rock  splits 
readily  into  broad,  thin  sheets. 
The  cause  of  slaty  cleavage  is 
to  be  found  in  the  great  lateral 
pressure  to  which  such  fine- 

VMHafiyaVa8penor0cs<     grained     sediments    as    clay    or 
(Modified  after  Pirsson.) 

(rarely)  volcanic  ash  are  sub- 
jected, especially  if,  when  compressed  in  one  direction,  they  are 
able  to  expand  to  some  extent  in  others.  A  rock  is  affected  by 
such  compression  in  three  ways  :  (i)  any  particles  capable  of 
compression  are  flattened  and  correspondingly  lengthened  at  right 


METAMORPHISM 


345 


angles  to  the  pressure ;  (2)  compression  also  turns  elongated  particles 
into  parallel  positions  so  that  they  take  a  direction  in  which  their 
longest  axes  are  at  right  angles  to  the  pressure;  (3)  as  a  result  of 
the  metamorphism  accompanying  compression  new  minerals,  such  as 
mica,  are  formed,  and  since  these  crystals  can  grow  more  easily 
in  the  direction  in  which  the  pressure  is  least  —  along  the  line  of 
least  resistance  —  they  also  will  have  their  longest  axes  at  right 
angles  to  the  pres- 
sure. The  combined 
effect  is  to  produce 
a  rock  which  will 
cleave  or  split  much 
more  readily  in  one 
direction  than  in  any 
other. 

Since  a  bed  of  shale 
is  seldom  perfectly 
homogeneous,  slate 
differs  in  the  perfec- 
tion of  its  cleavage. 
Sandy  layers,  for  ex- 
ample, are  contorted 
and  poorly  cleaved, 
while  the  layers  of 
pure  clay  have  a  perfect  slaty  cleavage.  Slaty  cleavage  will  be  per- 
pendicular to  the  bedding  if  the  rocks  were  subjected  to  pressure 
when  horizontal,  but  may  be  inclined  at  any  angle  to  the  bedding  if  the 
beds  were  folded  before  the  pressure  became  intense  (Figs.  337,  338). 

The  formation  of  slate  requires  much  less  extensive  metamorphic 
changes  than  does  that  of  schist  and  of  gneiss  (p.  346). 

Schist.  —  Schists  are  rocks  composed  of  thin,  wavy  leaves  or 
folia  in  which  the  foliation  (Latin,  foliatus,  leaved)  or  lamination  is 
due  to  the  abundance  and  parallel  position  of  such  minerals  as  mica, 
hornblende,  or  talc.  The  folia  are  not  of  uniform  thickness,  but  are 
flattened  lenses  of  the  minerals,  often  bent  and  wavy,  with  their  platy 
surfaces  in  parallel  planes.  The  characteristic  foliated  structure 
of  schists  is  developed  when  rocks  have  been  subjected  to  great  pres- 
sure. Schists  are  the  result  either  of  (i)  the  formation  of  new  min- 
erals which  developed  at  right  angles  to  the  pressure,  since  growth 
takes  place  more  readily  along  the  lines  of  least  resistance ;  or  of  (2) 


FIG.  338.  —  Illustration  showing  the  relation  of  slaty 
cleavage  (nearly  vertical)  to  bedding  (dipping  to  the 
right).  (Photo.  L.  E.  Westgate.) 


34-6 


PHYSICAL  GEOLOGY 


the  deformation  resulting  from  the  crushing  of  such  rocks  as  con- 
glomerates, granites,  or  basalts.  (3)  In  contact  metamorphism  the 
development  of  minerals,  especially  mica,  along  the  stratification 
planes  of  sedimentary  rocks  also  produces  a  schist. 

Schists  are  given  various  names,  depending  upon  their  most 
conspicuous  mineral.  Mica  schist  is  composed  principally  of  mica 
and  quartz  and  is  the  most  common  type  of  metamorphic  rock. 
Mica  schists  are  usually  metamorphosed,  fine-grained  sandstones 
and  shales.  Hornblende  schist  consists  largely  of  hornblende,  and 
varies  from  green  to  black  in  color.  In  some  cases,  the  characteristic 
needle  or  blade-like  crystals  are  readily  recognized,  but  in  others 
the  grain  is  so  fine  that  the  individual  crystals  cannot  be  seen.  Horn- 
blende schists  are  derived  from  diorites,  gabbros,  etc.,  by  pressure, 

and  it  is  probable 
that  impure  lime- 
stones containing 
sand,  clay,  and  iron 
oxides  also  produce 
hornblende  schists 
when  subjected  to 
metamorphism. 

Gneiss.  —  This  is 
a  banded,  crystalline 
rock  (Fig.  339)  in 
which  feldspar  is 
present.  It  is  a  rock 
with  the  composition 
of  granite,  but  with 
a  banded  structure. 

Gneiss  may  be  con- 
FIG.  339.  —  Gneiss,  showing  banding.     (U.  S.  National        -j         i     r 

Museum.)  Sldered    /or    conven- 

ience  as  intermediate 

between  an  igneous  rock,  such  as  granite  or  diorite,  and  schist.  It 
will  readily  be  seen  from  the  above  that  a  gneiss  may,  on  the  one 
hand,  so  closely  resemble  a  schist  that  one  will  be  in  doubt  as  to 
its  classification,  and  on  the  other  hand,  that  it  may  be  confused 
with  a  granite.  Typically,  however,  gneisses  are  easily  recogniz- 
able and  may  be  considered  for  convenience  as  banded  granites. 
As  in  the  case  of  schists,  various  qualifying  adjectives  are  used  in 
describing  gneisses,  as  biotite  gneiss,  hornblende  gneiss,  garnet  biotite 


METAMORPHISM 


347 


gneiss.  Gneisses  may  be  formed  either  (i)  by  the  metamorphism 
by  mashing  of  granite  or  other  igneous  rock;  (2)  by  the  meta- 
morphism of  sedimentary  beds;  or  (3)  when  a  granite  magma  is 
intruded  into  sedimentary  or  schistose  beds  under  pressure  operating 
from  a  distance,  the  molten  magma  spreading  along  the  sedi- 
mentary planes  or  between  the  folia  of  the  schists.  This  intimate 
admixture  permits  of  extensive  mineral  changes,  and  the  two  types 
of  rock,  very  different  in  geological  age,  become  welded  into  a  com- 
posite gneiss. 

TABLE  SHOWING  METAMORPHIC  CHANGES 


SEDIMENTS 

SEDIMENTARY  ROCKS 

METAMORPHIC  EQUIVALENTS 

Gravel 

Conglomerate 

Gneiss  and  various  schists 

Sand 

Sandstone 

Quartzite  and  quartz  schist  if  from 
pure    quartz    sand  ;    mica   schist 
if  certain  impurities  are  present 

Clay 

Shale 

Slate  and   schists,-  especially  mica 
schist 

Lime  deposits,  such  as 
chalk  or  shells 

Limestone 

Marbles 

IGNEOUS  ROCKS 

METAMORPHIC  ROCKS 

Granite,    syenite,   and    other    rocks    with 
much  feldspar 

Gneiss 

Fine-grained  feldspar  rocks,  such  as  felsite 
and  tuffs 

Slate  and  schists 

Diorite,  basalt,  and  other  basic  rocks 

Hornblende  schist  and 

other  schists 

SUMMARY  OF  CAUSES  OF  METAMORPHISM 

The  important  factors  to  be  considered  in  the  production  of  meta- 
morphism are  heat,  moisture  and  pressure,  mechanical  movements, 
and  the  nature  of  the  material  involved. 

Heat.  —  The  heat  necessary  for  metamorphism  may  come  (i) 
from  igneous  intrusions.  In  this  way  the  surrounding  rocks  are 
hardened  and  dehydrated.  The  process  is  shown  in  the  manufac- 
ture of  bricks,  in  which  the  clay  is  dehydrated  and  is  hardened  to  a 
rock-like  mass  by  partial  fusion.  New  minerals  are  often  developed 


348  PHYSICAL  GEOLOGY 

in  rocks  affected  by  intrusions.  (2)  The  heat  developed  by  pressure 
will  be  discussed  in  a  later  paragraph. 

Moisture.  —  When  moisture  is  present  in  considerable  quantity, 
as  is  the  case  with  sedimentary  rocks,  the  effects  of  heat  and  pressure 
in  producing  metamorphic  changes  are  greatly  increased.  This  is 
true  because  highly  heated  water,  especially  if  alkalies  are  present, 
readily  dissolves  minerals  which  would  otherwise  be  insoluble,  and 
from  the  solution  the  same  minerals  or  new  ones  may  be  formed. 
Water  also  takes  part  in  the  chemical  composition  of  some  minerals, 
such  as  mica,  and  is  therefore  necessary  for  their  formation.  The 
recrystallized  and  newly  formed  minerals  are  usually  arranged  with 
their  longer  axes  at  right  angles  to  the  pressure  (p.  349),  and  are  more 
stable  under  the  new  conditions  than  if  they  had  not  been  changed. 
The  potency  of  moisture  is  shown  by  the  fact  that  rock  which  re- 
quires a  temperature  of  2500°  F.  for  melting  when  dry,  becomes  pasty 
at  750°  F.  when  water  is  present.  The  effect  of  gaseous  emanations 
in  producing  metamorphism  is  sometimes  of  the  greatest  importance. 

Pressure.  —  Simple  downward  pressure,  such  as  that  which  results 
from  the  weight  of  overlying  rocks,  has  some  metamorphic  effect  and 
also  tends  to  consolidate  the  sediments  by  bringing  the  grains  closer 
together.  But  when  the  crust  is  under  enormous  lateral  pressure, 
as  a  result  of  the  contraction  of  the  earth,  the  strata  are  folded,  crushed, 
and  mashed  together,  and  metamorphism  takes  place.  In  this 
way  pebbles,  fossils,  and  crystals  are  flattened  and  elongated,  or 
broken  into  fragments.  By  this  agent  alone  the  texture  of  rocks 
can  be  changed,  but  it  is  in  combination  with  heat  and  moisture 
that  the  production  of  new  minerals  and  the  formation  of  highly 
metamorphic  rock  is  brought  about. 

The  importance  of  lateral  pressure  in  the  production  of  regional  metamorphism 
has  been  questioned  by  certain  French  geologists1  who  believe  that  it  is  brought  about 
by  heat,  moisture,  and  vertical  pressure  without  the  aid  of  lateral  pressure;  that  the 
sediments  in  the  lower  parts  of  thick  geosynclines  are  actually  fused  by  heat  from  the 
interior  of  the  earth,  and  upon  cooling  become  igneous  rocks,  capable  in  their  turn  of 
metamorphosing  by  contact  the  rocks  which  surround  them.  They  hold  that  this 
is  by  far  the  most  important  element  in  the  process  of  metamorphism  and  that  dynamic 
action  can  deform  but  cannot  transform  rock;  i.e.,  it  is  not  competent  by  itself  to  pro- 
duce metamorphic  changes.  This  theory  has  been  generally  abandoned  by  Ameri- 
can geologists  and  in  fact  by  many  eminent  French  geologists. 

How  the  Parallel  Arrangement  of  Minerals  is  Produced.  —  The 
conditions  favorable  for  the  production  of  metamorphism  having 

1  Haug,  —  Traitt  de  Gtdogie,  pp.  172-191 ;  234—235. 


METAMORPHISM  349 

been  discussed,  it  remains  to  be  shown  why  metamorphic  rocks  are 
usually  cleavable. 

1.  Crystallization. — A    study   of  a   mica    or   hornblende    schist 
shows  that  hornblende  and  mica  are  responsible  for  the  best  rock 
cleavage.     A   microscopic   examination   of  these   rocks   and   of  the 
sedimentary  rocks  from  which  they  were  derived  shows  that  horn- 
blende and  mica  were  built  up  chiefly  by  subsequent  recrystalliza- 
tion  from  substances  already  in  the  sedimentary  rocks  and  did  not 
exist  in  them  in  their  final  form.     This  fact  is  shown  by  a  general 
lack  of  fractures  in  the  minerals  of  the  cleavable  rock,  such  as  would 
have  been  developed  had  the  rock  been  formed  simply  by  crushing 
and  by  the  rotation  of  the  mineral  constituents  to  parallel  positions. 
Moreover,  most  of  the  mineral  particles  of  cleavable  rocks  are  larger 
than  those  of  the  same  rock  before  the  latter  was  metamorphosed. 
The  gradation  of  shale  to  phyllite  (a  metamorphic  shale)  means  an 
increase  in  the  size  of  the  grains.     The  parallel  arrangement  of  the 
mineral  constituents  of  a  metamorphic  rock  is  thus  seen  to  be  the 
result  of  crystallization,  and  the  rotation  of  the  original  particles 
to  a  parallel  position,  of  minor  importance. 

2.  Granulation.  —  Recrystallization    and     rotation    are    not    the 
only  processes  instrumental  in  the  production  of  easy  splitting  or 
cleavage  in  metamorphic  rocks.     In  the  early  stages  of  the  process 
the  larger  brittle  particles  are  broken  into  small  fragments  or  are 
granulated   and  elongated,  and    at    the  same  time  recrystallization 
builds  up  new  minerals  from  the  broken  particles.1     It  is  probable 
that  granulation  aids  crystallization  in  that  it  grinds  the  particles 
into  small  pieces  which  then  present  a  greater  surface  upon  which 
the  chemical  process  may  act. 

Relation  of  Cleavage  to  Pressure.  —  The  proof  that  the  planes 
of  easy  splitting  of  metamorphic  rocks  are  at  right  angles  to  the 
pressure,  or  in  other  words  parallel  to  the  rock  elongation,  is  seen  (i) 
in  the  distortion  of  the  pebbles  of  conglomerates,  (2)  in  the  distortion 
of  fossils,  the  lengthening  being  in  the  plane  of  the  cleavage,  and  (3) 
when  rock  is  intruded  by  igneous  masses  which  exert  a  great  pressure 
on  the  walls,  in  the  cleavage  which  is  developed  parallel  to  the  walls. 

From  Igneous,  through  Sedimentary,  to   Metamorphic  Rocks.  — 

The  history  of  a  metamorphic  rock,  formed  by  the  recrystallization 

of  sedimentary  rocks,  may  be  briefly  summarized.     If  a  great  mass 

of  granite  is  exposed  to  the  weather,  it  begins  to  decay ;   its  feldspar 

1  Leith,  C.  K.,  —  Structural  Geology,  1913. 


350  PHYSICAL  GEOLOGY 

and  mica  being  disintegrated  and  forming  simpler  compounds, 
some  of  which  are  dissolved  and  carried  away  by  the  water,  while 
the  remainder  is  left  as  clay.  This  clay  together  with  the  insoluble 
quartz  is  transported  by  streams  and  is  finally  deposited  in  the 
ocean,  the  clay  forming  mud  and  the  quartz  grains,  sand.  The 
lime  dissolved  from  the  feldspars  may  be  taken  up  by  organisms 
to  form  lime  ooze  or  limestone.  If  these  sediments  are  laid  down 
in  a  sinking  geosyncline  (p.  359),  they  may  in  time  be  buried  to 
a  depth  of  several  thousand  feet.  When  in  the  course  of  their 
burial  they  reach  the  belt  of  cementation  (p.  61),  they  will  be  con- 
solidated into  shales  and  limestones.  If  the  sediments  in  the  syn- 
cline  are  subjected  to  great  lateral  pressure,  heat  will  be  developed 
which  will  metamorphose  them,  changing  the  clays,  sandstones,  and 
limestones  to  schists,  quartzites,  and  marbles. 

For  a  discussion  of  the  formation  of  metamorphic  rocks  from 
igneous  rocks,  see  p.  347. 

Weathering  of  Metamorphic  Rocks.  —  Metamorphic  rocks  usually 
resist  weathering  better  than  sedimentary  ones,  because  they  have 
been  compacted  by  heat  and  pressure  and  have  a  crystalline  texture. 
Mica  schist,  for  example,  is  less  easily  disintegrated  than  the  impure 
shale  from  which  it  was  made ;  hard  quartzite  than  the  less  compact 
sandstone ;  marble,  however,  may  or  may  not  be  more  resistant  to 
weathering  and  erosion  than  the  limestone  from  which  it  was  derived  ; 
the  disintegration  of  slate  is  hastened  by  its  vertical  cleavage,  but 
is  hindered  by  its  greater  compactness.  As  a  result  of  prolonged 
weathering,  however,  metamorphic  sedimentary  rocks  are  reduced, 
in  time,  to  the  same  soil  which  their  sedimentary  equivalents  would 
have  made,  schists  weathering  to  clays;  quartzites  to  sands;  and 
marbles  to  calcareous  clays.  Because  of  their  greater  resistance  to 
weathering,  metamorphic  rocks  are  usually  associated  with  the  scen- 
ery of  mountains.  Quartzite  usually  resists  weathering  better  than 
any  other  rock  on  account  of  its  small  porosity,  its  insolubility,  and 
its  homogeneous  composition.  Because  of  the  last-named  character, 
it  is  little  affected  by  changes  in  daily  temperature  (p.  31),  and 
it  is  not  disintegrated  by  the  decay  of  a  weaker  constituent,  as  is 
the  case  with  igneous  rocks,  such  as  granite.  Quartzite  hills  are, 
consequently,  among  the  last  to  disappear.  In  regions  of  meta- 
morphic rocks,  where  schist  and  marbles  are  involved,  streams  have 
usually  cut  their  valleys  in  the  softer  and  more  soluble  marbles,  while 
the  more  resistant  schists  form  hills  and  mountains. 


METAMORPHISM  351 

Economic  Importance.  —  Gneisses  and  quartzites  are  often  used 
for  building  stones  and  road  material,  but  marble  is  the  metamorphic 
rock  which  is  the  most  prized,  both  for  building  purposes  and  for 
works  of  art. 

REFERENCES   FOR   METAMORPHISM 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  2d  ed.,  Vol.  i,  pp.  426-449. 
COLE,  G.  A.  J.,  —  Rocks  and  their  Origins. 
LEITH,  C.  K.,  —  Structural  Geology. 
PIRSSON,  L.  V.,  —  Rocks  and  Rock- Minerals. 


CLELAND   GEOL.  —  23 


CHAPTER  XI 
MOUNTAINS  AND   PLATEAUS 

THE  term  mountain  is  used  very  loosely  to  indicate  a  conspicuous 
height  of  land.  In  flat  regions  such  as  southern  New  Jersey  and 
the  plains  of  Texas,  heights  rising  more  than  100  to  200  feet  are 
dignified  by  the  name  mountain,  while  in  mountainous  regions  ele- 
vations of  1000  or  2000  feet  are  often  called  hills.  It  is  evident  that 
the  term  is  a  relative  one,  since  on  plateaus  a  mile  or  two  above 
the  sea  a  conspicuous  elevation  must  be  still  higher,  and  a  mountain 
in  such  a  situation  would  be  at  least  6000  feet  above  sea  level.  A 
mountain  ridge  or  range  is  usually  long,  with  a  narrow  crest ;  when 
numerous  ranges  are  associated,  they  constitute  a  mountain  chain. 
In  ancient  paintings  and  in  old  geographies,  the  slope  of  mountains 
was  usually  depicted  as  very  steep,  an  angle  of  60°  from  the  hori- 
zontal not  being  uncommon,  but  such  slopes  seldom  occur  in  nature, 
and  angles  as  high  as  35°  are  rare. 

Mountains  of  Accumulation. — Volcanoes  are  typical  of  this  class, 
as  they  are  built  up  by  the  accumulation  of  ash,  or  lava,  or  both. 
They  sometimes  occur  singly,  sometimes  they  are  arranged  along 
fracture  lines  (p.  267),  and  sometimes  no  definite  order  can  be  recog- 
nized. 

Since  sand  dunes  (p.  52)  occasionally  attain  a  height  of  600 
feet  and  moraines  (p.  159)  a  height  of  1000  feet,  they  are  sometimes 
called  mountains  in  regions  where  other  elevations  are  inconspicuous 
and  must  therefore  be  included  under  the  head  of  accumulation 
mountains. 

Residual  Mountains. —  These  are  formed  when  a  plateau  has  been 
extensively  dissected  by  rivers,  and  the  ridges  and  pyramids, 
the  remnants  of  the  plateau,  which  have  escaped  erosion,  stand 
so  high  above  the  valleys  as  to  constitute  mountains.  The  many 
"  temples  "  in  the  Grand  Canyon  of  the  Colorado  in  Arizona  (Fig. 
340)  show  at  a  glance  how  such  mountains  are  formed,  and  the 
Catskills  of  New  York  furnish  an  excellent  example  of  residual 

352 


MOUNTAINS   AND   PLATEAUS 


353 


354  PHYSICAL  GEOLOGY 

mountains  in  which  gentle  slopes  are  characteristic.  The  form  of 
mountains  of  this  class  depends  upon  the  nature  and  arrangement  of 
the  material  (Fig.  340)  out  of  which  they  were  sculptured,  and  to 
some  extent  upon  the  climate.  The  Catskills  owe  their  gentle  slopes 
to  the  fact  that  the  rocks  of  which  they  are  composed  do  not  differ 
greatly  in  hardness,  and  also  to  the  smoothing  effect  of  a  moist  climate. 
The  steep-sided  mesas  of  the  southwestern  United  States  are  often 
the  result  of  the  erosion  of  lava  plateaus,  the  hard  lava  forming  flat- 
topped  mountains  bounded  by  conspicuous,  vertical  cliffs.  Residual 
mountains  are  confined  to  those  formed  of  horizontal  rocks  or  slightly 
inclined  rocks.  The  external  form  of  complexly  folded  mountains 
(p.  356)  is  due  to  erosion,  and  they  are  in  a  sense  residual  moun- 
tains. They  have,  however,  been  placed  in  a  class  by  themselves  be- 
cause of  the  origin  of  the  folded  structure  which  gives  them  a  distinct 
character. 

Fault  or  Block  Mountains.  —  It  was  shown  in  the  study  of  fault- 
ing (p.  267,  Fig.  266)  that  important  topographic  features  are 
produced  in  this  way,  and  that  mountain  ridges  of  this  origin  have 
been  formed  either  by  uplift  along  one  side  of  a  fault,  or  by  sinking 
along  one  side,  or  by  a  combination  of  the  two  movements.  Moun- 
tains formed  by  the  elevation  of  wedge-shaped  blocks  are  called 
horsts  (p.  263,  Fig.  257).  In  southern  Utah  and  Oregon  block  or 
faulted  mountains  have  been  carefully  studied  and  have  been  found 
to  exhibit  all  the  stages  from  young  faulted  mountains,  in  which 
erosion  has  as  yet  been  able  to  accomplish  little,  to  ancient  fault 
mountains,  in  which  erosion  has  proceeded  so  far  that  their  origin 
can  merely  be  conjectured.  In  portions  of  these  regions  block  moun- 
tains 10  to  40  miles  long  and  1000  to  1200  feet  high  occur.  The 
ridges  are  steep  or  clifF-like  on  the  fault  side  and  have  a  gentle  slope 
on  the  opposite  side.  Between  the  faults  are  trough-like  depressions 
in  which  lakes  sometimes  rest.  The  steep  eastern  slope  of  the 
Sierra  Nevada  Mountains  marks  the  fault  along  which  a  great  block, 
500  miles  in  length  and  70  to  100  miles  broad,  has  been  raised,  the 
escarpment  thus  formed  rising  from  5000  to  6000  feet  above  the 
desert  valleys  to  the  eastward,  and  reaching  a  maximum  height  of 
14,000  feet  in  the  vicinity  of  Death  Valley.  (Russell.)  Well-known 
examples  of  block  mountains  are  the  Vosges  and  Black  Forest  of 
Germany  (p.  100,  Fig.  81). 

Laccolith  Mountains.  —  Under  the  discussion  of  laccoliths  (p. 
327)  it  was  seen  that  in  certain  localities  molten  material  has 


MOUNTAINS   AND   PLATEAUS 


355 


been  injected  into  the  earth's  crust  in  such  quantity  that  the  cover 
of  sedimentary  strata  has  been  lifted  into  dome-like  forms.  After 
prolonged  erosion  the  softer  strata  are  partially  or  wholly  removed, 
and  the  hard,  igneous  core  is  left  as  a  mountain  or  hill.  In  moun- 
tains of  this  origin,  the  strata  dip  away  in  all  directions  from  the 
center,  and  not  uncommonly  "hogbacks,"  with  steep  cliffs  facing 
towards  the  center,  form  one  or  more  broken  rings  about  the  moun- 
tain. Although  mountains  of  this  type  are  not  abundant,  a  large 


FIG.  341.  —  Little  Sundance  Dome,  Sundance,  Wyoming.     This  is  a  laccolith 
from  which  the  overlying  strata  have  been  eroded. 

number  are  known  to  exist,  of  which  those  in  Utah,  California,  Wyo- 
ming, South  Dakota,  British  Columbia,  and  Canada  might  be  men- 
tioned (Fig.  341). 

Domed  Mountains.  —  The  Black  Hills  of  South  Dakota  may 
be  taken  as  a  type  of  domed  mountains.  They  rise  2000  to  3000 
feet  above  the  surrounding  plains  and  about  7000  feet  above  sea 
level,  and  are  carved  from  a  dome-shaped  uplift  of  the  earth's  crust. 
The  length  of  the  dome  is  about  100  miles  and  the  width  about  50 
miles, —  about  the  size  of  Connecticut.  As  will  be  seen  from  the 
diagram  (Fig.  342),  the  eroded  central  part  is  composed  of  crystalline 
rocks  from  which  the  strata  dip  in  all  directions.  As  a  result  of  the 
more  rapid  erosion  of  a  stratum  of  shale,  a  trench  called  the  Red 
Valley,  in  many  places  two  miles  wide,  entirely  surrounds  the 
center,  except  where  it  is  cut  through  by  streams.  The  Red  Valley 
is  separated  from  the  flat  plains  surrounding  the  central  mountain 
mass  by  a  rim  or  "  hogback,"  which  presents  a  steep  face  towards 
the  valley  and  rises  several  hundred  feet  above  it. 


356 


PHYSICAL  GEOLOGY 


The  Uinta  Mountains  of  Utah  are  formed  from  a  flattened  dome 
or  broad  arch  150  miles  long  and  20  to  25  miles  wide,  which  rises 
about  10,000  feet  above  sea  level.  It  will  be  seen  from  the  diagram 


FIG.  342.  —  A  block  diagram  of  a  domed  mountain,  the  Black  Hills  of  South  Dakota. 
The  investing  valleys  with  their  steep,  infacing  cliffs  are  well  shown.  The  central 
mountain  mass  is  granite,  and  the  three  isolated  mountains  are  intrusive  masses  of 
igneous  rocks. 

(Fig.  343)  that  if  all  the  rock  which  has  been  carried  away  were 
restored,  the  mountains  would  be  three  and  a  half  miles  higher  than 
now.  This  does  not  prove  that  the  mountains  were  ever  as  high  as 
that,  since  the  denudation  of  a  mountain  mass  commences  as  soon 
as  it  begins  to  rise  above  the  surrounding  country,  and  the  rate  of 
erosion  in  all  probability  is  about  the  same  as  the  rate  of  upheaval. 


FIG.  343. — A  section  across  the  Uinta  Mountains,  Utah.  The  range  has  been 
formed  out  of  a  single  broad  arch  40  miles  wide,  which  has  been  greatly  eroded.  The 
original  surface  is  indicated  by  the  dotted  line,  showing  that  three  and  one  half  miles 
of  rock  have  been  removed  by  erosion. 

Complexly  Folded  Mountains.  —  It  is  to  this  class  that  the  great 
mountain  systems  of  the  world  belong,  the  Appalachians,  the  American 
Cordilleras  (the  Rocky,  Sierra  Nevada,  Coast,  and  Cascade  moun- 
tains), the  Alps,  Himalayas,  Pyrenees,  etc.  The  strata  which  compose 
them  may  consist  of  a  series  of  gentle  anticlines  and  synclines  (p.  254), 
or  may  be  intricately  folded  and  faulted.  Portions  of  the  Jura 


MOUNTAINS  AND   PLATEAUS 


357 


Mountains  of  Switzerland  present  a  classic  example  of  gently  folded 
strata ;  here  one  finds,  in  certain  places  across  the  system,  a  series 


FIG.  344.  —  Section  through  the  Juras,  showing  mountain  ridges  produced  by 
several  open  folds,  like  great  earth  waves. 

of  simple  anticlines  and  synclines  (Fig.  344).  A  portion  of  the 
Appalachian  Mountains  in  Pennsylvania  also  presents  a  similar  sim- 
ple structure  (Fig.  345).  In  the  Alps,  however,  the  folds  are  much 


FIG.  345.  —  Relief  map  of  the  Appalachian  Mountains. 
(See  Figs.  244  and  245,  p.  255.) 

more  pronounced  and  complicated,  and  it  is  often  extremely  difficult 
to  determine  the  structure  of  the  strata  (Fig.  346). 


358 


PHYSICAL  GEOLOGY 


It  is  evident  that  a  series  of  strata,  subjected  to  forces  sufficient 
to  produce  the  intense  folding  shown  in  the  Alps  and  in  the  southern 


FIG.  346.  —  Diagram  showing  a  cross  section  of  the  Alps  along  the  Simplon  tunnel. 
The  complicated  structure  and  former  extension  of  the  strata  are  shown.  (After 
Schmidt.) 

Appalachians  (Fig.  347),  will  often  break  and  fault  instead  of  fold- 
ing. It  is,  consequently,  seldom  that  folded  strata  are  free  from 
dislocations  over  a  distance  of  even  a  few  miles.  The  strata  of  folded 
mountains  have  often  been  so  compressed  that  cleavage  planes  parallel 


FIG.  347.  —  A,  section  across  the  southern  Appalachians  where  extreme  faulting  has  oc- 
curred (U.  S.  Geol.  Surv.) ;   B,  section  in  the  vicinity  of  Chattanooga,  Tennessee. 

to  the  folds  have  been  induced.     Metamorphism  is  in  proportion 
to  the  intensity  of  the  compression. 

ORIGIN  AND  DEVELOPMENT  OF  FOLDED  MOUNTAINS 

Four  points  have  been  established  with  reference  to  folded  moun- 
tains :  (i)  they  were  formed  from  thick  sediments  that  had  accumu- 
lated in  geosynclines ;  (2)  they  were  folded  as  a  result  of  lateral 
pressure;  (3)  the  rate  of  folding  was  slow;  and  (4)  their  outlines, 
after  prolonged  erosion,  are  determined  largely  by  the  character  of 
the  rocks  and  the  arrangement  of  the  strata.  A  discussion  of  these 
points  follows. 

There  is  also  reason  to  believe  that  mountains  of  this  class  are  situ- 


MOUNTAINS   AND  PLATEAUS 


359 


ated  at  the  junction  of  great  earth  segments  or  blocks  where,  as  a 
result  of  the  crowding  of  the  latter  upon  each  other  as  they  are 
drawn  toward  the  center  of  the  earth,  the  weak  strata  of  geosynclines 
are  folded. 

1.  Geosynclines.  — The  sedimentary  strata  of  which  folded  moun- 
tains are  formed  are  very  thick ;    in  the  Appalachians,  the  thickness 
is  about  25,000  feet;  in  the  Coast  Ranges  of  California,  30,000  feet; 
and  in  the  Alps,  50,000  feet.     When  a  stratum  is  traced  to  a  distance 
of  even  a  few  miles  from  the  mountain  chain,  it  is  found  that  it  rapidly 
becomes  thinner;   the  strata  that  have  a  thickness  of  about  25,000 
feet  in  the  Appalachians,  for  example,  are  only  about  2500  feet  thick 
in  the  Mississippi  Valley.     An  examination  of  the  rocks  of  moun- 
tain masses  often  shows  that  many  of  them  are  of  shallow  water 
origin,  as  the  occurrence  of  conglomerates  and  sandstones  testifies. 
Ripple  marks,  sun  cracks,  and  fossils  afford  similar  evidence.  *  The 
presence  of  limestones,  on  the  other  hand,  may  indicate  (p.  238) 
that  the  water  in  which  they  were  deposited  was  deep  or  far  from 
shore.     The  sediments  that  are  being  laid  down  in  the  seas  to-day 
are  deposited  in  a  belt  extending  from  the  shore  line  to  a  distance 
usually  considerably  less  than  50  miles  (p.  237).     Since  there  is  no 
reason  to  believe  that  the  conditions  of  sedimentation  in  the  past  were 
markedly  different  from  those  of  the  present,  it  is  generally  held  that 
the  strata  composing  the  great  mountain  ranges  were  laid  down  near 
shore  and,  since  many  of  them  are  of  shallow  water  origin,  that  sink- 
ing accompanied  and  for  the  most  part  kept  pace  with  the  deposi- 
tion, the  land  rising  as  the  geosyncline  sank.     Occasionally  uncon- 
formities occur,  which  indicate,  as  has  been  seen  (p.  270),  that  eleva- 
tion for  a  time  interrupted  the  deposit  of  sediment. 

2.  Lateral   Pressure.  —  When   sediments   have   accumulated   in   a 
geosyncline  to  a  depth  of  several  thousand  feet,  those  near  the  bot- 
tom of  the  deposit  are  somewhat  weakened  by  heat  (p.  347),  so  that 
they  are  compressed  and  thrown  into  folds  when  subjected  to  great 
lateral  pressure.     The  strata  composing  the  Appalachian  Mountains 
of  Pennsylvania,  between  Harrisburg  and  Tyrone  (Fig.  348),  were 
compressed  from  a  width  of  81  miles  to  one  of  66  miles;   i.e.,  the 
earth's  superficial  crust,  upon  being  folded,  was  shortened  15  miles, 
with  a  resulting  mean  elevation  of  three  miles.     It  has  been  estimated 
that,  if  the  folds  of  the  Alps  were  smoothed  out,  the  strata  would 
cover  an  area  74  miles  wider  than  the  mountains  do  now,  or  about 
twice  their  present  width.     The  shortening  of  the  Front  Range  in 


360 


PHYSICAL  GEOLOGY 


Colorado  is  estimated  to  be  about  25  miles,  and  that  of  the  Coast 
Ranges  of  California,  9  to  12  miles  (Fig.  349). 

A  careful  study  of  folded  regions  shows  that  the  strata  are  often 
broken  and  faulted,  the  folds  frequently  giving  place  to  thrust  or 


FIG.  348.  —  Folds  in  the  Appalachian  Mountains  between  Harrisburg  and  Tyrone. 
(As  restored  by  R.  T.  Chamberlin.) 

reverse  faults,  especially  where  the  strong  or  competent  stratum  is 
not  deeply  buried.  As  already  stated  in  the  discussion  of  reverse 
faults  (p.  263),  the  overriding  of  the  strata  is  sometimes  10  or  more 
miles.  In  fact,  in  the  southern  Appalachians  thrust  faults  are  so 
numerous  as  largely  to  determine  the  positions  of  the  mountain 
ridges  (Fig.  347,  p.  358),  and  the  elevation  in  the  Scottish  and 


FIG.  349.  —  Profile  of  the  Santa  Cruz  Mountains  in  the  Coast  Ranges  of  southern 
California.     (Arnold.) 

Scandinavian  Highlands  is  due,  to  some  degree,  to  the  fault  slices  piled 
one  on  top  of  another. 

Igneous  rock  is  often  associated  with  mountains  and  in  some 
ranges  is  an  important  factor  in  the  folding  and  metamorphism  of 
the  strata.  It  often  composes  the  cores  of  mountain  ranges  and 
frequently  forms  their  highest  portions  (p.  355).  (i)  The  igneous 
core  of  mountain  masses  is  often  derived  from  igneous  intrusions ;  (2) 
it  may  be  the  rock  of  the  floor  of  the  geosynclines ;  or  (3)  it  has  even 
been  suggested  that  it  is  sometimes  the  lower  portion  of  the  sediments 
of  the  geosyncline  which  have  been  fused  as  a  result  of  the  rise  of 
temperature  (p.  348)  from  the  interior  of  the  earth.  (Haug.)  In 
each  of  these  cases  the  igneous  core  is  exposed  only  after  erosion  has 
removed  a  great  thickness  of  overlying  sedimentary  rock. 

Experiments  in  Mountain  Building.  —  Experiments  have  been 
performed  to  determine  whether  the  folds  and  reverse  faults  observed 
in  such  mountains  as  the  Appalachians  can  be  reproduced.  In  these 
experiments  a  series  of  layers  composed  of  wax  and  other  substances 
varying  in  rigidity  and  elasticity  were  prepared  to  represent  rock 
strata  of  widely  different  character,  such  as  shale,  sandstone,  and 


MOUNTAINS  AND   PLATEAUS 


361 


limestone.  A  load  of  shot,  representing  the  weight  of  the  overlying 
strata,  was  then  placed  on  top  of  the  layers.  Upon  the  application 
of  lateral  pressure  it  was  found  that,  by  varying  the  rigidity  and 


FIG.  350.  —  Machine  for  experimenting  in  mountains  of  folded  structure. 
(U.  S.  Geol.  Surv.) 

thickness  of  individual  layers  and  of  the  layers  as  a  whole,  the  phe- 
nomena observed  in  folded  regions  were  reproduced.  A  study  of  the 
apparatus  (Fig.  350)  gives  a  better  idea  of  the  conditions  of  the  experi- 
ment than  a  written  description. 


SURFACE  FACTS  AND  UNDERGROUND    INFERENCES 
ZO/V£ 


ELATIONS  OF   LAND  AND  SEA  AND  OF  POSITION  OF  STRATA  PRIORTO  FOLDING 


FIG.  351.  —  Diagrams  showing  the  theoretical  history  of  a  folded  region.  The 
lowest  figure  shows  the  region  when  the  present  site  of  the  mountains  was  a  great 
syncline  in  which  a  load  of  sediment,  25,000  to  40,000  feet  thick,  had  been  laid  down. 
The  middle  figure  shows  the  region  after  it  had  yielded  to  great  lateral  pressure  and 
had  been  folded  and  faulted.  The  upper  figure  shows  the  region  as  it  is  to-day. 
(Redrawn  after  Willis.) 


362  PHYSICAL  GEOLOGY 

A  brief  and  incomplete  history  of  a  folded  region  is  shown  in  Figure 
351.  It  is  incomplete  because  many  of  the  important  chapters  of 
the  history  cannot  be  shown  without  too  great  confusion  of  detail. 

3.  Rate  of  Folding.  — The  rate  of  folding  must  necessarily  differ 
widely  in  different  geosynclines  and  in  the  same  geosyncline  at  vari- 
ous times.  In  certain  cases  it  seems  to  be  proved  that  rivers  have  been 
able  to  deepen  their  valleys  as  rapidly  as  the  land  surface  was  elevated 
(antecedent  rivers,  p.  102).  It  is  possible  that  the  general  denudation 
of  a  region  may  in  some  cases  have  proceeded  at  about  the  same 
rate  as  the  elevation,  so  that  at  no  time  was  the  'surface  far  above 
sea  level.  This  may,  for  example,  have  been  true  of  the  Appalach- 
ians, which  now  consist  of  comparatively  low  mountain  ridges 


FIG.  352.  —  Section  across  central  New  England,  showing  the  uplifted  peneplain 
and  Mt.  Monadnock.     (Hitchcock.) 

although  three  or  more  miles  of  sediment  have  been  removed  by 
erosion ;  and  also  of  the  folded  and  crumpled  rocks  of  New  Eng- 
land (Fig.  352). 

The  elevation  of  regions  of  folding  was  not  always  continuous, 
but,  as  is  shown  by  a  study  of  the  Appalachians,  the  folded  belts 
were  at  times  above  sea  level  and  suffered  erosion ;  upon  being  again 
depressed  they  received  more  sediment,  unconformities  marking 
the  sites  of  the  ancient  erosion  surfaces ;  and  later,  they  were  further 
folded  and  faulted  and  raised  above  the  sea.  The  present  height 
of  the  Appalachians  and  Sierra  Nevadas  was  brought  about  by 
vertical  elevation  and  not  by  lateral  compression. 

4.  To  What  the  Topographic  Features  of  Folded  Mountains  are  Due. 
—  A  comparison  of  the  external  form  of  mountains  and  their  geo- 
logical structure  shows  that  the  two  seldom  agree.  It  is  true  that 
the  mountain  ranges  in  general  are  parallel  to  the  strike  (p.  353)  of 
the  strata,  but  the  valleys  seldom  coincide  with  the  synclines  and 
the  ridges  with  the  anticlines.  This  coincidence  sometimes  occurs, 
but  the  reverse  is  as  frequently  seen.  In  the  Jura  Mountains, 
Switzerland,  excellent  examples  of  synclinal  valleys  may  be  seen 


MOUNTAINS  AND  PLATEAUS 


363 


(Fig.  344,  p.   357),  but  in  the  Appalachians   anticlinal  valleys  are 
perhaps  more  common  than  synclinal  ones.     This  lack  of  coincidence 
is  due  to  the  fact  that  when  strata  are  folded  the  crests  of  the  anti- 
clines are  stretched  and  consequently  weakened,  while  the  synclines 
are  correspondingly  compressed  and  strengthened.     Moreover,  when 
the     land     surface  ....... 

emerges  from  the  sea, 
the  crests  of  the  anti- 
clines are  first  attacked 
by  erosion,  and  their 
strata  may  be  worn 

through      while      the   „  ^'u^-j      r  u 

.  .    .         ric.  353. — The  slopes  of  the  sides  of  the  mountains  are 

synclines  are  receiving      determined  largely  by  the  dip  of  the  rock  forming  them, 
sediment  and  are  thus 

being  protected.  It  is  conceivable  that  a  syncline  may  never  have 
contained  a  stream,  since  before  its  surface  was  elevated  above  the 
sea,  valleys  had  already  been  established  in  the  anticlines. 

If  we  imagine  a  number  of  folds,  the  anticlines  and  synclines  of 
which  are  exposed  to  erosion  at  the  same  time,  it  will  readily  be  seen 
that  erosion  will  develop  valleys  in  the  anticlines,  as  it  is  now  doing 
•  in  the  Jura  Mountains.     In  in- 
tensely folded   mountains  where 
overturned   folds  occur,  as  is  so 
frequently  seen  in  the  Alps,  the 
variable  character  of  the  strata 
determines  the  cliffs  and  escarp- 
ments  of  the   mountains   (Figs. 
353,  354).     The  gentle  slopes  of 
mountains  of  this  structure  are 
most   likely   to   be   found    along 
the  dip  of  the  strata,  the  cliffs 
MILE                             along  the  strike. 
FIG.  354.  —  Section  showing  the  effect        The  effect  of  a  resistant  stratum 
of  the  dip  of  a  resistant  stratum  upon    •      determining   the   topography 

the  topography  of  a  mountain.  .         .  „     •  T 

of  a  region  is  well  shown  in  por- 
tions of  the  Appalachian  Mountains,  where  a  single  quartzite  stratum 
forms  long  mountain  ridges  wherever  it  outcrops  at  the  surface. 
The  canoe  valleys  of  the  Appalachians  and  other  folded  mountains 
are  formed  by  the  erosion  of  the  strata  of  a  plunging  anticline 
(Figs.  244,  245,  p.  255). 


364  PHYSICAL  GEOLOGY 

Cycle  of  Erosion  of  Mountains.  —  In  the  process  of  time  moun- 
tains may  be  wholly  reduced  by  erosion,  and  plains  and  plateaus  be 
formed  in  their  place,  which  will  have  all  the  structural  features  of 
folded  mountains.  Examples  of  such  plateaus  are  to  be  found  in 
the  Piedmont  of  Virginia,  in  New  England,  and  elsewhere. 

In  moist,  tropical  regions  the  luxuriant  vegetation  checks  erosion, 
with  the  result  that  the  forms  are  less  diversified  than  in  other  areas. 


THEORIES  OF  MOUNTAIN  BUILDING 

Mountain  chains  are  more  conspicuous  than  plateaus  because  of 
their  narrow  crests  and  great  length  in  proportion  to  their  width, 
but  when  the  heights  of  mountains  and  plateaus  are  compared,  it 
is  found  that  many  mountain  ranges  are  relatively  low  as  compared 
with  many  plateaus.  Portions  of  the  Appalachian  Mountains,  for 
example,  are  lower  than  portions  of  the  Allegheny  plateau  only  a 
few  miles  away.  The  Tibet  plateau  is  15,000  to  16,000  feet  high, 
being  higher  than  many  of  the  great  mountain  chains  of  the  world. 
The  highest  of  the  Colorado  plateaus  (Aquarius)  is  11,600  feet,  and 
that  at  Grand  Canyon,  Arizona,  is  6000  to  8000  feet  above  the  sea. 
It  is  evident  from  the  above  that  the  cause  of  the  elevation  of  the 
less  conspicuous  but  more  massive  plateaus  is  as  important  as  that 
of  the  more  spectacular  mountains. 

Cause  of  Lateral  Pressure.  —  In  the  discussion  of  the  interior 
of  the  earth  it  was  pointed  out  that  the  earth  is  composed  of  a  hot 
but  solid  core  with  a  cool  crust.  The  answer  to  the  question,  "What 
produces  lateral  pressure  ? "  will  be  found  to  depend,  to  some  degree, 
upon  this  relation. 

The  explanation  often  given  for  the  crumpling  of  the  earth's  crust  is  that,  as  the 
interior  heat  is  lost  very  slowly  by  conduction,  the  crust  wrinkles  to  accommodate  itself 
to  the  smaller  interior.  The  comparison  usually  made  is  that  of  an  apple  which  has 
been  left  in  a  warm,  dry  room.  Under  these  conditions  the  interior  of  the  fruit  loses 
water  by  evaporation,  while  the  dense  skin  shrinks  but  little  and  is  wrinkled  on  the 
contracted  interior.  The  efficacy  of  this  cause  has  been  proved  impossible  on  the 
ground  that  the  shrinkage  of  the  interior  of  the  earth  has  not  been  sufficient  to  produce 
the  lateral  compression  seen  in  the  great  folded  tracts  of  the  earth's  surface,  and  in 
proof  of  this  contention  it  is  pointed  out  that  during  a  single  era  of  the  earth's  history 
(Paleozoic,  p.  477)  the  folding  of  the  earth's  crust  resulted  in  a  shortening  of  between 
100  and  200  miles.  Since  a  lateral  shortening  of  six  miles  of  the  crust  is  produced  by 
one  mile  of  radial  shortening,  it  follows  that  a  minimum  estimate  would  require  a  radial 
shortening  of  16  miles,  and  a  maximum,  one  of  32  miles. 


MOUNTAINS  AND  PLATEAUS  365 

The  cause  of  the  great  deformations,  such  as  those  recorded  in  the 
Alps,  the  Appalachians,  and  other  ranges,  is  believed  to  be  found 
in  the  distribution  of  heat  beneath  the  surface.  It  is  thought  that 
the  heat  of  the  interior  "  would  be  conducted  from  the  deep  interior 
to  the  outer  zone  800  to  1200  miles  thick,  faster  than  from  the  latter 
outward,  with  the  result  of  raising  the  temperature  of  the  outer 
zone  while  that  of  the  deep  interior  falls.  The  result  of  this  should 
be  a  severe  crowding  of  the  outer  zone  upon  itself,  in  shrinking  to 
fit  the  deep  interior  as  it  loses  heat  and  shrinks."  (Chamberlin  and 
Salisbury.)  The  folding  of  great  areas  therefore  results,  according 
to  this  theory,  from  the  crowding  of  the  thick  outer  zone  on  itself. 

The  extrusion  of  lava  from  the  deeper  zones  of  the  earth  cooperates 
with  the  cooling  of  the  heated  interior  in  causing  a  shrinkage.  The 
outpouring  of  the  hundreds  of  thousands  of  square  miles  of  lava  in 
Oregon  and  neighboring  states,  and  in  the  Deccan  peninsula  of  Asia, 
undoubtedly  contributed  to  the  shrinkage  of  the  interior,  although 
the  total  effect  was  slight. 

The  Elevation  of  Plateaus  and  Mountains.  —  The  statement  is 
often  made  that  great  mountain  ranges  are  formed  solely  as  a  result 
of  lateral  pressure  and  also,  when  a  region  only  a  few  hundred  feet 
above  sea  level  is  found  to  be  underlain  by  much  folded  and  meta- 
morphosed rocks,  that  "erosion  has  laid  bare  the  mountains  to  their 
roots  and  that  the  ancient  heights  may  at  one  time  have  rivaled 
the  Alps  in  majesty."  Such  assumptions  must,  however,  be  accepted 
with  caution.  It  seems  safe,  at  least,  to  assume  that,  if  great  areas 
of  the  earth's  surface  can  be  raised  by  vertical  movements  to  form 
plateaus,  the  elevation  of  a  folded  region  may  be  largely  due  to  similar 
vertical  movements.  This  brings  us  to  the  modern  theory  of  isostasy 
(Greek,  isos,  equal,  and  stasis,  standing  still). 

The  Theory  of  Isostasy.1  —  If  oil  and  water  are  balanced  in  a 
U-tube,  it  is  evident  that,  since  water  is  the  heavier,  its  surface  will 
be  lower  than  that  of  the  lighter  oil.  It  is  upon  this  principle  that 
the  theory  of  isostasy  is  based.  The  ocean  basins  are  believed  to 
be  underlain  by  heavier  materials  than  the  continents  and  are  conse- 
quently lower,  since  they  are  drawn  more  strongly  toward  the  center 

1  Hayford,  J.  F.,  —  The  Figure  of  the  Earth  and  Isostasy  from  Measurements  in  the 
United  States:  U.  S.  Coast  and  Geodetic  Surv.,  1909. 

Hayford,  J.  F.,  —  The  Effect  of  Topography  and  Isostatic  Compensation  upon  the 
Intensity  of  Gravity :  Special  Publication  10,  U.  S.  Coast  and  Geodetic  Surv.,  1912. 

Reid,  H.  F.,  —  Isostasy  and  Mountain  Ranges,  Bull.  Am.  Geog.  Soc.,  Vol.  44,  1912, 
p.  354  et  seq. 


3  66 


PHYSICAL  GEOLOGY 


of  the  earth  by  gravity.  The  surface  of  the  earth  may,  therefore, 
be  considered  as  a  mosaic  of  great  polygonal  blocks  (Fig.  355),  which 
from  time  to  time  suffer  readjustment,  the  areas  occupied  by  the 
continents  being  the  continental  segments  and  those  by  the  oceans 
being  the  oceanic  segments.  Not  only  is  the  earth  divided  into  these 


Continent 


FIG.  355.  —  Diagrams  representing  the  conception  that  the  continents  were  lifted 
and  the  ocean  basins  sunk  by  movement  along  definite  sliding  planes  or  fault  planes. 
The  dotted  lines  may  be  taken  to  represent  a  somewhat  uniform  original  surface,  which 
may  be  looked  upon  as  the  surface  before  the  continents  and  ocean  basins  were  de- 
veloped. (After  Salisbury.) 

great  segments,  but  these  in  turn   are  made  up  of  smaller  blocks 
which  by  differential  movements  have  produced  the  high  plateaus 
and  low  plains  of  the  continents,  and  the  "  deeps  "  of  the  oceans. 
The  theory  of  isostasy  holds  that  every  segment  of  the  earth, 


MOUNTAINS  AND  PLATEAUS  367 

having  an  equal  area  of  surface  and  with  its  apex  at  the  center, 
contains  the  same  amount  of  material,  which  it  is  impossible  ma- 
terially to  increase  or  decrease.  When  a  large  quantity  of  material 
is  removed  from  the  land  by  erosion  and  deposited  in  the  ocean  by 
streams,  the  increased  weight  under  the  ocean  and  the  decrease 
under  the  mountains  will  cause  the  rock  at  a  great  depth  to  flow  from 
the  area  which  is  more  heavily  weighted,  to  that  from  which  the 
weight  has  been  removed,  and  the  approximate  equality  of  material 
in  the  segments  will  thus  be  restored. 

As  the  oceanic  and  continental  segments  are  drawn  toward  the 
center  of  the  earth,  the  surface  portions  are  subjected  to  great  lateral 
pressure  produced  by  the  crowding  of  the  segments  against  one 
another,  and  since  the  pressure  cannot  be  relieved  by  the  transfer 
of  material  by  rock  flowage  such  as  is  possible  at  great  depths,  it 
is  relieved  by  folding  and  thrust  faulting.  Since,  as  has  already  been 
shown,  the  materials  of  the  great  mountain  ranges  were  formed  from 
the  thick  sediments  of  geosynclines  whose  basal  portions  were  prob- 
ably weakened  to  some  extent  by  the  invasion  of  heat  from  the 
interior  of  the  earth,  it  is  clear  that,  if  such  thick  but  weak  strata 
are  subjected  to  great  horizontal  compression,  they  will  be  likely 
to  be  folded  and  faulted.  According  to  the  theory  of  isostasy,  how- 
ever, the  folding  of  strata  by  lateral  pressure  could  not  cause  the 
elevation  of  a  mountain  range  without  the  aid  of  the  expansion  of  the 
material  of  which  it  is  composed,  since  otherwise  the  quantity  of 
material  in  the  segment  would  be  increased  by  folding  and  this 
added  weight  would  cause  a  slow  sinking,  and  material  would  flow 
from  below  the  heavier  segment  to  the  lighter  one,  until  the  two  again 
balanced. 

This  theory  does  not  tell  us  definitely  the  cause  of  the  elevation 
of  mountains  and  plateaus,  but  it  positively  states  that  the  eleva- 
tion of  mountains  or  the  depression  of  oceanic  segments  must  be 
due  to  an  increase  or  decrease  of  density.  The  mountains  are  high 
because  their  material  is  light,  and  their  elevation  is  due  to  an  ex- 
pansion of  the  material  in  and  under  them ;  the  ocean  deeps  are 
depressed  because  the  material  under  them  is  dense  and  may  be 
sinking  because  this  material  is  becoming  denser. 

A  number  of  examples  of  mountain  ranges  which  owe  their  height 
to  vertical  elevation  can  be  cited.  The  present  altitude  of  the 
Appalachians,  as  has  been  stated,  is  the  result  of  vertical  movement 
without  the  aid  of  lateral  pressure,  the  folding  of  the  strata  long 

CLELAND   GEOL.  —  24 


368  PHYSICAL  GEOLOGY 

antedating  the  last  elevation.  The  Sierra  Nevadas,  after  folding, 
were  peneplained  and  were  later  elevated  along  a  great  fault  on  the 
east,  and  their  height  is  being  increased  at  the  present  time.  It  is 
thus  seen  that  the  elevation  of  high  mountains  may  be  due  to  verti- 
cal movements,  without  the  aid  of  folding. 

The  Distribution  of  Mountains.  —  Attention  has  long  been  called 
to  the  fact  that  the  mountain  ranges  of  the  Pacific  —  the  Andes, 
western  ranges  of  North  America,  etc.  —  are  situated  near  the  edges 
of  the  continents,  and  the  generalization  has  been  made  that  moun- 
tains are  usually  located  near  the  oceans,  the  higher  mountains 
bordering  the  deepest  basins.  It  is  also  to  be  noted  that  many 
exceptions  exist :  the  Alps,  Caucasus,  Urals,  and  Himalayas  are 
situated  at  considerable  distances  inland.  The  distribution  of  moun- 
tains has  led  to  two  theories  as  to  the  position  of  the  geosynclines 
in  which  the  sediments  forming  them  were  accumulated ;  one  hold- 
ing that  the  geosynclines  existed  at  the  edges  of  the  continents,  the 
other  that  they  were  between  land  masses.  The  apparent  exceptions 
to  the  latter  theory  are  attributed  to  the  subsequent  sinking  of  lands 
which  formerly  existed  near  the  present  shores  of  the  oceans  bordered 
by  mountains.  According  to  this  theory,  for  example,  the  Alpine 
geosyncline  existed  between  the  African  continent  and  the  ancient 
land  masses  on  the  north ;  the  Appalachian  geosyncline,  between 
the  Piedmont  land  on  the  east  and  other  ancient  lands  on  the  north 
and  west  (p.  477) ;  the  Himalayas,  between  the  Indian  peninsula 
and  land  to  the  north. 

Permanence  of  Continents  and  Ocean  Basins.  —  It  is  quite 
generally  agreed  by  geologists  that  the  ocean  basins  and  the  con- 
tinental platforms  have  been  very  much  as  now  for  many  millions 
of  years.  By  this  is  meant  that  the  present  continents  have  not  been 
covered  by  oceans  thousands  of  feet  deep,  nor  have  the  ocean  depths 
been  dry  land  over  wide  areas.  The  proof  of  the  former  lies  in  the 
fact  that  no  deep-sea  sediments  have  ever  been  found  in  the  sedi- 
mentary rocks  of  the  continents,  the  continents  having  been  covered 
repeatedly  by  shallow  seas  (called  epicontinental,  p.  405),  but  never 
by  any  of  great  depth.  Of  the  latter  no  positive  proof  has  been 
advanced,  but  on  the  contrary  the  distribution  of  animals  and  plants 
in  the  past  gives  reason  for  believing  that  land  connections  once 
existed  between  South  America  and  Africa,  North  America  and  Eu- 
rope, and  Australia  and  Africa. 

Age  of  Mountains.  —  This  subject  will  be  more  fully  discussed 


MOUNTAINS  AND  PLATEAUS  369 

later  (p.  519),  but  it  should  be  noted  in  this  connection  that  the  time 
at  which  a  region  was  raised  above  the  sea  was,  at  least,  not  previous 
to  the  youngest  rocks  of  which  the  region  is  composed.  For  example, 
the  Appalachian  Mountains  contain  coal  beds  which  show  that  the 
region  was  folded  after  their  formation,  i.e.,  after  the  Carboniferous 
(P-  477)- 

REFERENCES  FOR  MOUNTAINS 

DALY,  R.  A.,  —  Abyssal  Igneous  Injection  as  a  Causal  Condition  and  as  an  Effect  of 

Mountain-building:  Am.  Jour.  Sci.,  Vol.  22,  1906,  pp.  195-216. 
DALY,  R.  A.,  —  Mechanics  of  Igneous  Intrusion:   Am.  Jour.  Sci.,  Vol.   15,  1903,  pp. 

269-298;  Vol.  16,  1903,  pp.  107-126. 
DALY,  R.  A.,  —  Igneous  Rocks  and  their  Origin. 
GEIKIE,  J.,  —  Mountains;  their  Origin,  Growth,  and  Decay. 
GEIKIE,  A.,  —  Textbook  of  Geology,  4th  ed.,  Vol.  i,  pp.  672-702. 
GILBERT,  G.  K.,  —  Report  on  the  Geology  of  the  Henry  Mountains:   U.  S.  Geol.  and 

Geog.  Surv.  of  the  Rocky  Mountain  Region,  1877. 
READE,  T.  M.,  —  Origin  of  Mountain  Ranges. 
TARR  and  MARTIN, —  College  Physiography,  pp.  525-581. 
WILLIS,  B.,  —  The  Mechanics  of  Appalachian  Structure:  Thirteenth  Ann.  Rept.,  U.  S. 

Geol.  Surv.,  Pt.  2,  pp.  217-281. 

TOPOGRAPHIC  MAP  SHEETS,  U.  S.  GEOLOGICAL  SURVEY,  ILLUSTRATING  MOUNTAINS 

OF  VARIOUS  ORIGINS 

Folded  Mountains 

Delaware  Water  Gap,  Pennsylvania.  Estillville,  Kentucky. 

Harrisburg,  Pennsylvania.  Fort  Payne,  Alabama. 

Hollidaysburg,  Pennsylvania.  Tamalpais,  California. 

Residual  Mountains  Fault  Mountains  Laccolith  Mountains 

Kaaterskill,  New  York.  Alturas,  California.  Henry  Mts.,  Utah. 

Mt.  Mitchell,  North  Carolina.       Granite  Range,  Nevada. 
Monadnock,  New  Hampshire. 
Wausau,  Wisconsin. 


CHAPTER  XII 


ORE   DEPOSITS 

ORES  are  concentrations  in  the  earth's  crust  of  economically  valu- 
able minerals. 

Ores  in  Ready-made  Cavities.  —  A  common  form  of  deposit  is 
the  vein^  or  the  filling  of  a  fissure  in  a  rock.  The  contents  of  a  fissure 
may  consist  partly  or  wholly  of  minerals,  some  of  which  may  or  may 
not  be  of  economic  value.  When  mineral  veins  contain  ores,  they  are 
called  lodes  by  miners.  Fissures  and  other  cavities  are  formed  in 
several  ways,  as  has  been  seen  (p.  262).  (i)  Stretching  movements 
of  the  earth's  crust  fracture  it,  producing  open  cracks ;  (2)  faulting 
(p.  261)  forms  fissures  and  brecciated  zones ;  (3)  fissures  are  developed 
by  shrinkage,  such  as  occurs  when  igneous  rocks  cool  or  when 
limestone  is  changed  to  dolomite ;  (4)  the  joints  of  rocks  are  widened  ; 
(5)  cavities  are  formed  in  limestone  by  solution.  Cavities  formed 
in  any  of  these  ways  may  contain  ores. 

Fissure  Deposits.  —  Metalliferous  veins  are  not  composed  entirely 
of  metalliferous  minerals,  but  on  the  contrary  the  latter  often 

constitute  a  very  small  percent- 

ABCDDCBA  ri  •       rii-  T-I 

age  of  the  vein  filling.  The  use- 
less vein  material  is  called  gangue, 
the  common  gangue  minerals 
being  quartz,  calcite,  and  fluorite. 
In  some  veins  the  contents  are 
arranged  in  bands  parallel  to  the 
walls,  the  minerals  and  ores  of 
one  wall  being  represented  by 
corresponding  bands  on  the  op- 
posite wall  (Fig.  356).  This 

arrangement  is  the  result  of  the  deposition  of  minerals  from  solution 
on  the  two  walls  of  the  fissure  at  the  same  time.  Such  a  symmetrical 
arrangement,  however,  is  not  common,  the  layers  usually  being 
thicker  on  one  wall  than  on  the  other,  while  frequently  a  layer  on 


FIG.  356.  —  Banded  veins  :    A  and  Z), 
quartz;   By  sphalerite;   C,  galena. 


ORE   DEPOSITS  371 

one  side  has  no  corresponding  layer  on  the  other.  A  banded  struc- 
ture may  also  be  brought  about  when,  as  a  result  of  movements 
which  rend  the  vein  from  one  of  its  walls,  the  fissure  is  reopened 
and  minerals  are  subsequently  deposited  in  the  cavity  thus  formed. 
Several  such  movements  may 
take  place  and  two  or  more 
bands  may  be  formed.  In 
some  veins  the  filling  consists 
wholly  or  in  part  of  broken 
rock  (Fig.  357),  the  spaces 
between  which  are  filled  with 
quartz  or  other  minerals. 

Form  and  Extent  of  Veins. 
—  The  form  of  veins  usually         FIG.  357.  —  A  section  showing  a  fault  breccia, 
depends    Upon    the    shape    of     Such  breccias  have  sometimes  been  cemented 
i        r  i«   i       i  rn       together  by  precious  minerals  and  are  valuable 

the    fissures    which    they    fill,     ore  deposits>     (After  Ries  and  Watson.) 

and  their  width,  length,  and 

depth  consequently  vary  greatly.  Some  are  only  a  fraction  of  an 
inch  wide,  while  others  are  200  or  300  feet  in  width.  The  length 
is  even  more  variable,  being  in  some  cases  50  or  more  miles  and  in 
others  only  a  few  feet.  Some  veins  have  been  followed  to  a  depth 
of  more  than  5000  feet,  while  others  have  disappeared  a  few  feet 
beneath  the  surface. 

Source  of  Vein  Material.  —  It  has  been  shown  by  chemical  analysis 
that  nickel,  copper,  tin,  lead,  and  other  metals  occur  in  minute 
quantities  in  both  sedimentary  and  igneous  rocks,  and  it  is  generally 
believed  that  the  ores  which  are  now  concentrated  in  veins  were 
originally  disseminated  through  the  rocks ;  that  they  have  been  dis- 
solved out  by  water,  carried  to  fissures  or  other  cavities  and  there 
deposited.  This  theory  is  borne  out  by  the  fact  that  in  certain 
places  veins  are  actually  being  formed  at  the  present  time  by  deposi- 
tion from  water.  For  example,  near  Boulder,  Montana,  a  hot 
spring  is  depositing  gold-bearing  quartz  identical  with  the  gold  and 
silver-bearing  quartz  veins  of  the  region.  Steamboat  Springs,  in 
Nevada,  are  strongly  alkaline  and  are  depositing  quartz  in  fissures 
and  thus  forming  veins.  Sulphides  of  iron,  lead,  mercury,  and  zinc 
are  found  in  recently  filled  fissures. 

The  water  which  acts  as  a  transporting  agent  is  either  meteoric 
(rain  water)  or  magmatic  (the  waters  issuing  from  cooling  masses 
of  rock).  The  latter  are  believed  by  many  geologists  to  be  the  more 


372  PHYSICAL  GEOLOGY 

effective  carriers  of  metalliferous  minerals  in  the  majority  of  deposits, 
although  meteoric  waters  were  apparently  the  sole  agents  in  some 
cases.  Gases  and  vapors  given  off  by  molten  magmas  have  also 
formed  some  deposits. 

The  importance  of  igneous  intrusions  in  the  production  of  ore 
deposits  is  readily  understood  when  the  history  of  such  an  intrusion 
is  considered.  When  a  sedimentary  rock,  for  example,  is  penetrated 
by  a  molten  mass,  it  is  more  or  less  fractured.  In  these  fractures  the 
waters  heated  by  the  igneous  mass  can  circulate,  and  if  they  contain 
minerals  in  solution  the  latter  may  be  precipitated.  Moreover, 
igneous  rocks  are  often  rich  in  metallic  minerals,  and  the  water  derived 
from  them,  the  magmatic  waters,  may  be  the  chief  source  of  the  metal- 
lic minerals  of  the  ore  deposit. 

Cause  of  Precipitation.  —  Veins  exist  only  in  the  zone  of  fracture, 
that  is,  at  a  depth  seldom  as  great  as  10  or  n  miles,  and  in  most 
cases  within  a  mile  or  two  of  the  surface.  The  precipitation  of 
minerals  in  veins  may  be  brought  about  in  one  of  a  number  of  ways. 
(i)  It  may  be  caused  by  the  mingling  of  waters.  This  is  due  to  the 
fact  that  having  pursued  different  courses  the  waters  may  carry 
different  salts  which  may  react  to  cause  the  precipitation  of  metallic 
and  other  minerals.  (2)  Precipitation  may  also  be  brought  about 
by  the  contact  of  solutions  with  rocks  which  contain  carbon  or  other 
minerals  which  cause  precipitation  ;  (3)  by  a  decrease  in  temperature ; 
(4)  by  a  change  in  pressure;  and  (5)  by  oxidation  (if  the  solutions 
are  brought  near  the  surface).  (6)  If  two  rocks  differing  in  chemical 
composition  are  in  contact,  as,  for  example,  a  limestone  and  an  igneous 
rock,  precipitation  is  favored  at  or  near  the  plane  of  contact,  since 
the  waters  from  the  two  are  differently  mineralized. 

When  a  mineral  has  once  formed  on  a  fissure  wall,  it  may  act  as  a 
center  of  attraction  and  cause  a  further  accretion  of  the  same  mineral. 
This  process  is  called  mass  action. 

Replacement  Deposits.  —  Waters  carrying  minerals  in  solution 
sometimes  attack  the  rocks  which  they  penetrate,  dissolving  them 
and  at  the  same  time  depositing  some  of  their  load.  This  is  accom- 
plished molecule  by  molecule,  a  particle  of  vein  material  being  de- 
posited as  a  particle  of  the  rock  is  dissolved  out.  Many  of  the  rich 
ore  deposits  are  of  this  origin.  Replacement  deposits  often  occur 
along  faults  and  near  the  boundary  or  contact  of  igneous  with  sedi- 
mentary rocks. 

The  boundaries  of  veins  are  often  indefinite,  since  the  width  may 


ORE   DEPOSITS 


373 


depend  either  upon  the  width  of  the  original  fissure  or  upon  the 
amount  of  the  replacement  of  the  walls,  or  upon  both.  If  replace- 
ment has  not  occurred,  the  boundary  of  the  vein  may  be  distinct. 

Weathering  and  Concentration  of  Ores.  —  As  a  metalliferous 
vein  is  eroded,  it  is  attacked  by  the  agents  of  the  weather  and  under- 
ground water.  The  result  of  such  action  is  the  removal  of  the  more 
soluble  minerals  in  the  surface  zone,  (i)  If  the  minerals  removed  are 
worthless,  the  portion  remaining  will  be  richer.  For  example,  in 
gold-bearing  quartz  veins  in  which  the  gold  is  contained  in  pyrite, 
the  solution  of  the  pyrite  leaves  the  pure  gold  in  a  honeycombed, 
rusty  quartz.  It  was 
such  quartz  veins 
which  delighted  the 
old-time  prospector. 

(2)  If  the  minerals 
removed  are  valu- 
able and  are  depos- 
ited lower  in  the  vein 
by  the  percolating 
water,  a  rich  deposit 
may  result.  In  a 
vein  containing  chal- 
copyrite  and  pyrite, 
for  example,  the  iron 
may  be  left  in  the 
upper  part  of  the 
vein  in  the  form  of 
limonite.  This  is 


FIG.  358. — Vein  showing  three  zones:  A,  surface  or 
weathered  zone;  By  oxidized  or  middle  zone;  C,  unaltered 
or  sulphide  zone.  The  weathered  zone,  A,  is  often  largely 
composed  of  iron  hydroxide  and  is  called  gossan. 


called  the  gossan  (chapeau  de  fer  and  eisen  hut)  and  may  be  in  suffi- 
cient quantity  to  be  mined  as  iron  ore  (Fig.  358). 

Lower  in  the  vein,  in  the  oxidized  or  middle  zone,  the  ores  are  in 
the  form  of  oxides,  carbonates,  etc.,  and  may  be  enriched  by  the 
addition  of  metallic  minerals  brought  down  from  the  weathered 
zone.  In  some  deposits  the  oxidized  zone  is  the  only  portion  of  the 
vein  in  which  the  mineral  occurs  in  sufficient  quantities  to  be  ex- 
tracted with  profit. 

Beneath  the  oxidized  zone,  which  extends  to  or  below  the  level 
of  ground  water,  lies  the  unaltered  vein  material  of  the  unoxidized 
zone.  Here  the  ores  occur  as  they  were  originally  deposited.  These 
three  zones  are  not  usually  separated  by  well-defined  boundaries, 


374 


PHYSICAL  GEOLOGY 


the  change  from  one  to  the  other  being  sometimes  so  gradual  that 
it  is  difficult  to  say  where  one  begins  and  the  other  ends.  Veins 
are  known  in  which  oxidized  ores  occur  several  hundred  feet  below 
the  water  table. 

Magmatic  Segregation.  —  Certain  iron  and  nickel  deposits  which 
occur  in  igneous  rocks  were  probably  brought  together  while  the 
rocks  were  in  a  molten  condition  as  the  result  of  segregation.  Few 
workable  deposits,  however,  have  been  formed  in  this  way. 

Placer  Gold  Deposits.  —  In  the  early  days  of  gold  mining  in  many 
countries  the  first  gold  was  found  in  the  gravels  of  stream  beds.  The 
gold  of  the  Klondike  in  northwestern  Canada  and  the  majority  of 
the  early  finds  of  Alaska  were  located  in  stream  gravels,  while  that 


OUTCROP 


STREAM  GRAVELS 


FIG.  359  A.  —  Diagram  showing  the  development  of  eluvial  or  residual  placers, 
which  may  be  worked  like  ordinary  stream  placers,  and  stream  gravels.  In  this  case 
the  source  of  the  gold  is  the  quartz  vein.  (After  Lindgren.) 


of  Nome  was  found  in  the 
sands  of  the  seashore.  The 
gold  rush  to  California  in 
1849  was  due  to  the  find- 


FIG.  359  B.  —  Diagram  showing  ancient  aurif- 


erous  gravels  (dotted)  covered  by  a  lava  flow  ing  of  Stream  or  placer  gold, 
(vertical  lines).  In  mining  the  gravels  a  tunnel  The  source  of  the  nuggets  or 
is  driven  as  indicated.  just  m  strearn  gravels  is  evi- 

dently to  be  found  in  the  rocks  over  which  the  streams  or  their 
tributaries  now  flow  or  formerly  flowed. 

The  way  in  which  placer  gold  was  transported  and  deposited 
is  simple.  The  gold  occurred  either  in  veins  or  scattered  through 
the  country  rock  in  small  quantities.  When  these  rocks  were 
disintegrated  by  weathering  and  the  fragments  carried  away  by 


ORE   DEPOSITS 


375 


the  streams,  the  heavy  gold  particles  quickly  sank  to  the  bed  of  the 
streams,  while  the  lighter  minerals  were  borne  on  by  the  current. 
In  this  way  much  of  the  gold  contained  in  a  large  quantity  of  rock 
has  sometimes  been  concentrated  in  a  small  area.  It  consequently 
happens  occasionally  that  rich  placer  deposits  are  found  in  regions 
in  which  none  of  the  rock  contains  gold  in  sufficient  quantity  to  pay 
for  its  extraction. 

When  conditions  are  favorable,  gold-bearing  (auriferous)  gravels 
are  worked  by  dredging,  even  when  the  gravel  yields  only  twenty- 
five  or  thirty  cents  to  the  cubic  yard.  Ancient  gravels  which  have 
been  buried  beneath  sheets  of  lava  are  sometimes  mined  for  their 
gold  (Fig.  3  59  B). 

Sedimentary  Iron  Deposits.  —  Extending  in  a  broken  belt  from 
Nova  Scotia  and  New  York  to  Alabama,  beds  of  iron  ore  (Clinton 


FIG.  360.  —  Iron  deposits  in  the  Lake  Superior  region,  Mesabi  Range,  Minnesota. 

(U.  S.  Geol.  Surv.) 

iron  ore)  occur  which  have  the  same  position  and  much  the  same 
character  as  other  sedimentary  beds,  and  in  some  cases  contain  ma- 
rine fossils.  These  beds  of  iron  ore  may  have  been  precipitated 
from  salt  or  from  fresh  water,  just  as  iron  is  being  deposited  to-day 
in  fresh-water  ponds  and  lakes.  The  iron  contained  in  small  quan- 
tities in  the  rocks  (usually  igneous)  of  the  land  is  leached  out  by  per- 
colating waters  in  the  form  of  ferrous  compounds.  These  compounds 
upon  exposure  to  the  air  are  oxidized  and  ferric  oxide  (Fe2O3)  is 
precipitated,  usually  in  the  form  of  limonite  (2  Fe2O3  •  3  H2O).  In  this 
way  iron  accumulates  in  bogs  and  is  called  bog  ore,  and  similar  deposits 
are  laid  down  in  lakes.  Another  suggestion  which,  however,  is  not 
widely  accepted,  is  that  the  Clinton  iron  ore  has  been  derived  from 
lavas  rich  in  iron  minerals  which  were  extruded  beneath  the  sea. 

The  great  iron  deposits  of  the  Lake  Superior  region  (Fig.  360) 
are  believed  to  have  been  accumulated  in  beds  as  impure  iron  car- 
bonates and  silicates,  too  low  in  iron  to  pay  for  their  extraction. 
When  the  deposits  were  uplifted  to  form  land,  they  were  exposed 


376  PHYSICAL  GEOLOGY 

to  weathering  and  were  enriched  (i)  by  the  removal  of  the  impurities 
by  solution ;  (2)  by  the  replacement  of  the  impurities  by  iron  oxides 
as  the  former  were  dissolved  out;  or  (3)  by  concentration,  as  a 
result  partly  of  the  removal  of  impurities  and  partly  of  replacement. 
So  wide  and  deep  are  some  of  these  Lake  Superior  iron  deposits 
that  they  are  excavated  by  steam  shovels.  The  excavation  in  the 
Mesabi  region,  taking  into  account  both  the  removal  of  the  ore  and 
of  the  glacial  drift  which  overlies  it,  is  far  more  extensive  than  the 
work  conducted  at  the  Panama  Canal.  In  most  of  the  deposits 
underground  mining  methods  are  employed. 

REFERENCES  FOR  ORE  DEPOSITS 

LINDGREN,  W.,  —  Mineral  Deposits. 

RIES,  H.,  —  Economic  Geology. 

U.  S.  Geol.  Survey  Bulletins,  Professional  Papers,  and  Monographs. 


PART    II.     HISTORICAL   GEOLOGY 
CHAPTER  XIII 

HISTORICAL   GEOLOGY 

HISTORICAL  geology  deals  with  the  evolution  of  the  life  of  the  past, 
and  with  the  development  of  the  continents  and  oceans.  It  traces 
out,  as  accurately  as  our  present  knowledge  will  permit,  the  changes 
through  which  the  earth  has  passed;  it  endeavors  to  gather  from 
the  available  record  the  history  of  the  life  of  geological  times  and 
the  evolutional  changes  which  the  many  classes  of  animals  and  plants 
have  undergone  and,  as  far  as  possible,  to  determine  the  cause  or 
causes  of  these  changes.  This  section  of  geology  is  concerned  not 
only  with  the  recording  of  facts,  but  is  also,  to  an  important  degree, 
philosophical. 

Human  history  is  but  a  short  chapter  of  geological  history,  the 
former  being  measured  in  thousands  of  years  while  the  latter  extends 
over  millions  of  years.  The  immensity  of  geological  time  is  beyond 
our  comprehension,  but  some  conception  of  it  can  be  gained  when  it 
is  remembered  that  the  time  necessary  to  excavate  the  Grand  Canyon 
of  the  Colorado  was,  geologically,  comparatively  short;  that  a 
maximum  thickness  of  sediments  of  not  less  than  40  miles  has  been 
laid  down  in  the  seas;  that  great  mountain  ranges  have  not  only 
been  raised  but  have  been  worn  down  to  sea  level  during  portions 
of  the  smaller  divisions  of  geological  history.  Perhaps  the  most 
striking  evidence  of  the  length  of  geological  time  is  to  be  seen  in  the 
evolution  of  life. 

FOSSILS 

A  fossil  is  any  remains  or  trace  of  an  animal  or  plant  preserved  jp 
the  rocks  of  the  earth.  _  It  mav  consist  of  the  original  substance  of 
the  animal,  or  it  may  be  merely  an  impression,  such  as  a  footprint 
or  a  worm  trail.  Even  the  flint  implements  made  by  primitive  man 
may  be  considered  as  fossils. 

377 


378  HISTORICAL  GEOLOGY 

When  a  shell  or  other  organic  remain  is  buried  in  the  mud  or  sand 
of  an  ocean  or  lake  bottom,  in  the  dune  sand  of  a  desert,  in  volcanic 
dust,  in  a  peat  bog,  or  in  the  flood  plain  of  a  river,  the  record  of  its 
existence  may  be  preserved  in  a  number  of  ways. 

(1)  The  Original  Substance  may  be  Preserved.  —  In  recent  sediments 
the  shells  are  often  unchanged,  even  the  nacreous  luster  being  re- 
tained.    In  the  ice  of  Siberia  mammoths  have  been   found  whose 
flesh  had  been  so  perfectly  preserved  that  it  was  eaten  by  dogs  and 
wolves  and  possibly  by  the  natives  themselves.     Insects  are  found 
in  amber  —  the  fossil  gum  of  cone-bearing  trees  —  in  which  they  were 
entrapped  and  covered. 

(2)  Replacement.  —  The  original  substance  may  have  been  entirely 
replaced  by  some  other  mineral,  and  shells,  corals,  and  bones  are  often 
found  which,  although  bearing  little  external  evidence  of  alteration, 
are  composed  entirely  of  silica  or  some  other  mineral. 

As  alkaline  water  is  a  salient  of  silica  the  petrifaction  of  wood  (Fig  361)  is 
brought  about  when  such  water  containing  silica  in  solution  is  neutralized,  since  the 
silica  is  then  precipitated.  If  then  a  log  buried  in  a  bed  of  sand  or  volcanic  ash 


FIG.  361.  —  Petrified  log,  Adamana,  Arizona. 

is  saturated  with  underground  water  that  is  slightly  alkaline,  the  replacement  of 
the  wood  by  the  silica  will  be  slowly  brought  about  as  the  wood  decays.  As  each 
particle  of  wood  is  oxidized  carbon  dioxide  will  be  formed,  this  acid  (H2CO3)  will  neu- 
tralize the  alkali  of  the  water  and  will  cause  the  precipitation  of  the  silica  at  the 
point  where  the  wood  decayed.  By  some  such  slow  process  the  wood  may  be  replaced 
particle  by  particle  until  the  entire  tree  is  converted  into  a  solid  cylinder  of  silica. 

Silica  is  not  the  only  mineral  which  replaces  the  substance  of  shells, 
bones,  and  other  hard  parts.  Pyrite,  iron  oxide,  lime  carbonate,  and 
other  minerals  sometimes  occur. 


HISTORICAL  GEOLOGY 


379 


(3)  Casts  and  Molds.  —  The  original  substance  may  be  carried  away 
in  solution  by  underground  water,  leaving  a  cavity  in  which  only 
the  external  form  is  preserved ;  in  other  words,  a  mold  of  the  shell  or 
bone  is  left.  Often  natural  casts  of  these  molds  are  formed  by  mineral 
matter  carried  into  the  mold  or  by  the  infiltration  of  mud.  Molds 
of  the  interior  (Fig.  362)  and  exterior  (Fig.  363)  are  frequently  en- 
countered in  porous  rocks.  Some  fine  opals  in  Nevada  have  the  form 


FIG.  362.  —  Specimens  showing  the 
original  shell  (B)  and  a  natural  mold 
(^)  of  the  interior  of  a  similar  speci- 
men from  which  the  shell  has  disap- 
peared. (Turritella  mortoni.) 


FIG.  363.  —  One  half  of  a  con- 
cretion showing  the  leaf  which 
formed  the  nucleus. 


of  branches,  but  are  in  reality  casts  of  the  branches  of  trees,  the 
cavities  formed  by  the  decay  of  the  wood  having  been  filled  with 
silica. 

(4)  Footprints,  Trails,  etc.  —  Many  animals  are  known  from  their 
footprints,  trails,  burrows,  or  the  impressions  (Fig.  380,  p.  411)  made 
by  their  bodies  in  the  soft  mud. 

Entombment  of  Plants  and  Animals.  —  The  most  favorable  con- 
ditions for  the  preservation  of  animal  life  are  to  be  found  on  those 
portions  of  the  ocean  bottom  which  are  not  uncovered  by  tides  and 
where  sediments  are  accumulating.  When  under  such  conditions 
shellfish  or  other  animals  die,  their  bodies  may  be  buried  in  the  mud 
or  sand  and  preserved.  It  is  not  unusual  to  find  layers  of  rock  made 
up  largely  of  the  remains  of  shells  which  were  buried  in  this  way.  On 


380  HISTORICAL  GEOLOGY 

the  surface  of  some  slabs  of  rock  250  or  300  fossils  may  sometimes  be 
counted. 

Animal  and  plant  remains  are  often  well  preserved  in  lake  de- 
posits. In  deposits  of  this  class  are  found  leaves,  branches,  and 
flowers  which  were  carried  from  the  surrounding  land  by  the  streams, 
insects  which  were  beaten  down  by  the  wind  to  the  surface  of  the 
lake,  and  vertebrates  which  were  floated  down  the  streams  and 
found  a  burial  on  the  lake  bottom.  Some  of  the  most  beautiful 
fossils  were  made  in  this  way,  but  deposits  of  this  class  are  much  less 
important  than  those  of  marine  origin,  both  because  of  their  smaller 
extent  and  because  the  contained  fossils  seldom  afford  a  means  of 
exact  correlation  with  those  of  other  countries. 

The  fossils  preserved  in  delta  swamps  and  flood  plains  are  often 
numerous,  and  during  certain  periods  of  the  earth's  history  have 
afforded  the  chief  record  of  the  vertebrates  of  these  periods. 

Fossils  are  also  preserved  in  wind-blown  sand,  in  peat  bogs,  in  cav- 
erns, and  in  travertine. 

Imperfection  of  the  Record.  —  The  record  of  ancient  life  must 
necessarily  be  imperfect  for  two  reasons,  (i)  Only  a  small  per- 
centage of  the  life  of  any  one  period  is  preserved.  This  can  be  seen 
best  by  observing  the  proportion  of  the  plant  and  animal  life  of  to-day 
that  will  remain  as  a  record  of  the  life  of  the  twentieth  century.  Of 
the  life  of  the  sea  only  the  animals  with  shells  or  skeletons  will  be 
preserved  in  large  numbers ;  the  myriads  of  soft-bodied  animals  such 
as  jellyfish  and  protozoans  will  not  form  recognizable  fossils  except 
under  very  exceptional  conditions.  The  trees  of  the  forest  decay 
where  they  fall,  and  it  is  seldom  that  any  are  buried  and  leave  a  per- 
manent record.  The  same  fate  awaits  land  animals,  since  upon  their 
death  their  bones  are  soon  disintegrated  by  the  agents  of  the  atmos- 
phere and  they  crumble  to  dust.  It  is  only  the  bones  of  the  occasional 
carcass  which  floats  downstream  and  is  buried  under  favorable  con- 
ditions that  will  form  fossils. 

(2)  Even  after  being  buried,  the  record  is  not  always  preserved. 
Thousands  of  square  miles  of  sediments  have  been  metamorphosed 
and  the  contained  fossils  destroyed.  When  marine  sediments  have 
been  raised  to  form  land,  they  are  immediately  attacked  by  the 
weather  and  erosion  and  are  soon  carried  away.  We  consequently 
find  that  thousands  of  feet  of  rock  have  been  removed  and  the 
record  has  been  completely  lost.  Much  of  the  fossiliferous  strata  is 
also  either  buried  so  far  beneath  younger  rocks  as  to  be  inaccessible 


HISTORICAL  GEOLOGY 


or  is  under  the  waters  of  the  seas  and  so  beyond 
the  reach  of  the  geologist. 

GEOLOGICAL  CHRONOLOGY 

Relative  ages  of  strata  are  determined  in  two  ways. 

(1)  Order  of  Superposition.  —  If  a  series  of  strata 
or  beds  is  in  the  order  in  which  they  were  laid  down 
(Fig.  364),  it  is  evident  that  the  oldest  will  be  at 
the  bottom  and  the  youngest  at  the  top.     It  is  for 
this  reason  that  the  strata  of  a  geological  section  are 
always  placed  with  the  oldest  at  the  bottom  of  the 
column.     This  order  is  conclusive  proof  of  the  rela- 
tive age  of  rocks  unless  they  have  lost  their  original 
position  by  faulting  or  folding. 

(2)  Chronology  Determined  by  Fossils.  —  After  the 
true  order  of  a  series  of  beds  has  been  determined 
by  their  superposition,  their  contained  fossils  will 
usually    make   it    possible   to    correlate   them   with 
strata  which  may  be  hundreds  or  even  thousands 
of  miles  distant.     This  is  rendered  possible  by  the 
fact_that  the  inhabitants  of  the  earth  have  under- 
gone a  progressive  change  which  has,  as  a  whole, 
been  gradual,  but  which  has  taken  place  more  rapidly 
at   certain   times   than    at   others.     Certain    classes 
became  dominant  for  a  time,  and  then  declined  but 
seldom   entirely   disappeared.     As   a   result   of  this 
change   the   assemblage   of  animals   and   plants   of 
each  division  of  geological  history  differs  from  that 
of  every  other.     The  fact  that  life  has  suffered  such 
a   progressive   modification   is   of  the   greatest   im- 
portance, since,  as  already  indicated,  it  furnishes  a 
means   by  which  the  relative   age  of  the  rocks  in 
different    parts    of  the   world    can    be   determined. 
Since   certain   species   have    a   short   geological   life 
(their  vertical  range  is  short),  when  ''such  are  present 
the  relative  age  of  the  rock  is  readily  fixed. 

Although  fossils  are  the  surest  test  of  the  relative 
age  of  widely  separated  strata  it  should  not  be  con- 
cluded that  they  prove  exact  contemporaneity,  since 
in  favored  regions  an  old  fauna  may  live  thousands 


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382  HISTORICAL  GEOLOGY 

of  years  after  it  has  become  extinct  in  others.  An  example  is  found 
in  Australia  to-day,  where  the  indigenous  fauna  belongs  to  the  early 
Tertiary. 

Use  of  Fossils  in  Determining  Physical  Conditions.  —  A  study  of 
the  inclosed  fossils  usually  tells  definitely  whether  the  rocks  were 
laid  down  in  the  sea,  in  a  lake,  or  on  land.  Fossils  also  give  a  clue 
to  the  depth  of  the  water  and  the  proximity  of  the  shore.  Corals 
show  that  the  deposits  containing  them  were  laid  down  in  warm  seas 
some  distance  from  land,  or  that  the  land  was  so  low  that  little  sedi- 
ment was  carried  to  the  sea.  Leaves  and  stems  of  plants  as  well  as 
the  fossils  of  land  animals  indicate  nearness  to  shore. 

The  climate  of  the  past  is  also  told  with  considerable  certainty 
by  fossils.  For  example,  relatively  recent  travertine  deposits  of 
northern  France  contain  the  canary  laurel,  a  plant  which  blooms  in 
winter  and  which  now  grows  in  the  moist  climate  of  the  Canary  Is- 
lands, where  the  temperature  seldom  falls  below  59°  F.  It  is  evi- 
dent, therefore,  that  when  the  canary  laurel  grew  in  northern  France 
the  climate  of  that  region  was  probably  warm  and  moist.  The  occur- 
rence in  the  Pleistocene  deposits  of  Denmark  and  England  of  Arctic 
willows  which  now  grow  only  within  the  Arctic  Circle  is  evidence  of 
a  cool  climate  in  the  past  in  those  countries. 

A  typical  example  of  the  knowledge  to  be  gained  of  the  physical 
geography  and  climate  of  a  region  by  a  study  of  the  fossils  is  illus- 
trated by  the  limestones  of  Wisconsin.  These  strata  are  composed 
of  practically  pure  limestone,  being  free  from  land  sediments,  and 
contain  fossil  corals,  crinoids,  brachiopods,  and  the  remains  of  other 
marine  animals.  It  is  evident,  therefore,  that  when  the  limestone 
was  accumulating,  a  sea  spread  over  a  portion  at  least  of  Wisconsin, 
that  the  region  in  which  the  lime  ooze  was  deposited  was  probably 
far  from  land,  and  that  the  climate,  as  shown  by  the  corals,  was  prob- 
ably warm. 

Difficulties  in  Correlating  Strata.  —  (i)  When  rocks  have  been  over- 
turned or  faulted,  older  beds  are  sometimes  found  to  rest  on  younger 
ones.  (2)  In  some  regions  a  once  widespread  stratum  may  be  repre- 
sented now  only  by  isolated  patches  which  may  be  separated  by  dis- 
tances of  several  miles.  (3)  Strata  are  sometimes  separated  by  an 
unconformity  (p.  270)  which  may  represent  a  lost  interval  of  many 
years.  (4)  The  lithological  character  of  a  stratum  may  vary  greatly 
even  over  short  distances.  In  every  case,  however,  fossils  if  present 
will  usually  give  definite  knowledge  of  the  relative  age  of  the  rocks. 


HISTORICAL  GEOLOGY  383 

Since  in  no  one  region  are  the  strata  of  even  a  majority  of  the 
systems  of  the  earth  represented,  it  is  evident  that  one  of  the  diffi- 
culties of  geology  is  to  bring  together  the  data  and  place  them  in 
their  true  order  so  as  to  make  a  complete  and  accurate  record.  For 
example,  unconformities  representing  a  loss  of  two  or  three  systems 
may  occur  in  two  sections,  but  when  the  two  sections  are  compared 
it  may  be  found  that  they  complement  each  other,  that  which  is 
lacking  in  the  one  being  present  in  the  other  and  vice  versa.  It  is 
evident  that  when  such  sections  exist,  a  complete  record  of  a  portion 
of  geological  time  is  available. 

The  difficulties  may  be  seen  by  a  study  of  the  rocks  upon  which 
the  city  of  Paris  is  situated.  An  examination  of  these  strata  shows 
that,  at  least  ten  times  in  the  past,  this  region  was  covered  by  the 
sea  and  sediments  accumulated  on  the  sea  floor,  and  as  many  times 
the  sea  bottom  was  raised  above  the  water  and  was  subjected  to 
erosion.  When  the  latter  occurred,  no  sediments  preserve  the  fossils 
of  the  periods  during  which  land  existed,  and  it  is  only  by  studying 
the  fossils  in  strata  of  other  regions  that  the  whole  history  can  be 
read  and  the  age  of  the  strata  which  are  present  be  determined. 

DIVISIONS  OF  GEOLOGICAL  TIME 

The  broad  outlines  of  the  earth's  history  have  been  learned  as  a 
result  of  such  studies  as  those  indicated  above,  and  have  been  ar- 
ranged in  chronological  order  and  separated  into  more  or  less  clearly 
marked  divisions  which  correspond  to  the  cjiapters  of  human  history. 
The  divisions  of  time  and  corresponding  divisions  of  the  rocks  have 
been  given  the  following  terms : 

TIME  SCALE  ROCK  SCALE 

Era Group 

Period System 

Epoch* Series 

Age       ' Stage 

• 

An  era  consists  of  several  periods  during  which  a  group  composed  of 
several  systems  of  strata  were  accumulated.  During  an  epoch  a  series 
composed  of  one  or  more  stages  was  laid  down.  When  one  speaks  of 
the  Cambrian  System,  he  means  the  succession  of  strata  which  were 
laid  down  in  the  Cambrian  Period;  when  he  speaks  of  the  Miocene 
Series,  he  refers  to  strata  deposited  during  the  Miocene  Epoch,  i.e., 
during  a  definite  portion  of  the  Tertiary  Period. 

CLELAND    GEOL.  —  25 


384 


HISTORICAL  GEOLOGY 


The  following  table  includes  the  more  important  divisions  of  the 
geological  record : 


f  Era 

Cenozoic  I     and 
[  Group 


TEra 

Mesozoic  <      and 
[  Group 


f  Period    ( 

Quaternary  {      and     { 
{  System  { 


Recent 
Pleistocene 


Epochs 

and 
Series 


I_.    .    .  f  Pliocene  f  „       . 

Period  Ayr.  Epochs 

,  Miocene  . 

and  <  ~..  <      and 

0  Ohgocene  _    . 

System     ^  Series 

J  ( Eocene  [ 

I  Cretaceous  Period  and  System 

{  Jurassic  Period  and  System 

[  Triassic  Period  and  System 


Carboniferous  Period  and  System 

Permian 

{Era  Pennsylvanian 

and      •  Mississippian 

Group       Devonian  Period  and  System 

Silurian  Period  and  System 

Ordovician  Period  and  System 

Cambrian  Period  and  System 

_,     _      .    .      ,,       f  Proterozoic  Era  and  Group 
rre-Cambnan  Eras  {   .     ,          .    ,-  ,  ^ 

{  Archaeozoic  Kra  and  Group 


REFERENCES 

GRABAU,  A.  W.,  —  Principles  of  Stratigraphy,  pp.  1073-1095. 
LECONTE,  JOSEPH,  —  Elements  of  Geology,  5th  ed.,  pp.  197-209. 
SCOTT,  W.  B.,  —  An  Introduction  to  Geology,  pp.  516-531. 
SHIMER,  H.  W.,  —  An  Introduction  to  the  Study  of  Fossils,  pp.  8-28. 


CHAPTER  XIV 
THE  EARTH  BEFORE  THE   CAMBRIAN 

SPECULATIONS  and  theories  as  to  the  origin  of  the  earth  are  as  old 
as  the  human  race,  but  of  the  many  that  have  been  offered  only  two 
are  based  upon  physical  laws,  and  these  only  demand  our  attention. 

THEORIES  OF  THE  EARTH'S  ORIGIN 

Nebular  Hypothesis.  —  The  theory  of  a  molten  earth  is  generally 
associated  with  the  Laplacian  or  nebular  hypothesis  of  the  earth's 
origin,  although  in  a  modified  form  the  hypothesis  holds  that  the 
earth  has  never  been  in  a  molten  condition.  This  hypothesis,  in 
brief,  holds  that  the  material  of  which  the  sun,  earth,  and  other  plan- 
ets are  composed  was  once  in  the  form  of  a  vapor  (nebula)  diffused 
throughout  the  space  now  occupied  by  the  solar  system  and  extending 
some  distance  beyond  the  orbit  of  the  outermost  planet  (Neptune). 
This  great  mass  of  vapor  began  to  rotate  because  of  contraction  and 
formed  a  much-flattened  spheroid.  As  contraction  continued,  a  time 
came  when  the  centrifugal  force  of  the  particles  near  the  equator  of 
the  mass  was  equal  to  the  pulling  force  of  gravity,  and  they  were  left 
as  a  ring,  while  the  remainder  of  the  mass  continued  to  contract. 
This  process  being  repeated  successive  rings  were  abandoned.  Each 
of  the  rings  thus  formed  probably  revolved  for  a  time  as  a  whole, 
but  finally  broke  up,  the  material  of  each  ring  being  concentrated 
into  a  single  planet  with  its  satellites,  the  satellites  being  formed  from 
rings  abandoned  by  the  planets  as  they  contracted,  just  as  the  planets 
were  formed  from  the  parent  nebula.  In  this  way,  according  to  the 
nebular  hypothesis,  the  planets  were  originated,  the  sun  remaining 
as  the  central  and  uncooled  portion  of  the  nebula. 

According  to  this  hypothesis,  therefore,  the  earth  was  first  a  globe 
of  highly  heated  vapor  which  was  later  condensed  to  a  liquid  and  was 
finally  cooled  sufficiently  to  permit  of  the  formation  of  a  crust  over 
the  surface,  while  the  interior  was  still  a  liquid.  In  this  early  stage 
the  atmosphere  was  very  heavy  and  hot,  and  contained  not  only  all 
the  water  now  on  the  earth,  but  many  of  the  gases  that  are  now  united 

385 


3  86  HISTORICAL  GEOLOGY 

with  other  elements  to  form  the  rocks.  When  the  crust  finally  cooled 
to  such  an  extent  that  water  could  remain  on  its  surface,  the  oceans 
were  formed  and  the  atmosphere  gradually  lost  its  gases  until  its  pres- 
ent composition  and  character  were  attained. 

Planetesimal  Hypothesis.  —  The  planetesimal  hypothesis  has 
been  offered  as  a  substitute  for  the  nebular  hypothesis.  Omitting 
the  astronomical  considerations,  this  theory  assumes  that  the  earth 
was  never  in  a  molten  condition,  but  grew  gradually  by  the  ingather- 
ing of  small  particles  (Fig.  365)  called  planetesimals  (little  planets). 


FIG.  365. — Spiral  nebula.     (Yerkes  Observatory.) 

In  its  early  stages,  if  this  hypothesis  is  true,  the  earth  had  no  atmos- 
phere, since,  on  account  of  its  small  mass,  the  attraction  of  gravity 
was  insufficient  to  hold  the  gases  which  were  lost  in  space  because  of 
their  activity.  With  its  present  mass  the  earth's  attraction  is  suffi- 
cient to  prevent  the  escape  of  most  of  the  gases,  but  such  gases  as 
hydrogen  and  helion  are  still  superior  to  its  'attraction.  The  moon 
appears  to  be  devoid  of  an  atmosphere,  the  gravity  of  its  mass  being 
insufficient  to  hold  the  gases  to  it.  When  the  earth  was  as  small  as 
the  moon  is  now,  it,  too,  probably  had  no  atmosphere,  but  as  it  grew 
by  the  addition  of  meteoric  matter,  gravitational  attraction  increased, 
permitting  it  to  bind  to  itself  more  and  more  gases  until  the  present 
condition  was  reached.  The  gases  first  held  by  the  attraction  of  the 
earth  were  the  heavier  ones,  such  as  carbon  dioxide,  nitrogen,  and 
water  vapor.  Of  these,  carbon  dioxide,  being  chemically  active 


THE  EARTH   BEFORE  THE  CAMBRIAN  387 

probably  entered  into  combination  with  the  rocks  to  an  important 
degree,  while  nitrogen,  an  inactive  gas,  has  accumulated  until  it  now 
constitutes  about  79  per  cent,  of  the  atmosphere.  The  percentage 
of  carbon  dioxide  in  the  atmosphere  is  about  .03  of  one  per  cent. 

The  atmosphere  as  now  composed  has  been  derived  either  directly 
from  space,  as  when  gases  came  within  the  influence  of  the  earth's 
attraction  and  were  held  by  it,  or  from  the  interior  of  the  earth  from 
which  they  were  forced  by  the  increasing  heat  produced  by  com- 
pression as  well  as  by  the  pressure  itself.  When  the  water  vapor 
condensed  to  form  rain,  streams  began  to  cut  down  the  land,  under- 
ground waters  began  their  work  of  solution  and  deposition,  the  de- 
pressions of  the  land  were  filled  with  water,  and  the  earth's  surface  as 
we  now  know  it  began  its  long  series  of  transformations. 

Nebular  and  Planetesimal  Theories  Contrasted.  —  The  nebular 
and  planetesimal  theories  differ  in  a  number  of  fundamental  features. 
Under  the  former,  the  earth  was  once  hotter  and  larger  than  now  and 
by  cooling  and  contraction  has  become  smaller  and  solid,  or  nearly  so. 
Under  the  planetesimal  hypothesis,  the  earth  became  continually 
larger  as  its  bulk  was  increased  by  the  gathering  in  of  planetesimals. 
The  atmosphere  of  the  earth,  according  to  the  theory  of  a  molten 
globe,  was  heaviest  at  first;  according  to  the  planetesimal  theory, 
it  was  lightest  at  the  beginning  and  grew  denser  as  the  earth  increased 
in  size  and  mass.  According  to  the  one  (nebular),  we  may  hope  to 
find  the  original  igneous  crust  of  the  earth ;  according  to  the  other 
(planetesimal),  a  "  crust  "  never  existed,  but  after  the  appearance 
of  an  atmosphere  the  surface  was  composed  of  lava  flows,  volcanic 
ash,  meteoric  matter,  and  sedimentary  deposits. 

The  advocates  of  each  of  these  theories  agree  that  at  present  the 
earth  is  essentially  a  solid  mass  more  rigid  than  steel  or  glass.  This 
is  shown  by  two  lines  of  evidence  (p.  272)  :  (i)  earthquake  shocks 
pass  directly  through  the  earth  and  travel  at  a  rate  which  shows  that 
the  transmitting  medium  is  an  extremely  rigid  substance;  (2)  had 
the  earth's  interior  been  in  a  molten  condition  for  a  long  period  of 
time,  the  rotation  of  the  earth  upon  its  axis  would  long  since  have 
ceased  because  of  the  internal  friction  of  the  liquid. 

REFERENCES   FOR  THEORIES  OF  THE  ORIGIN  OF  THE  EARTH 

CHAMBERLIN  and  SALISBURY,  —  Geology,  Vol.  2,  1906,  pp.  1-132. 
CROLL,  JAMES,  —  Climate  and  Cosmology. 
MOULTON,  F.  R.,  —  An  Introduction  to  Astronomy. 
YOUNG,  C.  A.,  —  A  Textbook  of  General  Astronomy. 


388 


HISTORICAL  GEOLOGY 


PRE-CAMBRIAN  ERAS 

In  no  connection  is  the  saying,  "All  beginnings  are  difficult,"  more 
true  than  in  the  study  of  the  earliest  chapters  of  the  earth's  history. 
This  is  the  case  not  only  because  the  rocks  which  preserve  the 
record  in  a  given  region  have  been  subjected  to  all  the  foldings  and 
metamorphisms  which  have  affected  all  the  subsequent  rock  forma- 
tions of  that  region  as  well  as  earlier  ones,  but  also  because  no  fossils 
have  been  found  except  near  the  close  of  the  Pre-Cambrian,  and  these 
are  few  and  fragmentary. 

A  brief  classification  of  the  Pre-Cambrian  *  of  the  Lake  Superior  re- 
gion of  North  America  is  as  follows  : 


Proterozoic  One  or  more  series  separated  by  unconformities  to  which 

(Greek,  proteros,          \  local  names  have  been  given  since  it  has  not  been  possible  to 
early,  and  zoa,  life)     [  determine  their  equivalents  in  distant  regions. 


Upper  Proterozoic  (Keweenawan) 

Unconformity 

Middle  Proterozoic  (Upper  Huronian) 

Unconformity 

Middle  Huronian 
Lower  Proterozoic 


Great  Unconformity 


Unconformity 
Lower  Huronian 


Proterozoic  in  the 
Lake  Superior  region. 


Archaozoic 
(Greek,  arche,  begin- 
ning, and  zoa,  life) 


(Laurentian) 


(Keewatin) 


Mainly  light-colored  (acid)  granites,  gneisses, 
and  schists.  These  are  largely  intrusive,  but 
some  may  represent  the  surface  upon  which 
the  Keewatin  was  laid  down. 


Mainly  dark-colored  (basic)  metamorphic 
rocks,  composed  largely  of  metamorphic  lava 
flows  and  tuffs,  with  small  amounts  of  meta- 
morphic sediments. 


1  The  United  States  Geological  Survey  includes  all  of  the  Pre-Cambrian  rocks  under  the 
term  Proterozoic  and  uses  Archaean  for  the  Lower  (Archaeozoic)  and  Algonkian  for  the  Upper 
Proterozoic. 

F.  D.  Adams  considers  that  the  Pre-Cambrian  rocks  have  a  threefold  division  which  he 
designates  as  Eo-Proterozoic  (Archaeozoic),  Meso-Proterozoic  (Middle  and  Upper  Huronian), 
and  Neo-Proterozoic  (Keweenawan),  Proterozoic  being  used  independent  of  any  consideration 
of  the  presence  or  absence  of  life. 


THE  EARTH   BEFORE  THE  CAMBRIAN  389 

THE  ARCHEOZOIC  ERA 

Distribution  of  the  Af chaeozoic  Rocks.  —  The  rocks  constituting 
the  Archaeozoic  system  are  the  oldest  of  which  we  at  present  have 
any  knowledge  and,  as  far  as  known,  underlie  all  the  younger  rocks 
of  the  earth's  crust.  In  regions  which  have  been  repeatedly  uplifted 
and  eroded  the  Archaeozoic  rocks  are  uncovered,  and  it  is  in  such  places 
that  they  have  been  studied.  In  North  America  the  greatest  area  of 
Archaeozoic  rocks  lies  in  the  eastern  half  of  Canada,  where  they  have 
an  area  of  about  2,000,000  square  miles,  forming  an  irregular  mass 
around  Hudson  Bay  and  extending  south  into  Wisconsin  and  Minne- 
sota. This  is  often  designated  as  the  "  Laurentian  shield."  In  the 
Adirondacks  of  New  York,  in  New  England,  and  in  a  belt  stretching 
from  Maryland  south  into  Alabama  (Piedmont  Plateau)  are  crystal- 
line rocks  which  are  partly  of  Archaeozoic  age.  In  the  cores  of  the 
mountains  of  the  western  half  of  the  continent  and  in  other  isolated 
patches  they  also  appear  at  the  surface. 

Our  detailed  knowledge  of  the  Pre-Cambrian  of  North  America 
is  largely  confined  to  the  region  about  the  Great  Lakes  and  the  St. 
Lawrence  River.  Here  excellent  and  fresh  exposures  have  been  de- 
veloped by  glacial  erosion,  and  the  presence  of  valuable  deposits  of 
copper,  iron,  nickel,  cobalt,  and  silver  has  led  to  a  careful  study 
of  the  region. 

Archaeozoic  rocks  apparently  corresponding  to  the  Archaeozoic  of 
North  America  occur  in  Scandinavia  and  other  parts  of  Europe,  over 
a  large  area  in  Brazil,  in  central  Africa,  in  China,  in  India,  and  else- 
where, but  the  determination  of  the  age  of  the  crystalline  rocks  of 
many  regions  is  yet  in  doubt.  It  has  been  roughly  estimated  that  the 
Pre-Cambrian  rocks  appear  at  the  surface  over  one  fifth  of  the  land 
area.  The  term  "  surface  "  is  used  to  mean  that  the  formation  is 
not  covered  by  younger  rock  formations,  although  it  may  be  hidden 
in  many  places  by  soil  or  glacial  deposits. 

The  difficulty  of  any  attempt  to  correlate  the  Archaeozoic  rocks  of 
distant  or  isolated  regions  is  obvious,  since  fossils  are  absent,  and  this 
exact  method  of  determining  the  age  of  rocks  is  consequently  un- 
available. Moreover  the  fact  that  the  lithological  character  of  the 
rocks  of  a  formation  may  vary  greatly,  even  in  short  distances,  makes 
such  characters  of  a  formation  an  extremely  uncertain  criterion  upon 
which  to  base  a  correlation.  However,  since  fossils  are  lacking,  the 
lithological  character,  superposition,  and  the  degree  of  metamorphism 


390 


HISTORICAL  GEOLOGY 


and  deformation  must  be  taken  advantage  of  in  making  provisional 
correlations. 

Characteristics  of  Archaeozoic  Rocks.  —  Nb  rocks  are  more  complex 
than  those  of  this  system.  In  fact,  their  very  complexity  is  a  char- 
acter which  aids  in  their  determination.  In  the  Lake  Superior  re- 
gion (Fig.  366)  the  system  is,  in  general,  composed  of  a  great  series 
(Keewatin)  made  up  predominantly  of  dark-colored  (basic)  schists 
and  great  masses  of  granitoid  gneisses  and  light-colored  (acid)  schists 
(Laurentian)  which  have  apparently  for  the  most  part  been  intruded 
into  the  Keewatin  schists.  The  Keewatin  schists  are,  therefore,  as 
far  as  present  investigation  shows,  the  oldest  rocks  of  the  earth's 
crust  (unless  some  of  the  gneisses  prove  to  be  of  even  greater  age). 

They  are  composed 
largely  of  lava  flows 
and  tuffs,  with  oc- 
casional conglomer- 
ates, shales,  and 
beds  of  iron  ore, 
which  have  been 
folded,  contorted, 
and  so  metamor- 
phosed that  their 
former  condition  is 
with  difficulty  rec- 
ognized. They  have,  moreover,  been  broken  by  faults  and  by 
massive  intrusions.  Dikes  through  which  was  forced  the  lava  that 
flowed  over  surfaces  that  have  since  been  worn  away  now  cut  both 
the  schists  and  the  Laurentian  granites  and  gneisses.  Great 
batholiths  (p.  328)  of  granite  (Laurentian)  occur  so  frequently  as 
to  make  them  almost  characteristic  of  the  Archaeozoic  systems, 
and  in  certain  regions  they  constitute  the  larger  part  of  the  surface 
rock.  These  batholiths  have,  in  turn,  been  broken,  faulted,  and  in- 
truded by  lavas  of  later  age,  and  these  by  even  younger  intrusions. 
Formerly,  before  they  were  recognized  as  intrusive  masses,  the 
granites  and  gneisses  of  the  Archaeozoic  systems  were  considered  to 
be  portions  of  the  original  crust  of  the  earth.  That  surfaces  must 
have  existed  upon  which  the  lava  flows  and  ash  deposits  spread  and 
from  which  the  material  was  derived  to  form  the  sedimentary  beds 
is  obvious.  Nevertheless,  no  such  surface  has  yet  been  recognized 
with  certainty,  either  because  it  is  still  buried  beneath  the  overlying 


/IXCHEOZO/C 


PftOTEffOZOfC 


FIG.  366.  —  Block  diagram  showing  the  occurrence  and 
complicated  structure  of  Pre-Cambrian  rocks  in  the  Lake 
Superior  region. 


THE  EARTH   BEFORE  THE   CAMBRIAN  391 

rocks  or  because  it  has  been  so  welded  into  them  by  heat  and  pressure 
that  it  cannot  be  determined. 

Thickness.  —  The  rocks  referred  to  the  Archaeozoic  systems  are 
of  great  but  unknown  thickness.  The  lower  limits  of  the  system,  as 
stated,  have  never  been  observed,  even  where  they  have  been  cut 
down  many  hundreds  of  feet  in  mountain  ranges  or  in  the  great 
"  Pre-Cambrian  shield  "  of  Canada,  which  has  apparently  been  re- 
peatedly subjected  to  prolonged  and  profound  erosion  such  perhaps 
as  few  other  regions  of  the  world  have  experienced. 

Causes  of  Metamorphism  and  Deformation. — The  cause  of  the 
metamorphic  character  of  the  Archaeozoic  rocks  is  readily  understood 
when  the  disturbances  which  have  affected  them  are  considered. 
The  Archaeozoic  was  a  time  (i)  of  unusual  volcanic  activity,  as  well  as 
(2)  of  great  deformations.  Moreover  (3)  the  rock  that  now  appears 
at  the  surface  was  probably,  for  the  most  part,  deeply  buried  be- 
neath younger  formations.  These  three  factors  alone  would  produce 
metamorphic  changes  of  the  first  order.  Deformation  was  produced 
in  a  number  of  ways,  (i)  The  great  masses  of  lavas  which  were  in- 
truded into  the  rocks  caused  them  to  fold  and  crumple.  Moreover 
(2)  as  the  lava  was  withdrawn  from  below  the  surface  and  poured 
out  upon  it,  settling  resulted  which  caused  a  further  folding.  These 
elements,  taken  in  connection  with  (3)  the  lateral  pressure  resulting 
from  the  contraction  of  the  interior  of  the  earth  (p.  359),  must  have 
altered  profoundly  the  original  structure  and  composition  of  the  rocks, 
changing  the  lavas  and  tuffs  and  sedimentary  rocks  to  schists  of  vari- 
ous kinds,  and  the  granites  to  gneisses  and  even  schists. 

Conditions  during  the  Archaeozoic  Era.  —  A  few  deductions  can 
be  made  concerning  the  conditions  which  prevailed  in  this  earliest 
era.  The  presence  of  successive  lava  flows  and  of  volcanic  ash  and 
cinders  shows  that  volcanoes  were  abundant  and  active,  at  least 
locally.  The  conglomerates  and  shales  prove  that  the  surfaces  of  the 
land  were  worn  down  by  running  water  and  that  the  rocks  were  weath- 
ered, since  the  clays  of  which  shales  are  formed  were  produced  by  the 
weathering  of  igneous  or  other  rocks.  The  presence  of  limestone 
suggests  the  possibility  that  shell-bearing  animals  were  in  existence, 
but  since  limestone  is  known  to  be  formed  by  chemical  precipitation 
as  well  as  by  organic  remains,  the  evidence  is  not  conclusive.  The 
Grenville  series  of  the  St.  Lawrence  valley,  estimated  to  be  50,000 
feet  thick,  is  distinctly  stratified  and  is  one  of  the  greatest  limestone 
series  in  the  earth's  crust,  a  part  if  not  all  of  which  is  believed  to 


392  HISTORICAL  GEOLOGY 

be  of  Archaeozoic  age.  Graphite,  which  may  be  met amorphic 'organic 
matter,  indicates  the  presence  of  plants.  Graphite,  however,  may  be 
of  inorganic  origin,  derived  perhaps  from  petroleum.  No  remains 
that  can  be  positively  identified  as  fossils  have  been  found  in  the 
Archaeozoic  rocks. 

It  has  been  suggested  (Daly)  that  the  Pre-Cambrian  limestones  were  entirely  prod- 
ucts of  chemical  precipitation.  This  is  based  on  the  assumption  that  the  land  areas 
were  at  first  relatively  small,  and  that  the  abundance  of  decaying,  soft-bodied  organ- 
isms on  the  sea  floor  produced  ammonium  carbonate,  which  led  to  a  continuous  precipi- 
tation of  such  lime  as  was  available.  Hence  the  ocean  was  limeless,  and  it  was  not 
until  the  lands  became  more  extended  that  a  sufficient  quantity  of  lime  salts  was 
brought  in  by  rivers  to  counterbalance  that  thrown  down  by  the  ammonium  carbon- 
ate and  sodium  carbonate  on  the  sea  floor.  If  this  be  true,  the  earlier  organisms 
could  not  form  calcareous  shells  or  skeletons.  The  fact  that  Pre-Cambrian  and 
Cambrian  limestones,  even  when  unaltered,  show  no  signs  of  having  originated  from 
shell  remains  is  offered  in  proof. 

Duration.  —  An  immense  but  unknown  duration  is  assigned  to  the 
Archaeozoic  era,  an  era  so  vast  that  even  if  it  were  possible  to  state 
the  duration  in  terms  of  years  the  number  would  be  so  large  as  to 
convey  little  meaning  to  the  human  mind.  If  millions  of  years  were 
consumed  by  the  later  eras,  tens  of  millions  must  be  ascribed  to  this 
era.  In  fact,  it  is  possible  that  the  Archaeozoic  may  have  been  longer 
than  all  the  subsequent  eras  taken  together. 

Bearing  upon  the  Theories  of  the  Earth's  Origin.  —  (i)  According  to  the  theory  that 
the  earth  was  originally  a  molten  globe  (nebular  hypothesis)  which  upon  cooling  first 
formed  a  crust,  we  should  expect  to  find  the  earliest  sedimentary  rocks  underlain  by 
an  igneous  or  metamorphic-igneous  floor,  provided  that  igneous  activity  was  slight 
after  the  crust  became  cool  enough  to  permit  the  operation  of  the  agencies  of  erosion 
and  the  weather.  It  seems  more  probable,  however,  that  in  these  early  stages  igneous 
activity  would  be  unusually  prevalent,  with  the  result  that  lava  flows,  volcanic  prod- 
ucts of  enormous  thickness,  as  well  as  great  intrusions  might  completely  hide  the 
original  crust,  if  indeed  it  were  not  remelted. 

(2)  According  to  the  planetesimal  theory,  the  matter  gathered  in  from  space  became 
so  hot  at  the  (a)  center  that  it  recrystallized  to  form  an  essentially  igneous  core,  (b)  A 
thick  zone  outside  of  the  central  core,  made  up  largely  of  planetesimal  matter,  partly 
of  igneous  rock  erupted  from  below,  theoretically  underlies  the  (c)  next,  and  relatively 
thin  zone  which  is  composed  largely  of  extrusive  igneous  rock,  with  smaller  amounts 
of  sediments  and  of  planetesimal  matter  gathered  from  space.  This  is  the  zone  which, 
according  to  the  planetesimal  theory,  appears  at  the  surface  and  is  known  as  the 
Archaeozoic. 

It  will  be  seen  from  the  above  that  according  to  either  the  planetesimal  or  the 
modified  nebular  hypothesis  the  "crust"  of  the  earth  would  have  practically  the 
same  characters  and  that,  therefore,  even  though  fundamentally  different,  no  means 
is  afforded  of  testing  the  two  theories. 


THE   EARTH   BEFORE  THE   CAMBRIAN  393 

THE  PROTEROZOIC  ERA 

Archaeozoic  and  Proterozoic  Contrasted.  —  The  Archaeozoic  sys- 
tems are  separated  from  the  overlying  Proterozoic  by  a  great  and 
widespread  unconformity  (p.  270)  upon  which  rests  a  series  of  rocks 
of  enormous  thickness,  which  extend  to  the  fossiliferous  Cambrian. 
The  two  groups  differ  in  a  number  of  particulars.  "The  Archaean 
[Archaeozoic]  is  a  group  dominantly  composed  of  igneous  rocks,  largely 
volcanic,  and  for  extensive  areas  submarine.  Sediments  are  subor- 
dinate. The  Algonkian  [Proterozoic]  is  a  series  of  rocks  which  is 
mainly  sedimentary.  Volcanic  rocks  are  subordinate.  The  Algon- 
kian [Proterozoic]  sediments,  where  not  too  greatly  metamorphosed, 
are  similar  in  all  essential  respects  to  those  which  occur  in  the  Paleo- 
zoic and  later  periods.  When  the  Algonkian  [Proterozoic]  rocks  were 
laid  down  essentially  the  present  conditions  prevailed  on  earth.  The 
Archaean  [Archaeozoic]  rocks,  on  the  other  hand,  indicate  that  during 
this  era  the  dominant  agencies  were  igneous.  On  the  whole,  the  def- 
ormation and  metamorphism  of  the  Archaean  [Archaeozoic]  are  much 
farther  advanced  than  the  Algonkian  [Proterozoic].  The  two  groups 
are  commonly  separated  by  an  unconformity  which  at  many  localities 
is  of  a  kind  indicating  that  the  physical  break  was  of  the  first  order  of 
importance."  (Van  Hise.) 

The  Proterozoic  in  Different  Regions.  —  Lake  Superior  Region. 
(Table,  p.  388.)  South  of  the  "  Pre-Cambrian  shield  "  (Fig.  367)  the 

Ahlt-m      JUil    ^<m^          Ahm Akm 


FIG.  367.  —  Section  through  a  portion  of  northern  Minnesota,  showing  the  relation  of 
the  Pre-Cambrian  rocks.     (U.  S.  Geol.  Surv.) 

T>  •        Akm       Keweenawan. 

rroterozoic        ,,,         u 

Ahl  m     Huroman. 

.     ,  •        /Rl        Granites  and  gneisses  of  the  Laurentian. 

/Rk       Schists  and  iron-bearing  formations  of  the  Keewatin. 

lowest  member  of  the  great  series  which  constitutes  the  Proterozoic, 
is  the  Lower  Proterozoic  (Huronian,  named  from  the  fine  develop- 
ment north  of  Lake  Huron),  and  is  composed  of  quartzites,  slate, 
schists,  interbedded  lava  flows,  and  igneous  intrusions,  together  with 
limestone  and  beds  of  iron  ore.  The  rocks  are  usually  much  folded 
and  occur  in  the  form  of  long,  narrow  belts,  separated  by  the  Archaeo- 
zoic schists  and  gneisses,  being  small  remnants  of  a  once  extensive 
system.  Locally,  at  least,  the  Lower  Proterozoic  (Huronian)  is 


394  HISTORICAL  GEOLOGY 

divided    into    two    systems    (Lower    and    Middle)    by    an    uncon- 
formity. 

Resting  unconformably  upon  the  Lower  Proterozoic  (Middle  Hu- 
ronian)  is  the  Middle  Proterozoic  (Upper  Huronian),  which  resembles 
the  lower  system  lithologically  in  that  it  is  composed  of  similar  sedi- 
mentary rocks  and  lava  flows,  but  is  somewhat  less  metamorphic. 
In  this  system  occur  the  largest  and  richest  deposits  of  iron  in  North 
America.  The  unconformity  which  separates  the  Lower  Proterozoic 
(Lower  and  Middle  Huronian),  and  Middle  Proterozoic  (Upper  Hu- 
ronian), is  considered  by  some  geologists  to  be  of  an  importance  almost 
equal  to  that  between  the  Archaeozoic  and  Proterozoic  systems. 

The  closing  system  of  the  Proterozoic  (Keweenawan),  separated 
from  the  Middle  Proterozoic  (Upper  Huronian)  by  an  unconformity, 
differs  from  the  preceding  Proterozoic  systems  in  the  presence  of 
numerous,  and  in  the  aggregate  enormously  thick  lava  beds,  which  ap- 
parently welled  up  through  fissures  (much  as  in  Iceland  to-day)  and 
did  not  flow  from  distinct  volcanoes.  The  total  thickness  of  these 
lava  flows  is  estimated  at  nearly  six  miles,  making  this  the  most  no- 
table time  of  local  volcanism  in  geological  history.  In  northwestern 
Minnesota  and  contiguous  portions  of  Wisconsin  there  are  sixty-five 
distinct  lava  flows  and  five  conglomerate  beds,  none  of  the  former  be- 
ing more  than  100  feet  thick.  In  the  section  cited  neither  the  upper 
nor  the  lower  limits  are  known.  The  maximum  thickness  of  the 
Keweenawan  is  estimated  at  50,000  feet,  of  which  sedimentary  beds 
constitute  about  15,000  feet.  Towards  the  close  of  the  period  the 
igneous  outbursts  became  less  frequent,  with  a  corresponding  increase 
in  the  proportion  of  sedimentary  deposits.  The  great  Lake  Superior 
copper  deposits  which  have  up  to  this  time  yielded  many  millions  of 
dollars  in  profits  to  their  owners  occur  in  the  lavas  and  conglomerates 
of  this  system.  (The  Keweenawan  is  by  some  writers  considered  to 
be  Cambrian.) 

The  unconformities  which  separate  the  various  systems  of  the 
Proterozoic  in  the  Lake  Superior  region  are  well  marked.  They  are 
evidenced  (i)  by  basal  conglomerates  (p.  240)  that  represent  the 
shores  of  an  encroaching  sea,  (2)  by  the  irregular  erosion  surfaces  of 
the  underlying  rocks,  (3)  by  differences  in  the  amount  of  volcanism, 
and  (4)  by  the  differences  in  the  metamorphism  of  the  sediments  of  the 
overlying  and  underlying  formations. 

In  the  Grand  Canyon  of  the  Colorado  the  Pre-Cambrian  forma- 
tions (Fig.  368)  are  more  than  10,000  feet  thick  and  differ  in  many 


THE   EARTH   BEFORE  THE   CAMBRIAN 


395 


FIG.  368.  —  Photograph  of  the  wall  of  the  Grand  Canyon  of  the  Colorado  River, 
Arizona,  showing  two  unconformities.     (See  Fig.  369.) 


respects  from  those  of  the  Lake  Superior  region.  The  lower  portion 
of  the  gorge  is  sunk  into  the  Archaeozoic  gneisses.  These  are  overlain 
unconformably  by  a  strongly  dipping  series  of  sedimentary  (Proter- 
ozoic)  strata  separated  by  minor 
unconformities,  and  they,  in  turn, 
underlie  unconformably  the  Cam- 
brian strata.  Some  measure  of 
the  length  of  time  represented  by 
the  unconformities  is  shown  by 
the  flatness  of  the  floor  (Fig.  369) 
upon  which  the  Proterozoic  rests, 
and  also  of  that  above  the  tilted 
Proterozoic  sediments  upon  which 
the  Cambrian  lies. 

In    the    Black    Hills   of    South 
Dakota    (Fig.   370,   also   see  Fig. 
342,  p.  356),  in  the  cores  of  many 
of   the 
west, 


mountain 
as  well   as 


ranges   of   the 
in   the  Adiron- 


FIG.  369.  —  Section  of  the  Grand 
Canyon  of  the  Colorado  River,  Arizona. 
The  lower  portion  shows  the  complex 
schists  of  the  Archaeozoic.  Upon  them, 
separated  by  an  unconformity  CD,  rest  a 
series  of  Proterozoic  strata.  The  Prot- 
erozoic strata  are  separated  from  the 
overlying  Cambrian  by  the  unconformity 
AB.  (See  Fig.  368.) 


396  HISTORICAL  GEOLOGY 

dacks  and  the  Piedmont  Plateau  of  eastern  North  America,  Proter- 
ozoic  rocks  have  been  identified  with  some  certainty. 

Rocks  of  this  age  are  believed  to  occur  on  other  continents,  but 
their  correlation  has  not  yet  been  definitely  determined.     In  China, 


FIG.  370.  —  A  generalized  section  through  the  Black  Hills,  South  Dakota,  showing 
the  basal  Archeozoic  rocks  underlying  the  Cambrian  and  younger  strata. 

for  example,  the  Pre-Cambrian  rocks  have  a  threefold  division,  the 
upper  two  of  which  are  believed  to  be  Proterozoic. 

Iron  and  Copper  Deposits.  —  A  discussion  of  the  Proterozoic 
would  be  incomplete  without  mention  of  the  valuable  deposits  of 
iron  which  they  contain.  In  the  five  years  previous  to  1914, 
216,981,280  long  tons  of  iron  ore  were  mined  from  the  Proterozoic 
rocks  of  the  Lake  Superior  region  alone,  making  this  the  most  im- 
portant iron-ore  center  in  the  world.  The  ore,  chiefly  as  hematite 
(Fe2O3),  occurs  in  the  form  of  thick  beds  in  the  sedimentary  strata. 
Originally  some  of  the  formations  contained  large  quantities  of  iron 
minerals  intermingled  with  silica  and  other  non-metallic  minerals, 
and  if  it  had  remained  in  this  state  would  probably  not  have  been 
of  commercial  value.  The  iron  ore  was  later  concentrated  through 
the  agency  of  underground  waters  which  dissolved  out  and  carried 
away  the  silica  and  other  impurities,  leaving  pure,  or  nearly  pure, 
iron  ore.  Some  deposits  were  further  enriched  by  "  replacement  " 
(p.  372),  ore  being  deposited  as  the  non-metallic  minerals  were 
removed. 

One  of  the  greatest  known  deposits  of  native  copper  occurs  in  the 
rocks  of  the  Keweenawan  system  of  the  Lake  Superior  region.  The 
copper  occurs  in  the  cracks  of  igneous  rocks,  in  the  pores  of  some  of 
the  lava  flows,  and  in  the  spaces  between  the  pebbles  and  grains  of 
sand  of  the  conglomerates  and  sandstones.  The  copper  was  originally 
diffused  in  small  quantities  through  the  lava,  but  was  partly  dissolved 
out  by  underground  water,  carried  into  porous  layers,  and  there  de- 
posited, in  some  cases  in  such  quantities  as  to  constitute  a  cementing 
material. 

Life  of  the  Proterozoic  Era.  —  The  indirect  evidences  of  life  in  the 
Proterozoic  are  more  abundant  than  in  the  Archaeozoic,  although  of 
much  the  same  character.  Limestones  imply  but  do  not  prove  the 


THE  EARTH   BEFORE  THE  CAMBRIAN 


397 


(Beltina  danai) 
from  the  Prot- 
erozoic.  This 
is  one  of  the 
known 
fossils.  (After 
Walcott.) 


existence  of  shell-bearing  animals,  such  as  are  now  forming  the  cal- 
careous ooze  and  shell  deposits  of  the  ocean  bottom.  Graphite  and 
black  shales  are  suggestive  of  plant  remains.  The  great  deposits  of 
iron  ore  are  thought  to  indicate  the  existence  of  life, 
since  organic  matter  seems  necessary  to  have  furnished 
the  carbon  dioxide  by  means  of  which  the  insoluble 
iron  minerals  were  decomposed,  and  as  soluble  iron 
carbonates  were  carried  away  and  redeposited  where 
the  further  movement  of  the  underground  water  was 
prevented.  It  is  possible,  however,  that  decomposing  An  I°'  enja~ 
organic  matter  may  not  have  been  essential  to  this  of  a  crustacean 
process. 

Direct  evidence  is  furnished  by  a  few  fossils  that 
have  been  found  in  the  Proterozoic  rocks  of  the 
Grand  Canyon  of  the  Colorado  in  Arizona,  and  in  oldest 
rocks  of  this  age  in  Montana  and  Ontario.  The 
known  animal  life  consists  of  several  species  of  worms, 
a  large  crustacean  (Fig.  371),  a  sponge-like  fossil  (Atikokania),1  some 
of  which  are  15  inches  in  diameter,  and  a  brachiopod.  Abundant 
fossils  of  a  calcareous  alga  (Fig.  372),  individuals  of  which  are  more 
than  two  feet  in  diameter,  form  layers  of  limestone  three  feet  thick. 
It  is  probable  that  when  all  parts  of  the  world  become  geologically 

better  known,  fossils  will  be  dis- 
covered in  Proterozoic  formations 
as  distinctive  in  character  as  those  of 
the  Cambrian  and  overlying  systems. 
Duration.  —  The  fossils  of  the  Prot- 
erozoic, though  few  and  fragmentary, 
show  that  some  forms  of  life  were  well 
up  in  the  scale  of  life.  Crustaceans, 
worms,  and  brachiopods  (p.  414)  are  so 
high  in  the  scale  as  to  force  the  con- 
clusion that  life  had  been  in  existence 

FIG.  372.  — Hemispherical  bodies     many  millions  of  years  prior  to  this 

time.      Moreover,   judging   from   the 
extreme  slowness  with  which  evolu- 
tional changes  take  place,  the  great  differentiation  in  the  life  proves 
a  great  antiquity.     When  this  evidence,  even  though  theoretical,  is 
taken  in  connection  with  the  great  thickness  of  the  sediments  and 
1  Atikokania  is  probably  not  a  sponge  but  a  calcareous  alga. 


believed  to  have  been  formed  by  blue- 
green  algae.     Proterozoic,  Montana. 


398  HISTORICAL  GEOLOGY 

lava  flows,  as  well  as  the  long  periods  represented  by  the  uncon- 
formities, it  seems  probable  that  the  Proterozoic  was  very  much 
longer  than  all  of  Paleozoic  time.  In  fact,  if  the  degree  of  life 
development  is  taken  as  a  basis  by  which  to  measure  time,  it  is 
thought  that  the  appearance  of  the  Cambrian  fauna,  although  many 
millions  of  years  ago,  was  a  comparatively  recent  event. 

Climate.1  —  Little  can  be  said  of  the  climatic  conditions  of  this 
remote  age.  The  presence  of  fossils  in  Montana,  Arizona,  and  On- 
tario indicates  a  climate  that  was  certainly  not  frigid.  The  presence 
of  scratched  bowlders  in  formations  believed  to  be  Proterozoic  in 
Norway,  China,  and  Australia,  and  perhaps  in  southern  Africa,  some- 
times resting  upon  a  striated  rock  pavement  possessing  such  char- 
acters as  to  make  the  glacial  origin  of  the  deposits  undoubted,  leads%to 
the  surprising  conclusion  that  even  at  this  time  the  earth  was  visited 
by  periods  of  glaciation  such  as  that  of  the  Great  Ice  Age.  There  is 
some  question  as  to  the  age  of  these  glacial  formations,  some  investi- 
gators believing  that  they  belong  to  the  Lower  Cambrian.  According 
to  the  theory  of  a  cooling  earth,  with  an  atmosphere  that  was  at  first 
heavy,  it  is  difficult  to  explain  the  presence  of  continental  ice  sheets 
in  this  early  era. 

Life  before  Fossils. — The  earliest  rocks  in  which  an  abundance 
of  fossils  of  which  any  records  have  been  found  occur  in  the  Cambrian.2 
These  fossils  are  highly  organized  and  are  not  the  simple,  unspecialized 
ancestors  of  modern  animals  that  the  theory  of  evolution  demands. 
They  are  of  a  degree  of  specialization  which  indicates  a  long  period 
of  preceding  life.  Can  the  life  which  antedates  the  first  known  fos- 
sils be  inferred  ? 

In  the  seas  of  to-day  the  number  and  aggregate  bulk  of  minute 
and  microscopic  soft-bodied  animals  and  plants  which  live  near  or 
at  the  surface  of  the  ocean  is  astonishing.  Small  jellyfish  sometimes 
cover  the  ocean  for  many  miles,  tiny  crustaceans  live  in  myriads  and 
microscropic  animals  in  countless  numbers.  The  reasons  for  the 
abundance  of  microscropic  life  near  the  surface  (i.e.,  within  a  few  hun- 
dred feet  of  the  surface)  are  evidently  to  be  found  in  the  abundance 
and  uniform  distribution  of  mineral  food  in  solution,  in  the  presence 
of  sunlight,  and  in  the  uniformity  of  temperature.  Practically  all 
of  the  life  of  the  ocean  depends  upon  these  simple  forms,  either  directly 

1  Schuchert,  Chas.,  —  Climates  of  Geologic   Time,   Carnegie    Institution  of    Washington 
Publication  192,  1914,  pp.  263—298. 

Walcott,  C.  D.,  —  Smithsonian  Misc.  Coll.,  Vol.  64,  1914,  pp.  80-84. 

2  Unless  the  problematical  Eozoon  proves  to  be  organic. 


THE  EARTH   BEFORE  THE  CAMBRIAN  399 

or  indirectly.  Yet,  with  the  conditions  as  they  are  to-day,  almost 
nothing  of  this  profuse  life  would  be  preserved  in  a  fossil  state  as  a 
record  of  their  existence,  since,  with  few  exceptions,1  fossils  are  con- 
fined to  such  forms  as  possessed  some  hard  parts,  such,  for  example, 
as  shells  or  skeletons.  The  study  of  embryology  teaches  that  all 
classes  of  life  were  descended  from  minute,  possibly  swimming  crea- 
tures. The  starfish,  coral,  shellfish,  and  other  marine  animals  all 
began  life  as  minute,  free-swimming  forms.  It  seems  probable  that 
preceding  the  Cambrian  the  oceans  were  tenanted  by  such  small, 
soft-bodied  animals  as  those  which  populate  the  surface  to-day,  and 
that  they  were  in  equal  abundance. 

The  question  next  to  be  answered  is  :  Why  did  any  of  these  animals 
seek  the  bottom  of  the  ocean  and  become  stationary  forms  ?  The  first 
settlers  on  the  bottom  probably  did  not  secure  more  or  better  food 
than  their  swimming  relatives,  but  they  had  one  advantage  :  they  were 
able  to  devote  their  superfluous  energies  to  growth  and  multiplica- 
tion and  thus  to  become  larger  and  to  increase  in  numbers  faster  than 
the  swimming  forms.  Consequently  those  which  first  acquired  the 
habit  of  resting  on  the  bottom  soon  began  to  multiply  faster  than  their 
swimming  relatives.  But  this  rapid  increase  must  soon  have  given 
rise  to  crowding  and  competition  which  led  to  a  struggle  for  existence. 
Thus  the  stronger  forms  increased  at  the  expense  of  the  weaker. 

The  development  of  hard  coverings,  such  as  the  shell  of  the  mollusk 
(the  clam  is  an  example,  p.  413)  and  the  crustacean  (the  crawfish  is  an 
example,  p.  410),  may  have  been  due  largely  to  such  competition, 
since  the  animal  which  was  protected  in  some  way  would  have  a  better 
chance  to  escape  being  devoured.  Or  the  development  of  hard  pro- 
tective coverings  may  have  been  due  to  the  appearance  of  some  es- 
pecially voracious  creature,  and  the  trilobite  (p.  410),  the  largest  and 
most  active  of  the  inhabitants  of  the  early  ocean  bottom,  has  been 
suggested  as  the  aggressive  animal.  Later,  however,  some  animal 
arose  more  formidable  and  active  than  the  trilobite,  such  perhaps  as 
the  ancestor  of  the  fish,  and  may  have  caused  the  development  of  still 
heavier  armor. 

REFERENCES   FOR  THE   PRE-CAMBRIAN   ERAS 

ADAMS,  F.  D.,  —  Basis  of  Pre-Cambrian  Correlation:    Outlines  of  Geologic  History 

(Willis  and  Salisbury),  pp.  9-27. 

BROOKS,  W.  K., —  The  Origin  of  the  Oldest  Fossils:  Jour.  Geol.,  Vol.  2,  1894,  PP-  455" 
479- 

1  Some  jellyfish,  worm  borings,  worm  casts,  trails,  etc. 
CLELAND    GEOL.  —  26 


400  HISTORICAL  GEOLOGY 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  2,  1906,  pp.  133-217. 

COLEMAN,  A.  P., —  The  Lower   Huronian    Ice   Age:    Jour.    Geol.,    Vol.    16,    1908, 

pp.  149-158- 
MORRIS,  CHARLES,  —  Life  before  Fossils:  Am.  Naturalist,  Vol.  30,  1896,  pp.  188-194; 

279-285. 

RIES,  H.,  —  Economic  Geology,  3d  ed.,  Iron  Ore,  pp.  364-371;  Copper,  pp.  402-405. 
SCHUCHERT,  CHAS., —  Climates  of  Geologic  Time:  Carnegie  Institution  of  Washington, 

Publication  192,  1914,  pp.  263-293. 
STEIDMANN,  E.,  —  Summaries  of  Pre-Cambrian  Literature  of  North  America;   Jour. 

Geol.,  Vol.  23,  1915,  pp.  81-92. 
I).  S.  Geological  Survey  Folios. 
VAN  HISE,  C.  R.,  —  Pre-Cambrian  Geology  of  North  America:   Bull.  U.  S.  Geol.  Surv. 

No.  360,  1909. 
VAN  HISE,  C.  R.,  —  The  Pre-Cambrian  Rocks:   Outlines  of  Geologic  History  (Willis 

and  Salisbury),  pp.  1-8. 
VAN  HISE,  C.  R.,—  The  Problem  of  the  Pre-Cambrian:    Bull.  Geol.  Soc.  America, 

Vol.  19,  1908,  pp.  1-28. 


CHAPTER  XV 
THE   CAMBRIAN  PERIOD 

The  Paleozoic  Era.  —  Lying  above  the  Pre-Cambrian  formations 
is  the  Paleozoic  group,  which  includes  the  following  systems : 

7.  Permian  1 

Younger  Paleozoic  '     „  ,        .          /-.    L      •/• 

„,..,.  f  6.  Pennsylvania!!  \  Carboniferous 

Characterized  by  the  presence  of  verte-  ,,.    .    .     . 

,     ,  /»  ,         ,.,.  5.  Mississippian 

brates,  both  fishes  and  amphibians  ^         .  J 

4.  Devonian 

Older  Paleozoic  (  3.  Silurian 

Characterized  by  the  scarcity  of  verte-  <  2.  Ordovician 
brates  [  I.  Cambrian1 

The  first  three  of  these  systems  (the  Cambrian,  Ordovician,  and  Si- 
lurian) are  sometimes  grouped  together  as  the  older  Paleozoic,  since 
they  are  characterized  by  invertebrate  life,  vertebrate  remains  being 
absent  in  the  first  half  and  rare  in  the  second  half.  The  younger 
Paleozoic  (Devonian,  Mississippian,  Pennsylvanian,  and  Permian)  is 
characterized  by  the  higher  forms  of  life,  such  as  fishes,  amphibians, 
and  reptiles,  although  invertebrates  were  as  abundant  as  in  the  older 
Paleozoic.  These  seven  systems  2  are  not  of  equal  length,  nor  are 
they  of  equal  importance  in  the  evolution  of  life,  but  may  be  recog- 
nized in  any  portion  of  the  world  in  which  they  occur  by  their  pecul- 
iar fauna.  Locally,  the  systems  are  often  clearly  marked  by  un- 
conformities (p.  270),  but  it  is  upon  differences  in  the  faunas  that  the 
separations  are  ultimately  based  (p.  417). 

The  maximum  thickness  of  the  rocks  of  this  group  in  Europe  is 
estimated  at  100,000  feet,  and  in  the  Appalachian  region  of  this 
country  a  maximum  up  to  40,000  feet  is  exposed.  The  duration  of 
the  Paleozoic  era  was  immense,  exceeding  that  of  all  subsequent  time. 

1  Ozarkian  is  a  proposed  system  which  includes  the  Upper  Cambrian  and  part  of  the  Lower 
Ordovician. 

2  Schuchert,  Chas.,  —  The  Delimitation  of  the  Geologic  Periods,  illustrated  by  the  paleogeog- 
raphy  of  North  America:  International  Geological  Congress,  1913. 

401 


402  HISTORICAL  GEOLOGY 

THE  CAMBRIAN  PERIOD 

Divisions  of  the  Cambrian.  —  The  first  great  period  of  the  Paleo- 
zoic is  the  Cambrian  (Latin  name  for  Wales),  so-called  because  of  its 
development  in  Wales,  where  it  was  first  studied  with  care.  This  is 
the  oldest  fossiliferous  system  known  at  present  (although  a  new  series 
of  fossils  may  yet  be  discovered  in  the  youngest  Proterozoic  rocks), 
if  one  excepts  the  few  fossils  found  in  the  Proterozoic ;  and  upon  it  we 
must  depend  to  a  large  extent  for  our  knowledge  of  the  early  life  of 
the  world. 

The  Cambrian  is  usually  separated  into  three  subdivisions :  the 
Lower  (Waucobian),  the  Middle  (Acadian),  and  the  Upper  (Croxian). 
These  divisions  are  based  upon  differences  in  the  character  of  the  sedi- 
ments in  certain  regions,  but  chiefly  upon  the  differences  in  the  faunas. 
A  study  of  the  fossils  of  the  Cambrian  formations  has  shown  (as  is 
true  of  all  later  systems)  that  the  fossils  of  the  earliest  and  latest 
formations  of  the  system  differ  markedly,  although  some  of  them  are 
the  same.  This  is  due  to  the  gradual  disappearance  of  some  species 
and  the  introduction  of  others.  Among  the  trilobites  in  the  Lower 
Cambrian  (p.  412)  is  a  world-wide  genus  (Olenellus,  Fig.  382  A,  p.  412) 
which  is  not  found  in  the  Middle  Cambrian,  while  in  the  Middle  Cam- 
brian a  trilobite  appears  (Paradoxides,  Fig.  382  5,  p.  412)  at  about 
the  time  that  the  Olenellus  drops  out.  The  Upper  Cambrian  like- 
wise is  distinguished  by  the  presence  of  another  genus  (Dicellocepha- 
lus,  Fig.  382  C,  p.  412).  The  fact  that  these  trilobites  are  practically 
restricted  to  one  series  each  has  given  rise  to  the  use  of  their  names  in 
indicating  the  divisions  of  the  system.  Thus  the  life  of  the  Lower 
Cambrian  is  spoken  of  as  the  Olenellus  fauna,  that  of  the  Middle 
Cambrian  as  the  Paradoxides  fauna,  and  that  of  the  Upper  Cambrian 
as  the  Dicellocephalus  fauna.  Not  only  are  certain  trilobites  char- 
acteristic of  these  three  epochs  of  the  Cambrian,  but  other  forms  of 
life  as  well,  so  that  even  though  trilobites  are  absent,  the  age  of  the 
rocks  can  be  determined  by  other  genera  and  species.  Although 
certain  genera  and  species  are  practically  confined  to  one  formation, 
others  have  a  wide  vertical  range,  i.e.,  are  found  in  several  formations. 
Such  fossils,  while  showing  that  the  rocks  are  of  Cambrian  age,  do 
not,  without  the  presence  of  those  of  more  restricted  vertical  range, 
tell  to  which  series  they  belong. 

Location  of  Cambrian  Rocks.  —  The  Cambrian  formations  outcrop 
around  the  borders  of  the  Pre-Cambrian  rocks;  as,  for  example,  on 


THE   CAMBRIAN   PERIOD 


403 


the  border  of  the  Pre-Cambrian  shield  (p.  389)  and  the  Pre-Cambrian 
mass  of  the  Adirondacks,  and  in  regions  where  the  Cambrian  has  been 
exposed  by  the  deep  erosion  of  regions  which  have  been  raised  and 
folded,  as  in  the  folded  Appalachians,  from  the  St.  Lawrence  to  Ala- 
bama, and  in  portions  of  the  West.  For  the  most  part,  however, 
Cambrian  rocks  in 
North  America,  al- 
though of  wide  ex- 
tent, are  not  exposed 
at  the  surface  over 
large  areas,  being 
deeply  buried  under 
younger  strata. 

Physical  Geog- 
raphy of  Ancient 
Periods.  —  The  de- 
termination of  the 
distribution  of  land 
and  water  in  such 
remote  periods  as 
the  Cambrian  is  very 
difficult,  and  at  best 
the  outlines  of  the 
continents,  oceans, 
and  seas  are  only  ap- 
proximately known. 
Maps  of  the  kind 
shown  here  (Figs. 

J/J>    J/4/    «•  •  pIG    373. —  Map  showing  the  probable  distribution  of 

upon  several  lines  ot     land  and  water  in  North  America  during  Lower  Cambrian 

evidence.  times.     The  shaded  portion  is  land.    The  Lower  Cambrian 

(i)  When  the  fos-    sedimentswere  laid  down  in  long,  narrow  straits.   (Modified 

after  Schuchert.) 
sils  of  a  formation  of 

known  age  are  found  to  be  of  practically  the  same  species  in  outcrops 
that  are  widely  separated,  it  is  assumed  that  the  waters  in  which 
they  lived  were  either  connected  by  broad  straits,  or,  if  nothing 
points  to  a  different  conclusion,  that  they  inhabited  the  same  seas. 
If,  however,  they  are  found  to  differ  widely  in  species  in  regions 
which  may,  for  example,  be  less  than  fifty  miles  apart,  although 
the  conditions  under  which  they  lived  were  apparently  the  same, 


4o4 


HISTORICAL  GEOLOGY 


it  is  assumed  that  the  seas  which  they  inhabited  were  separated  by 
dry  land  or  other  barrier  to  their  spread.  Here,  however,  is  an 
opportunity  for  error,  since  currents  of  cold  water  are  favorable  for 
one  fauna,  while  in  the  warm  waters  of  the  same  sea,  a  short  distance 
away,  a  very  different  assemblage  of  animals  may  flourish.  Such 

a  distribution  has 
often  been  reported 
from  the  seas  of  to- 
day. This  objection 
is  not  as  serious  as  at 
first  appears,  since 
during  much  of  the 
geologic  past  climatic 
zones  were  probably 
not  as  well  established 
as  now. 

(2)  The  character 
of  the  deposits  fur- 
nishes aid  in  deter- 
mining ancient  shore 
lines.  If  a  certain 
formation  is  a  con- 
glomerate, it  is  evi- 
dent that  it  was  laid 
down  at  or  near  the 
shore,  since  only 
strong  waves  and 
currents,  such  as  are 
effective  in  shallow 
waters,  are  able  to 
move  coarse  gravel. 
Sandstones  are  also 

good  indicators  of  shores,  or  at  least  nearness  of  land.  Muds  point 
to  shallow  seas,  while  limestones  are  indicative  of  seas  of  wider  ex- 
tent, with  more  distant  shores  in  which  the  accumulation  of  lime 
carbonate  from  the  remains  of  shell-bearing  and  coral-secreting  an- 
imals, and  that  chemically  precipitated,  was  built  up  with  little  inter- 
mixture of  muds  and  sands. 

(3)  The  above,  as  Well  as  other  evidences  of  which  space  will  not 
permit  mention,  taken  in  connection  with  the  distribution  of  the 


FIG.  374.  —  Map  showing  the  probable  distribution  of 
land  and  water  in  the  Upper  Cambrian.  The  shaded 
portions  are  land.  (Modified  after  Schuchert.) 


THE  CAMBRIAN  PERIOD 


405 


formations  as  shown  on  geological  maps,  gives  a  clue  to  the  extent 
of  the  continents  and  the  positions  of  the  shallow  seas  (epiconti- 
nental ;  Greek,  epi,  upon)  which  at  various  times  in  the  past  covered 
large  areas  of  what  is  now  land.  These  maps,  showing  the  distri- 
bution of  the  land  and  water  in  ancient  periods,  must  be  considered 
as  mere  approximations,  since  (i)  the  absence  of  strata  does  not  al- 
ways prove  the  absence  of  seas  in  the  past  in  any  particular  region, 
because  if  the  strata  had  been  laid  down  they 
might  have  been  subsequently  carried  away 
by  erosion.  For  example  (Fig.  375),  80  miles 
from  the  nearest  rocks  of  a  certain  age  (De- 


JLJL 


vonian)  in  Illinois,  fossils  of  this  age  were  found  ,FlGl  37S'  ~  Devonian 
'          .            ,         r     i  j                Jo-i      •      \      i  sediments,   A,   found   in 
in  a  nssure  in  rocks  ot  older  age  (Silurian),  the  fissures  of  Silurian  lime- 
strata   of    the    former    having    been    entirely  stone-    This  is  the  prin- 

eroded    away.      If  this    accidental    discovery     cipal  evidence  that  De~ 

J      vonian  strata  at  one  time 

had  not  been  made,  there  would  have  been  covered  an  area  in  Illinois, 
doubt  as  to  the  extension  of  the  seas  in  De- 
vonian times.  Also,  in  the  buried  extensions  of  strata  there  may  be 
many  interruptions  where  islands  and  peninsulas  formerly  existed. 
(2)  Much  of  the  strata  is  often  buried  deeply  under  younger  forma- 
tions, and  its  distribution  in  such  regions  is  uncertain. 

Basal  Unconformity.  —  The  lower  layers  of  the  Cambrian  forma- 
tions usually  rest  upon  the  eroded  surface  of  older  rocks,  showing 
that  at  the  close  of  the  Pre-Cambrian  the  continent  of  North  America 
was  probably  even  larger  than  at  present.  The  comparative  levelness 
of  the  Pre-Cambrian  surface,  except  where  it  has  been  deformed  by 
later  movements,  indicates  that  erosion  had  been  active  and  that 
the  land  had  been  reduced  to  a  comparatively  level  plain  (peneplain, 
p.  114).  Upon  such  a  surface  the  sea  appears  to  have  gradually  en- 
croached. The  reason  for  the  spread  of  the  water  may  be  found  either 
(i)  in  the  actual  sinking  of  the  land  or  (2)  in  the  raising  of  the  sea 
level  in  an  amount  equal  to  the  volume  of  the  sediments  which  were 
being  carried  into  the  sea,  displacing  the  water  and  causing  it  to  over- 
flow the  land.  As  the  sea  encroached  upon  the  land,  it  left  upon  its 
ancient  surfaces  the  coarse  gravels  and  sands  composed  of  fragments 
of  the  older  rocks,  which  occur  at  the  base  of  the  Cambrian  system  and 
constitute  the  "  basal  conglomerate." 

Physical  Geography  of  the  Cambrian.  —  On  evidence  such  as  that 
already  mentioned  (p.  403),  it  has  been  found  that  at  the  beginning 
of  the  Cambrian  (Fig.  373)  the  continent  of  North  America  was  much 


406  HISTORICAL  GEOLOGY 

expanded,  the  Atlantic  shore  being  farther  east  than  now.  On  the 
east  a  narrow  sea  stretched  from  Alabama  northeast  to  Labrador, 
separated  from  the  ocean  by  a  land  of  unknown  eastern  extent  called 
Appalachia,  but  whose  western  shore  line  was  drawn  near  the  site 
of  the  present  Blue  Ridge.  In  the  west  a  similar  sea  existed  which, 
at  its  greatest  extent,  reached  from  California  to  the  Arctic  Ocean. 

The  submergence  of  the  continent  continued  in  the  Middle  Cam- 
brian, at  which  time  a  portion  of  the  central  United  States  was  covered 
by  seas  whose  shallowness  is  shown  by  ripple  marks  in  the  sandstone, 
and  even  by  sun  cracks  made  by  the  drying  out  of  sediments  exposed 
to  the  sun's  heat.  In  the  Upper  Cambrian  (Fig.  374)  the  seas  spread 
over  portions  of  the  continent  which  were  land  in  the  Middle  Cam- 
brian and  were  withdrawn  from  others  which  had  been  covered  by 
the  Middle  Cambrian  seas,  and  in  still  other  portions  the  sedimenta- 
tion continued,  showing  that  the  seas  remained  as  before.  Conse- 
quently near  the  close  of  the  Cambrian  the  physical  geography  was 
very  different  from  that  in  the  early  epoch,  the  water  covering  a  much 
larger  area  than  in  the  latter. 

Character  of  the  Cambrian  Rocks.  —  The  Cambrian  formations  are 
composed  of  sedimentary  rocks  which  vary  in  character  from  place 
to  place.  Where  the  sea  advanced  over  a  low  shore  in  which  there 
was  an  abundance  of  soil  or  other  loose  material,  the  waves  and  cur- 
rents worked  them  over  and  spread  them  upon  the  sea  bottom.  Such 
was  doubtless  the  origin  of  the  Middle  and  Upper  Cambrian  sand- 
stones which  are  so  widespread  in  the  interior  of  the  United  States. 
The  occurrence  of  limestones  and  shales  in  the  West  and  in  the 
Appalachian  Mountains  indicates  either  that  the  shores  were  distant 
in  these  regions,  or  were  so  low  that  the  gradients  of  the  streams  were 
insufficient  to  permit  the  latter  to  move  any  but  fine  material  and  such 
salts  as  were  in  solution. 

The  thickness  of  the  formations  of  the  period  varies  from  a  few 
hundred  to  twelve  thousand  feet.  This  variation  is  due  to  the  fact 
(i)  that  in  some  places  deposition  took  place  longer  than  in  others, 
and  (2)  that  in  other  places  where  erosion  was  rapid  and  the  condi- 
tions favorable  to  sedimentation,  the  ocean  bottom  was  built  up 
rapidly  by  the  sand  and  gravel  brought  in  by  the  streams  and  waves. 
(3)  In  other  regions,  where  the  land  was  low,  or  (4)  in  portions  of  the 
seas  distant  from  the  shore,  the  sedimentation  may  have  taken  place 
with  extreme  slowness,  so  that  in  thousands  of  years  the  thickness 
of  sediment  accumulated  was  a  small  fraction  of  that  laid  down  in  an 


THE   CAMBRIAN   PERIOD 


407 


equal  time  in  more  favorable  locations.  This  should  be  kept  in  mind 
in  future  discussions,  since  too  often  the  student  forgets  that  a  com- 
paratively thin  formation  may  have  required  in  its  upbuilding  as  long 
or  a  longer  time  than  a  much  thicker  one  of  different  material. 

Present  Condition  of  the  Sediments.  —  Some  of  the  Cambrian  sedi- 
ments have  undergone  important  changes  since  their  deposition.    The 


FIG.  376.  —  Section  showing  the  relation  of  the  Cambrian,  Cs,  and  overlying  strata  to 
the  Pre-Cambrian  gneiss,  gn.     Crested  Butte,  Colorado. 

gravels  have  been  changed  to  hard  conglomerates ;  the  sands  to  sand- 
stones, and  where  the  quartz  grains  have  been  cemented  by  quartz, 
into  flint-like  quartzites ;  the  calcareous  ooze  of  the  clear  seas  into 
limestone.  When  metamorphism  has  been  intense,  shales  have  been 
converted  into  slates  and  schists,  sandstones  into  schists,  and  lime- 
stones into  marble.  All  of  the  Cambrian  formations,  however,  have 
not  been  metamorphosed,  some  having  been  little  changed.  In  many 
places  the  Cambrian  strata  have  been  intensely  folded,  tilted,  and 
faulted  (Fig.  376).  Some  of  the  mountain  ridges  of  the  Appalachians 
are  formed  of  the  hard,  upturned  edges  of  the  quartzites  of  this  age. 
In  other  regions,  as  in  Wisconsin  and  northern  Minnesota  (Fig.  377), 


DRIFT 


FIG.  377.  —  A  section  in  northern  Minnesota,  showing  the  relation  of  the  Cambrian 
to  the  Pre-Cambrian  strata. 

where  the  comparatively  thin  beds  are  not  folded,  the  formations 
spread  over  a  wide  extent  of  territory. 

Volcanism.  —  The  Cambrian  seems  to  have  been  a  time  of  little 
volcanic  activity  over  the  greater  part  of  the  world.  In  North 
America  scarcely  a  trace  of  volcanic  material  has  been  discovered. 
Scotland  and  Wales,  however,  were  the  scenes  of  intense  volcanic 
activity. 

Close  of  the  Cambrian.  —  The  Cambrian  is  not  separated  from 
the  rocks  of  the  overlying  system  (Ordovician)  by  great  unconformi- 
ties, although  local  ones  exist,  but  so  gradual  was  the  change  that  it 


4o8  HISTORICAL  GEOLOGY 

is  often  difficult  to  draw  a  line  between  them.  In  fact,  it  has  been 
suggested  (Ulrich)  1  that  the  former  dividing  line  be  disregarded  and 
the  Upper  Cambrian  and  a  portion  of  the  Lower  Ordovician  consti- 
tute a  separate  system  called  the  Ozarkian. 

Other  Continents.  —  The  Cambrian  system  is  represented  in  Wales 
(20,000  feet)  and  Brittany  by  formations  of  great  thickness.  It  also 
occurs  in  Scandinavia,  Russia,  Siberia,  China,  India,  Australia,  Ar- 
gentina, and  other  parts  of  the  world. 

There  appears  to  have  been  a  land  connection  between  North 
America  and  Europe,  or  at  least  a  chain  of  islands  separated  by  shal- 
low water,  in  the  Cambrian.  This  is  shown  by  the  strong  resemblance 
of  the  fossils  of  this  age  in  eastern  North  America  and  Europe,  a 
similarity  which  would  not  have  been  possible  had  the  animals  in- 
habiting the  shallow  waters  of  the  shores  been  unable  to  migrate  from 
one  continent  to  the  other. 


LIFE  OF  THE  CAMBRIAN 

The  richness  of  the  life  of  the  Cambrian  is  in  marked  contrast  to 
that  of  the  Pre-Cambrian,  although  the  presence  of  worm  trails  and  a 
highly  developed  crustacean  (Beltina  danai,  Fig.  371,  p.  397)  in  the 
latter  indicates  that  the  life  of  that  ancient  time  comprised  many 
forms  of  invertebrates.  However,  so  few  specimens  have  been  found 
and  so  obscure  is  the  evidence  that  little  more  can  be  said  at  present 
than  that  the  facts  indicate  that  life  was  well  developed  before 
Cambrian  times  began. 

The  apparently  abrupt  appearance2  of  the  earliest  known  Cambrian 
fauna  is  probably  to  be  explained  by  the  absence  on  our  present  land 
areas  of  the  sediments  and  fossils  of  the  period  between  the  Protero- 
zoic  and  the  Cambrian.  This  resulted  from  the  continental  area's 
being  above  sea  level  during  the  development  of  the  unknown  ances- 
try of  the  Cambrian  fauna,3  and  consequently  the  sediments  of  that 
time  are  now  covered  by  the  sea  and  cannot  be  studied. 

The  indirect  evidence  of  the  existence  of  life  long  antedating  the 
Cambrian  is  even  stronger  than  the  direct.  A  comparison  of  the  life 
of  the  Cambrian  with  that  of  to-day  shows  that  of  the  eight  branches 

1  Ulrich,  E.  O.,  —  Revision  of  the  Paleozoic  Systems:  Bull.  Geol.  Soc.  America,  Vol.  22, 
IQII,  pp.  281-680. 

2  Walcott,  C.  D.,  —  Abrupt  Appearance  of  the  Cambrian  Fauna  on  the  North  American  Conti- 
nent: Smithsonian  Misc.  Coll.,  Vol.  57,  No.  i,  1910,  pp.  1-16. 

3  The  term  fauna  means  the  total  animal  life  of  a  certain  region  or  period. 


THE  CAMBRIAN  PERIOD  409 

of  the  animal  kingdom  all  except  the  vertebrates  have  representatives 
in  the  former.  If,  as  is  generally  believed,  this  differentiation  was  the 
result  of  slow  evolutional  changes,  it  is  probable  that  a  greater  length 
of  time  was  required  to  produce  such  a  divergence  than  for  all  the 
changes  in  life  that  have  taken  place  since  the  Cambrian. 

Another  indirect  evidence  is  found  in  embryology.  Each  animal 
in  its  development  from  the  egg  to  the  adult  condition  passes  through 
a  series  of  stages  which  resemble  those  through  which  the  race  passed 
in  its  evolution,  many  embryonic  stages  representing  those  of  mature 
but  remote  ancestors.  It  is  evident,  therefore,  that  the  embryonic 
and  larval  stages  of  the  individual  furnish  somewhat  of  a  basis  upon 
which  to  estimate  the  length  of  the  evolutional  history  of  the  race 
to  which  the  individual  belonged.  Some  of  the  larval  stages  of  the 
trilobites  are  preserved  and  give  firm  ground  for  the  belief  that  this 
class  had  a  long  line  of  ancestors  previous  to  the  Cambrian. 

PLANTS 

Since  all  animals  depend  directly  or  indirectly  upon  vegetation  for 
their  food,  it  is  evident  that  plants  must  have  been  in  existence  in 
large  numbers  in  the  Cambrian  in  order  to  supply 
with  food  the  abundant  marine  animal  life  of 
that  time.  When,  however,  a  search  for  plant 
fossils  is  made,  none  are  found  that  can  with 
certainty  be  recognized  as  plant  remains.  The 
inference  is  forced  upon  us  that  Cambrian  plants 
were  not  highly  organized,  and  that  they  possessed  FIG  _ A  prob- 
little  or  no  woody  tissue  and  were,  consequently,  iematical  fossil,  Old- 
incapable  of  fossilization.  Some  poorly  defined,  hamia  antiqua,  which 
,.,  .  .  r  j  i  r^  u  '  has  been  sometimes 

stemlike    impressions    found    in    the    Cambrian    considered    to    be    a 

strata    at    Burlington,    Vermont,    and    elsewhere    piant. 
strongly  suggest  the  stems  of  seaweeds,  but  some 
of  these  may  be  worm  tracks ;   some,  rill  marks ;  and  some,  tradings 
made  by  animals.     The  difficulty  in  determining  such  "  fossils  "  is 
well  shown   in   the  controversy  over  the  determination  of  certain 
specimens  (Oldhamia)  found  in  the  Cambrian  rock  of  Ireland,  which, 
as  the  illustration  shows  (Fig.  378),  have  the  appearance  of  vege- 
table growth.     Some  investigators  have  classed  them  as  the  remains 
of  animals,  some  as  plants,  and  some  as  inorganic  markings. 

The  scarcity  of  plant  fossils  may  perhaps  be  attributed  to  the  fact 
that  only  Cambrian  rocks  of  marine  origin  have  been  studied,  since 


410 


HISTORICAL  GEOLOGY 


it  is  seldom,  even  in  the  sediments  that  are  being  deposited  to-day, 
that  plants  are  embedded  in  marine  sediments. 

Small  calcareous  algae  have  been  found  in  the  Cambrian  rock  of 
the  Antarctic  Continent. and  elsewhere,  and  it  appears  probable  that 
plants  which  secreted  lime  played  an  important  part  in  the  formation 
of  the  Cambrian  limestone.1 

ANIMALS 

The  oldest  known  fossiliferous  rocks  of  the  Cambrian  contain  a 
varied  fauna.  Corals,  sponges,  worms  (in  the  form  of  trails  and 
borings  and  impressions),  brachiopods,  pteropods,  and  crustaceans 
have  been  identified,  and  there  is  no  doubt  that  the  discoveries  have 
brought  to  light  only  a  fraction  of  the  life  of  the  time.  Doubtless  the 
lowly  protozoans  were  in  existence  at  that  time,  as  well  as  other  classes 
which  have  not  been  found.  This  diverse  life,  as  has  been  shown,  did 
not  arise  suddenly,  but  was  derived  from  a  long  line  of  ancestors  which 
lived  during  Proterozoic  times. 

Crustacea 

Trilobites.  —  The  highest  and  most  striking  form  of  life  of  the 
period  was  the  trilobites  (Greek,  tri-,  three,  and  lobos,  a  lobe  or  rounded 


FIG.  379.  —  Dorsal  and  ventral  views  of  an  Ordovician  trilobite,  showing 
the  restored  appendages.     (Beecher.) 

1  Garwood,  E.  J.,  — Nature,  Vol.  92,  1913,  p.  114. 


THE  CAMBRIAN  PERIOD 


411 


projection),    a   group   of 

animals  belonging  to  the 

same     phylum     as     the 

crabs  and  lobsters.     The 

name  trilobite  is  a  very 

descriptive  one,  since  the 

animal   was  marked    by 

two      grooves      running 

lengthwise  of  the   body, 

which    divided     it     into 

three,  usually  well- 
marked  lobes.  Trans- 
versely, there  were  also 

three  divisions  :  the  head 

shield     (cephalon) ;     the 

body,  composed  of  jointed 

segments    (thorax) ;    and 

the  tail  shield  (pygidium). 

Trilobites    were    marine 

animals  and  had  delicate 

antennae,    doubtless    for 

touch,  and  numerous  legs 

and      breathing     organs 

(Fig.    379).     They   were 

probably    able    both    to 

walk   and    swim.     Their 

trails  and  burrows  show 

that  they  burrowed  and  pushed  their  way  through  the  muds  and  soft 

sands.  A  series  of  tracks  probably 
made  by  a  trilobite  is  shown  in 
Figure  380. 

The  eyes  of  trilobites  were  usually 
raised,  crescent-shaped  elevations 
and  were  compound  like  those  of  an 
insect  (Fig.  381),  the  number  of 
lenses  in  each  age  varying  in  differ- 
ent species  from  14,000  to  15,000. 
A  few  species  were  eyeless.  Cam- 
brian trilobites  (Fig.  382  A,  B,  C,  D) 
varied  greatly  in  size,  in  form,  and  in 


FIG.   380.  —  Tracks  supposed  to  have  been  made  by 
a  trilobite. 


FIG.  381.  —  Eye  of  a  Devonian 
trilobite,  much  enlarged,  showing  the 
many  eyelets  forming  the  compound 
eye.  (N.  Y.  Geol.  Surv.) 


4I2 


HISTORICAL  GEOLOGY 


ornamentation  :  some  were  a  fraction  of  an  inch  long  (Agnostus,  Fig. 
382  D),  while  others  (Paradoxides,  Fig.  382  B)  attained  a  length  of 
from  one  to  two  feet ;  some  had  a  smooth  surface  and  few  body  seg- 
ments (two  in  Agnostus),  while  others  were  ornamented  with  spines 
and  had  a  large  number  of  body  segments  (Paradoxides  had  from  17 


0 


FIG.  382.  —  Cambrian  crustaceans.  Trilobites :  A,  Olenellus  thompsoni;  B, 
Paradoxides  harlani;  C,  Dicellocephalus  minnesotensis ;  D,  Agnostus  inter slrictus. 
Other  crustaceans  :  .£,  Hymenocaris  perfecta;  F,  Naraoia  compacta.  (After  Walcott.) 

to  20) ;  some  had  large  eyes,  while  others  were  eyeless.  These  features 
show  that  the  trilobite  race  must  have  extended  far  back  into  Pre- 
Cambrian  times,  since  such  a  great  diversity  of  form  and  structure 
could  only  be  developed  as  a  result  of  long  evolution.  Although 
trilobites  varied  greatly,  it  should  be  distinctly  understood  that  in 


THE  CAMBRIAN  PERIOD 


413 


nearly  every  particular  they  were  very  primitive  or  simple  in  struc- 
ture and  closely  agree  with  a  theoretical  crustacean  ancestor. 

Since  trilobites  moulted  their  shells  at  certain  times  and  the  great 
majority  of  their  fossils  consist  of  these  fragments,  a  complete  speci- 
men usually  indicates  the  death  of  an  individual. 

Not  only  were  trilobites  the  most  conspicuous  animals  of  the  period, 
but  since  new  species  and  genera  appeared,  while  the  older  became  ex- 
tinct, they  furnish  the  best  means  of  correlating  the  formations  of 
different  continents  and  of  widely  separated  portions  of  the  same  con- 
tinent. The  three  divisions  of  the  system,  as  has  been  seen  (p.  402), 
are  consequently  named  for  the  three  dominant  genera  of  trilobites : 
the  Lower  or  Olenellus  zone,  the  Middle  or  Paradoxides  zone,  and  the 
Upper  or  Dicellocephalus  zone. 

Other  Crustaceans.  —  In  addition  to  trilobites  a  number  of  crus- 
taceans (Fig.  382  E,  F)  of  a  different  group,  representatives  of 
which  are  living  to-day,  have  been  found  in  the  Middle  Cambrian 
of  British  Columbia.  That  a  large  and  varied  crustacean  fauna  pre- 
ceded these  is  certain. 

Mollusca 

Gastropods  (Univalves). — This  class  is  now  represented  by  snails, 
conchs,  and  winkles.  The  most  conspicuous  feature  of  the  shelled 

the    single, 
spiral     shell. 


is 


forms 
usually 

Gastropods  lived 
throughout  the  period 
but  were  seldom  abun- 
dant. The  earliest 
forms  were  chiefly 
simple,  conical  shells 
(Fig.  383  C,  E),  while 
later  in  the  period 
coiled  and  spiral  forms 
(Fig.  383  B,  D)  be- 
came  more  common. 
Some  of  the  spiral 
forms  bear  a  close  resemblance  to  some  modern  gastropods. 

A  division  of  the  gastropods,  the  pteropods,  was  well  represented  in 
the  Cambrian.  The  fossils  usually  consisted  of  simple,  conical  shells 
(Fig.  383  A).  Several  specimens  have  been  discovered  with  distinct 
impressions  of  the  characteristic  fleshy  portions. 


A 


FIG.  383.  —  Cambrian  gastropods:  A,  Hyolithes  cari- 
natus;  By  Pelagiella  (Platyceras)  primcevum;  C,  Scenella 
varians;  D,  Trochus  sarat ogen sis ;  E,  Stenotheca  rugosa. 


HISTORICAL  GEOLOGY 


Molluscoidea 

Brachiopods.  —  This  great  class  was  especially  important  in  the 
Paleozoic,  not  only  because  of  the  abundance  of  individuals,  but  also 
because  certain  species,  though  prolific,  were  short-lived,  being  abun- 
dant in  one  period  or  a  subdivision  of  one  period  and  becoming  ex- 
tinct at  its  close.  As  a  result,  when  the  fossil  remains  of  such  species 
are  found  in  a  stratum,  proof  is  offered  of  the  age  of  the  formation. 
Brachiopods  or  Lamp  Shells  (so-called  because  of  the  resemblance  of 
some  of  them  (Fig.  384  A,  F)  to  Roman  lamps)  are  inclosed  by 
two  shells  or  valves  and  can  usually  be  readily  distinguished  from 


FIG.  384.  —  Cambrian  brachiopods  :  A,  Billingsella  color adoensis ;  B,  Lingulepis 
acuminata;  Cy  Obolella  atlantica;  "  D,  Acrothele  subsidua;  E,  Micromitra  bella;  F, 
Kutorgina  cingulata. 

other  shellfish  by  two  characteristic  features  :  (i)  the  bilateral  sym- 
metry of  their  shells,  i.e.,  a  line  drawn  from  the  beak  to  the  front  di- 
vides them  into  equal  parts;  and  (2),  in  most  cases,  by  the  dissimi- 
larity and  unequal  size  of  the  two  valves.  The  name  brachiopod 
(Greek,  brachion,  arm,  and  pous,  foot)  refers  to  the  two  long  spiral 
"  arms  "  inclosed  between  the  valves  by  means  of  which  food  is  ob- 
tained and  respiration  carried  on.  These  "  arms  "  are  attached  to  a 
shelly  apparatus,  sometimes  in  the  form  of  loops  and  sometimes  in 
spirals  (Fig.  403  M,  p.  432). 

Brachiopods  are  divided  into  two  great  subdivisions  :  the  hingeless 
(Fig.  384  B,  D,  E)  or  inarticulate,  with  phosphate  of  lime  shells  the 
two  valves  of  which  were  held  together  only  by  the  muscles  of  the 
animal ;  and  the  hinged  (Fig.  384  A,  C)  or  articulate,  with  calcareous 


THE  CAMBRIAN  PERIOD 


415 


shells  and  well-developed  hinges  and  dissimilar  valves. 
Of  these  two  subdivisions  the  first  and  most  primitive 
was  more  abundant  in  the  Cambrian,  and  the  second, 
later  in  the  Paleozoic.  In  the  Lower  Cambrian  22 
genera  of  brachiopods  have  been  found  in  Europe  and 
North  America,  showing  that  the  class  was  probably 
well-developed  in  the  preceding  era  (Proterozoic). 
The  two  subdivisions  of  brachiopods  are  living  in  the 
seas  of  the  present,  having  undergone  many  changes 
during  their  long  existence;  yet  the  class  as  a  whole 
has  been  little  modified  since  Cambrian  times. 

Echinodermata 

Cystoids  (Fig.  385)  of  very  simple  structure  lived  in 
the  Cambrian,  and  sea  cucumbers  have  been  discovered    toid :  Eocystites 
in  Middle  Cambrian  strata  in  British  Columbia.  longidactylus. 

Worms 

Perhaps  the  fossjk  next  in  abundance  to  the  trilobites  are  the  trails 
and  borings  of  worms.  In  certain  Lower  Cambrian  beds  vertical 
tubes  (Scolithus)  are  so  common  as  to  give  a 
striped  appearance  to  the  rock.  Many  fossils 
which  were  at  one  time  thought  to  be  fossil 
marine  plants  are  now  known  to  be  the  trails 
of  worms,  or  borings  which  have  been  filled 
with  sand  or  clay.  Since  worms  are,  as  a  rule, 
destitute  of  hard  parts,  it  is  seldom  that  any 
traces  of  the  actual  animals  have  been  pre- 
served. In  some  fine  shales  of  the  Middle 
Cambrian  in  British  Columbia,  however,  the 
fleshy  parts  of  the  animal  are  sometimes  pre- 
served as  a  glistening  surface,  even  to  the  fine 
details  of  the  structure  (Fig.  386  A,  B). 


FIG.  386.  —  Cambrian 
worms :  A,  Ottoia  -pro- 
lific a;  B,  Wortkenella 
cambria.  These  fossils 
are  represented  by  a 
thin  film  which  is  darker 


Ccelenterata 

Corals.  —  Cambrian   corals   (Fig.  387)   were 

than  the  shale  contain-    so  simple  in  structure  that  some  of  them  have 
ing  them,  the  contents    ^een  C2L\\ed  sponges  by  certain  writers  and  corals 


face. 


by  others.     This  group  was  abundant  locally, 
(After  Walcott.)      but  in  general  was  rare  throughout  the  period. 

CLELAND   GEOL.  —  27 


416  .    HISTORICAL  GEOLOGY 

Graptolites.  —  This  group  will  be  discussed  more  fully  in  the  next 
chapter  (p.  427),  since  it  reached  its  greatest  abundance  and  develop- 
ment in  the  Ordovician.  The  word  graptolite  (Greek,  graptos,  written, 
and  lithos,  a  stone)  is  descriptive,  since,  when 
preserved  in  shale,  as  is  usually  the  case,  grapto- 
lites  have  the  appearance  of  lead-pencil  marks 
(Fig.  397,  p.  428)  with  saw  teeth  on  one  or  both 
sides.  Graptolites  were  slender  organisms,  plant- 
like  in  appearance,  usually  resembling  the  modern 
hydroids.  As  is  true  of  hydroids,  they  were  com- 
FIG.  3.87. —  Cam-  posite  animals  in  which  the  individuals  lived  in 
«*U.  strung  on  one  or  both  sides  of  a  slender, 
horny  axis  which  united  the  "  colony."  The 
form  of  these  colonies  varied  greatly,  as  will  be  shown  later  (p.  428). 
Graptolites  appear  for  the  first  time  in  the  Upper  Cambrian. 

Jellyfish.  —  The  discovery  of  fossil  jellyfish  in  Cambrian  rocks  is 
most  surprising,  since  these  animals  have  no  bony  skeletons  or  shells. 
Specimens  preserving  the  external  form  as  well  as  something  of  the 
interior  structure  have  been  found.  They  musl^nve  been  buried  in 
mud  soon  after  they  died,  for  otherwise  they  would  have  been  de- 
stroyed by  the  worms  and  predatory  crustaceans  associated  with  them. 
Sponges  lived  in  some  abundance  in  portions  of  the  Cambrian  and 
were  represented  by  several  genera.  They  are  known  by  their  sili- 
ceous spicules,  which  were  either  embedded  in  horny  fibers  or  inter- 
laced into  a  supporting  framework,  and  which  were  preserved  because 
of  their  resistant  character. 

Protozoa 

Theoretically  there  is  every  reason  to  believe  that  the  simple 
unicellular  protozoa  were  as  abundant  in  the  Cambrian  seas  as  in  the 
present  oceans,  but  no  fossils  have  been  discovered  which  are  known 
with  certainty  to  belong  to  this  group. 

SUMMARY 

Evolution  during  the  Cambrian.  —  It  has  been  seen  that  at  the 
beginning  of  the  Cambrian  many  classes  of  animals  were  already 
in  existence,  and  that  the  advanced  stage  of  development  of  some  of 
them,  notably  the  trilobites,  taken  in  connection  with  the  traces  of 
Pre-Cambrian  life,  indicates  that  life  was  well-advanced  before 
Cambrian  time  began.  Whether  or  not  the  evolution  of  this  Pre- 
Cambrian  life  was  rapid  is  not  known.  Evolution  during  the  Cam- 


THE   CAMBRIAN   PERIOD  417 

brian  was  continuous,  but  more  rapid  at  certain  times  than  at  others, 
the  fauna  at  the  close  of  the  period  being  distinctly  more  advanced 
than  that  at  the  beginning.  The  evolution  of  life  was  profoundly 
influenced  by  environment,  this  perhaps  more  than  any  other  cause 
being  responsible  for  the  marked  difference  between  the  faunas  of 
the  Lower  and  Upper  Cambrian. 

Since  the  life  of  the  Cambrian  changed  from  time  to  time  during 
the  period,  a  study  of  the  fossils  of  any  stratum,  as  has  been  said, 
gives  definite  information  as  to  the  relative  age  of  the  beds  containing 
them.  This  change  in  the  fauna  was  brought  about  (i)  by  the  slow 
evolution  of  species  when  conditions  were  somewhat  uniform ;  (2)  by 
rapid  evolution  due  to  changes  in  environment,  such  as  occurred 
when  seas  were  enlarged,  shore  lines  shifted,  and  new  conditions  of 
food  and  temperature  imposed ;  (3)  by  competition  resulting  from  the 
immigration  of  large  numbers  of  new  species  from  other  regions,  which 
caused  the  extinction  of  many  species  and  the  modification  of  others. 

Climate  and  Duration.  —  The  widespread  occurrence  of  coral-like 
organisms  in  the  Lower  Cambrian  and  the  vast  numbers  of  individuals 
of  various  species^ltrilobites  and  other  classes  indicate  a  warm  and 
more  or  less  unifdmclimate.  In  fact,  throughout  at  least  the  greater 
part  of  the  period  the  character  and  distribution  of  the  fossils  imply 
nearly  uniform  climatic  conditions  over  the  entire  world. 

The  duration  of  the  Cambrian  is  to  be  expressed  in  terms  of  hun- 
dreds of  thousands  of  years.  The  time  required  to  remove  and  de- 
posit thousands  of  feet  of  rock  must  have  been  enormous.  If  lime- 
stone is  deposited  on  an  average  of  one  foot  a  century,  it  would  re- 
quire 600,000  years  for  the  accumulation  of  the  6000  feet  of  Cambrian 
limestone  of  some  portions  of  the  West,  omitting  the  time  necessary 
for  the  formation  of  the  thick  sandstones  of  the  same  regions.  Per- 
haps 1,000,000  years  may  be  placed  as  the  minimum  duration  of  the 
period  and  4.000,000  years  as  the  maximum. 

REFERENCES  FOR  THE  CAMBRIAN  PERIOD 
WALCOTT,  C.  D.,—  The  Cambrian  Faunas  of  North  America:  Bull.  U.  S.  Geol.  Surv. 

No.  10,  1885. 
WALCOTT,  C.  D.,  —  Studies  in  the  Cambrian  Faunas  of  North  America:    Bull.  U.  S. 

Geol.  Surv.  No.  30,  1886. 
WALCOTT,  C.  D.,  —  Fauna  of  the  Lower  Cambrian:  Tenth  Ann.  Rept.,  U.  S.  Geol. 

Surv.,  1890,  pp.  515-629. 

WALCOTT,  C.  D.,  —  Cambrian  Brachiopoda:   Mon.  51.  U.  S.  Geol.  Surv.,  1912. 
WALCOTT,  C.   D.,  —  Cambrian  Geology  and  Paleontology:    Smithsonian  Misc.  Coll. 

Vol.  53,  1908,  pp.  1-230;  and  Vol.  57,  1910,  pp.  1-16,  231-431. 


CHAPTER  XVI 


1 


THE   ORDOVICIAN   PERIOD 

THE  system  next  younger  than  the  Cambrian  is  the  Ordovician.1 
The  name  Lower  Silurian  has  been  replaced  by  the  above,  although 

still  occasionally  used 
by  writers. 

Ordovician  Physi- 
cal Geography.2  —  In 
North  America  dur- 
ing this  period  the 
epicontinental  seas 
^05)  varied  greatly 
and  position 
in  the  different  stages 
(Figs.  388,  389,  390, 
391),  shifting  so  often 
and  to  such  an  ex- 
tent that  an  attempt 
to  define  their  bor- 
ders would  require  a 
more  extended  de- 
scription than  seems 
advisable. 

In  general,  it  can 
be  said  that  the  lands 
about  the  epiconti- 
nental seas  were  low 
and  that  the 

shallow,   as 


seas 


were 


s 


FIG.  388.  —  Probable  distribution  of  land  and  water  in  the 
Lower  Ordovician.     (Modified  after  Schuchert.) 

1  Ordovici,  an  ancient  tribe  in  Wales ;  a  name  given  because  the  rocks  of  the  period  are  well- 
developed  in  Wales. 

8  For  the  physical  geography  of  the  different  epochs  of  this  and  subsequent  periods  the 
student  is  referred  to  Chas.  Schuchert, — Paleogeography  of  North  America,  Bull.  Geol.  Soc. 
America,  Vol.  20,  1910,  pp.  427-606,  which  contains  the  most  accurate  maps  of  these  remote 
periods. 

418 


THE  ORDOVICIAN  PERIOD 


419 


shown  by  the  character  of  the  sediments,  perhaps  not  exceeding  200 
to  300  feet  in  depth.  It  is  impossible  to  characterize  the  rocks  of  a 
system  in  a  few  words,  since  at  all  times  in  the  earth's  history  sedi- 
mentary deposits  of  every  description  were  being  laid  down  in  some 
portion  of  the  world.  This  is  also  true  of  the  Ordovician,  during 
which  gravels  and  sands  were  deposited  in  certain  places,  but  lime- 
stones and  shales  form  a  much  larger  proportion  of  the  deposits 
than  perhaps  in  any  other  Paleozoic  period.  One  of  the  physi- 
cal conditions  which  brought  this  about  was  the  limited  area  and 
probably  slight  elevation  of  the  land,  which  consequently  yielded 


FIG.  389.  — Probable  distribution  of  land  and  water  in  the  Middle  Ordovician. 
The  continent  has  probably  never  been  so  completely  submerged  since  that  time. 
(Modified  after  Schuchert.) 

little  sediment,  leaving  extensive  areas  of  the  seas  free  from  sands 
and  muds.  However,  although  built  up  largely  of  the  remains 
of  lime-secreting  animals,  such  as  brachiopods,  corals,  and  bryo- 


420 


HISTORICAL  GEOLOGY 


zoans,   yet    the   lime   was   ultimately    derived    from   the   land    by 
solution. 

In  the  Appalachian  trough  (Figs.  392,  393),  which  was  first  formed 
in  the  Cambrian,  sands,  muds,  and  limestones  were  laid  down.  The 
Appalachian  trough  was  separated  from  the  Atlantic  by  the  great 


I 


FIG.  390.  —  A  later  stage  of  the  Ordovician,  when  the  land  was  greatly  extended. 
(Modified  after  Schuchert,) 

island  or  continent  of  Appalachia,  whose  eastern  extent  was  unknown. 
Its  western  border  was  a  shifting  shore  which  during  certain  times  was 
west  of  the  present  Mississippi  River.  Sandstones  occur  in  the  West, 
where  islands  formerly  existed,  but  are  seldom  extensive.  In  New- 
foundland, Ottawa,  and  west  of  the  Adirondacks,  limestone  is  the 
prevalent  rock  of  the  system,  although  shales  are  abundant,  showing 
either  that  the  land  was  low  or  covered  by  vegetation  which  prevented 
rapid  erosion,  or  that  the  drainage  of  the  land  was  not  discharged  in 
the  direction  of  these  regions. 


THE  ORDOVICIAN  PERIOD 


421 


The  Ordovician  was  a  period  of  quiet,  during  which  the  epicon- 
tinental  seas  gradually  increased  until  the  middle  of  the  period  (Fig. 
389),  at  which  time  a  larger  portion  of  the  continent  was  under  water 
than  at  any  stage  since  the  Pre-Cambrian,  more  than  half  of  the  con- 
tinent being  at  this  time  submerged,  the  epicontinental  seas,  broken 


FIG.  391.  — A  stage  later  than  Fig.  390,  when  the  continent  was  again  greatly 
submerged.     (Modified  after  Schuchert.) 

by  peninsulas  and  large  and  small  islands,  extending  at  certain  times 
from  ocean  to  ocean.  The  seas  were  for  the  most  part  much  less  ex- 
tensive in  the  latter  part  of  the  period  (Fig.  390),  at  which  time  de- 
posits of  mud  were  laid  down  over  extensive  areas.  A  submergence 
almost  equal  to  that  earlier  in  the  period  (Fig.  389),  however,  again 
occurred  (Fig.  391),  and  epicontinental  seas  spread  widely  over  the 
continent.  At  the  close  of  the  period  these  seas  were  again  drained, 
and  the  outlines  of  the  continent  were  probably  not  unlike  those  of 
to-day. 


422 


HISTORICAL  GEOLOGY 


c  t 


il 


•€<-> 

a 


<U 

-S3 


s 


§1 
c  > 


schists. 


The  classic  section  of  the  Ordovician  in  the  United   States  is  in 
New  York,  where  it  was  first  extensively  studied. 
Pulaski  stage  (shale) 
Frankfort  stage  (shale) 
Utica  stage  (shale) 

Ordovician  I  Trenton  stage  (limestone) 
system         Black  River  stage  including  Lowville  limestone 
Chazy  stage  (limestone) 
Beekmantown  stage  (limestone) 
Tribes  Hill  stage  (limestone) 

In  this  region  limestone  deposits  prevailed  during  the  Lower 
(Canadian)  and  Middle  (Mohawkian)  Ordovician,  but  shales  in  the 
Upper  (Cincinnatian)  Ordovician. 

Close  of  the  Ordovician.  —  The  close  of  the  Ordo- 
vician was  marked  by  horizontal  and  vertical  move- 
ments of  considerable  importance  in  eastern  North 
America  (Taconic  deformation)  and  Great  Britain. 
During  the  Cambrian  and  Ordovician  in  North  America 
sediments  had  been  accumulating  in  a  subsiding  trough 
lying  between  the  Adirondack  land  mass  and  a  land 
mass  in  New  England,  and  which  stretched  from  the 
St.  Lawrence  River  to  the  City  of  New  York  and  to 
the  south  (Fig.  374,  p.  404).  These  sediments,  after 
having  been  accumulated  to  a  thickness  of  more  than 
a  mile,  were  subjected  to  great  lateral  pressure  which 
folded  them  and  brought  them  above  sea  level  to  form 

!  ^  a  mountain  range  of  which  the  present  Taconic  Moun- 
tains of  western  New  England  are  perhaps  rather 

$       insignificant   remnants.     The    folding  was    so    intense 

j  that  limestones  were  recrystallized  to  marbles,  of  which 
the  most  famous  are  those  of  Vermont  and  Massa- 
chusetts; the  sandstones  were  changed  to  quartzites 
and  schists,  and  the  muds  and  shales  to  slates  and 

These  disturbances  affected  the  region  east  of  the  Taconics, 


but  were  comparatively  local,  as   is   shown  by  the  slightly  folded 


FIG.  393.  —  An  east-west  section  through  the  Appalachian  Mountains  near  Mercers- 
burg,  Pennsylvania,  showing  the  relation  of  the  Ordovician,  Osr,  Ocy  Om,  Oj,  and  the 
Silurian,  Sc  and  St.  (U.  S.  Geol.  Surv.) 


THE  ORDOVICIAN  PERIOD 


423 


rocks  of  this  period  in  New  York,  New  Jersey,  and  Canada,  only 
short  distances  from  the  scene  of  maximum  deformation.  The 
region  north  of  the  St.  Lawrence  seems  to  have  been  little  affected, 
since  the  sedimentation  continued  from  the  Ordovician  to  the 
Silurian  with  slight  interruption,  almost  the  entire  record  being 
preserved  in  the  strata  of  Anticosti  Island  in  the  Gulf  of  St. 
Lawrence.  The  date  at  which  the  Taconic  deformation  occurred 
is  known,  because  the  Silurian  rocks  rest  upon  the  eroded  and 
upturned  edges  of  the  Ordovician  (Fig.  394),  snowing  that  after  the 
deformation  the  strata  were  elevated  and  eroded  for  many  years,  and 
were  again  covered  by  the  sea  and  Silurian  deposits  laid  down  on 


FIG.  394.  —  An  east-west  section  in  eastern  New  York,  showing  the  Silurian  rest- 
ing unconformably  upon  the  upturned  edges  of  the  Ordovician.  (After  W.  J.  Miller.) 
We  have  here  the  proof  that  this  portion  of  the  continent  was  raised  above  the  sea 
during  the  Ordovician,  and  that  after  the  land  was  eroded  it  sank,  and  upon  this  old 
land  surface  Silurian  sediments  were  deposited. 

them.  These  disturbances,  which  culminated  in  the  Taconic  deforma- 
tion and  in  the  draining  of  the  interior  seas,  were  of  long  duration. 

Cincinnati  Anticline.  —  The  first  evidence  of  a  deformative  move- 
ment in  the  Middle  States  is  found  in  the  formation  of  the  Cincinnati 
and  other  anticlines,  which  appeared  as  low  folds  in  the  Middle  Or- 
dovician (Trenton).  The  Cincinnati  arch,  though  later  submerged, 
was  again  elevated  and  greatly  enlarged  at  the  close  of  the  period. 
This  fold  extends  over  an  oval  area  in  Ohio,  Indiana,  and  Kentucky, 
with  the  longer  axis  in  a  north  and  south  direction. 

The  withdrawal  of  the  epicontinental  seas  at  this  time  may  have 
been  due  to  the  sinking  of  the  ocean  bottoms  (oceanic  segments, 
p.  366)  or  to  the  raising  of  the  land. 

Volcanism.  —  There  is  little  evidence  of  igneous  activity  in  North 
America  during  the  period,  although  in  England  and  Wales  great 
masses  of  lava  and  volcanic  ash  form  thick  strata.  Indeed  the 
Ordovician  volcanism  of  Great  Britain  was  one  of  the  most  extensive 
in  Europe  since  Pre-Cambrian  times. 

Ordovician  of  Other  Continents.  —  Ordovician  rocks  occur  in  Great 
Britain,  where  a  thickness  of  24,000  feet  has  been  measured.  A 


424  HISTORICAL  GEOLOGY 

deformation  comparable  to  that  in  North  America  folded,  crumpled, 
and  metamorphosed  the  Ordovician  strata  at  the  close  of  the  period. 
In  Scotland  the  folding  was  exceptionally  severe,  producing  over- 
turned folds  and  faults,  in  one  locality  thrusting  strata  ten  miles 
along  a  fault  plane. 

In  Europe,  although  the  Ordovician  often  underlies  the  Silurian 
unconformably,  the  disturbances  which  ushered  in  the  latter  appear 
to  have  been  slight.  On  both  continents  the  important  disturbances 
took  place  where  thick  beds  of  sediment  had  accumulated. 

PETROLEUM  AND  NATURAL  GAS 

Conditions  Favoring  the  Accumulation  of  Oil  and  Gas.  —  The  im- 
portance of  the  oil  and  gas  industry  is  such  that  the  essential  features 
of  their  geological  occurrence  demand  attention. 

Petroleum  and  natural  gas  occur  in  varying  quantities  in  all  of  the 
fossiliferous  rocks,  from  the  Ordovician  through  the  Tertiary,  but  oil 
never  occurs  in  paying  quantities  unless  there  is  a  porous  stratum 
overlain  by  an  impervious  one,  in  this  respect  resembling  artesian 
wells.  In  an  artesian  well,  however,  it  is  essential  that  the  porous 
stratum  be  open  to  the  surface  in  order  that  the  supply  of  water 
may  be  replenished.  In  an  oil  well,  on  the  contrary,  if  the  porous 
stratum  reaches  the  surface  the  oil  is  lost  by  evaporation,  since  the 
supply  of  oil  comes  from  below. 

Oil  and  gas  usually  occur  at  or  near  the  crest  of  broad  anticlines 
(p.  254)  or  other  "  reservoirs,"  where  their  further  movement  upward 
is  prevented,  the  oil  moving  up  the  porous  stratum  through  the 
water  which  permeates  the  bed,  since  oil  and  gas  are  lighter  than  the 
former.  If,  however,  water  is  absent  from  the  porous  stratum,  the 
oil  will  be  at  the  bottom  of  the  syncline  and  the  gas  in  the  anticline. 

One  of  the  modes  of  occurrence  common  in  the  eastern  United 
States  and  Canada  is  shown  in  Fig.  395  A,  in  which  the  oil  and  gas 
gradually  move  up  the  bed  until  (i)  the  anticline  is  reached.  If  the 
stratum  is  saturated  with  water,  the  oil  and  gas  will  accumulate  under 
the  anticline,  but  (2)  if  water  is  absent,  the  oil  will  accumulate  in  the 
syncline  (Fig.  395  B),  while  the  gas  passes  on  to  the  highest  attain- 
able point. 

(3)  Oil  is  accumulated  also  when  the  strata  are  domed  up,  as  in 
Texas.  The  principle  of  the  accumulation  of  the  oil  is  the  same  as 
in  the  anticline.  (4)  In  Mexico  there  are  numerous  volcanic  necks 


THE  ORDOVICIAN  PERIOD 


425 


(p.  316)  which  have  burst  through 
the  strata  (Fig.  395  C),  raising 
them  and  producing  a  dome-like 
structure  which  thus  brings  about 
favorable  conditions  for  the  con- 
centration of  oil.  (5)  Oil  some- 
times accumulates  also  when 
porous  strata  are  faulted  (Fig. 
395  E)  (California).  The  oil 
ascending  along  an  inclined, 
porous  layer  is  prevented  from 
escaping  to  the  surface  by  a 
fault  which  has  displaced  the 
strata  to  such  an  extent  that  the 
porous  layer  is  sealed  by  an  im- 
pervious one.  (6)  In  Oklahoma 
big  lenses  of  sandstone  covered 
over  by  impervious  beds  (Fig. 
395  D)  sometimes  yield  large 
quantities  of  oil.1 

The  size  of  oil-producing  re- 
gions is  usually  comparatively 
small. 

Origin  of  Oil  and  Gas.  —  Pe- 
troleum has  not  a  definite  chemi- 
cal composition,  but  is  made  up 
of  a  large  number  of  substances 
(hydrocarbons),  ranging  from 
gases  to  solids,  the  gas  in  oil  wells 
being  merely  a  liquid  that  is 
volatile  at  low  temperatures. 

Since  oil  and  gas  are  probably 

FIG.  395.  —  Diagrams  showing  the  more  important  modes  of  the  occurrence  of  oil 
and  gas.  Oil  is  represented  in  solid  black.  A,  the  oil-bearing  stratum  (dotted)  con- 
tains water,  and  the  oil  and  gas  are  consequently  near  the  crest  of  the  anticline 
(Pennsylvania  and  Illinois).  B,  the  oil-bearing  stratum  is  devoid  of  water.  The 
oil  is  in  the  syncline.  C,  the  strata  are  bent  upward  around  a  volcanic  neck,  and  the 
oil  has  accumulated  around  the  latter  (Mexico).  D,  oil  and  gas  occur  in  lenses  of 
sandstone  (Oklahoma).  E,  oil  is  accumulated  as  a  result  of  faulting  (California). 

^osworth,  T.  O.,  —  Outlines  of  Oilfield  Geology:  Geol.  Mag.,  Vol.  9,  1912,  pp.  16-24,  53~6o. 
Clarke,  F.  W.,  —  Data  of  Geochemistry:  Bull.  U.  S.  Geol.  Surv.  No.  491,  1911,  pp.  681-704. 
Ries,  H.,  —  Economic  Geology,  3d  ed.,  pp.  50-100. 


D 


426  HISTORICAL  GEOLOGY 

rarely  indigenous  to  the  rock  containing  them  their  origin  has  given 
rise  to  much  speculation.  There  are  two  principal  theories  of  the 
origin  of  oil,  (i)  the  organic  and  (2)  the  inorganic,  of  which  the' 
former  is  more  generally  held.  The  inorganic  theory  is  based  upon 
laboratory  experiments  with  metallic  carbides,  and  holds  that  when 
water  percolating  downward  through  the  earth's  crust  reaches  heated 
rocks  it  becomes  converted  into  steam  which  attacks  the  iron  car- 
bides, believed  to  exist  there,  generating  hydrocarbons  (oil).  Ac- 
cording to  the  organic  theory,  petroleum  and  its  products  are  derived 
from  animal  or  plant  remains  or  both,  which  were  embedded  in  the 
sediments  and  were  later  decomposed  to  oil.  It  is  often  stated  that 
oil  and  gas  were  derived  from  beds  of  shale,  either  underlying  or 
overlying  the  oil-bearing  rock. 

Life  of  Oil  Wells  and  Fields.  —  The  amount  of  oil  yielded  by  single 
wells  in  various  parts  of  the  world  in  one  year  has  exceeded  100,000 
tons,  but  such  an  enormous  production  lasts  but  a  few  weeks  at  the 
most.  The  oil  wells  of  Pennsylvania  have  an  average  life  of  about 
seven  years,  those  of  Texas  about  four  years,  and  those  of  California 
about  six  years.  The  average  production  of  the  wells  of  the  Appala- 
chian region  was  less  than  two  barrels  in  1907,  and  that  of  the  Cali- 
fornia field  was  forty-two  and  a  half  barrels.  Since  the  discovery  of 
oil  in  the  United  States  the  production  has  increased  from  decade  to 
decade,  but  this  increased  yield  has  been  the  result  of  the  sinking  of 
new  wells  and  the  discovery  of  new  fields.  The  reason  for  the  short 
lifeof  oil  and  gas  wells  is  that,  unlike  water,  there  is  no  perennial  supply. 
The  great  spouting  wells,  or  "  gushers  "  are  the  fortunate  tappings 
of  the  accumulations  of  ages  which,  though  enormously  productive 
when  first  opened,  are  also  in  about  the  same  proportion  rapidly 
exhausted. 

Oil  is  more  commonly  found  in  the  younger  rocks  than  in  the 
older,  although  some  of  the  richest  "  pools  "  are  in  the  Ordovician 
and  Devonian.  The  reason  for  this  seems  to  be  that  the  older  a  for- 
mation is,  the  more  opportunity  there  has  been  for  the  escape  of  the  oil 
and  gas  (i)  by  faulting  which  permits  the  escape  of  the  oil  and  gas 
from  the  porous  rock,  and  (2)  by  the  erosion  of  the  edges  of  the  oil- 
bearing  strata  when  it  is  lost  by  evaporation.  Only  those  Paleozoic 
strata  which  have  been  deeply  buried  and  sealed  by  newer  formations 
and  have  remained  practically  undisturbed  are  likely  to  yield  large 
quantities  of  petroleum.  The  Ordovician  limestones  of  Ohio  have 
yielded  large  quantities  of  high-grade  oil  and  gas;  the  Devonian 


THE  ORDOVICIAN   PERIOD 


427 


sandstones  of  New  York,  Pennsylvania,  and  West  Virginia,  however, 
have  furnished  the  richest  oil-bearing  strata  of  the  eastern  United 
States. 

LIFE  OF  THE  ORDOVICIAN 

The  life  of  the  Ordovician  differed  from  that  of  the  Cambrian  in  the 
abundance  of  certain  classes  which  were  rare  in  the  latter,  and  in  the 
higher  level  of  development  in  many  cases.  Graptolites,  although 
rare  in  the  Cambrian,  attained  their  greatest  abundance  in  the  Ordo- 
vician. The  primitive  corals  of  the  Cambrian  were  followed  by  well- 
developed  forms ;  the  cephalopods  became  the  largest  animals  of  the 
period ;  gastropods  were  much  more  modern  in  appearance ;  and 
brachiopods  show  a  great  increase  in  variety  and  abundance. 

Protozoa 

Siliceous  protozoa  (Radiolaria)  are  found  in  the  Ordovician  strata 
of  some  regions  in  sufficient  numbers  to  show  that  they  were  abun- 
dant in  the  seas  of  that  period. 

Ccelenterata 

Sponges  are  well  represented  by  forms  that  secrete  siliceous  skeletons 
(Fig.  396  A^  B),  and  some  of  them  attained  a  diameter  of  a  foot  or 


B 

FIG.   396. — Ordovician  sponges:   Ay  Brachiospongia  digitata;  B,  Receptaculites 

ohioensis. 

more.     Certain  beds  of  the  Ordovician  (Chazy),  of  New  York,  are 
composed  almost  entirely  of  sponges. 

Graptolites.  —  This  class  (Fig.  397  A-K]  can  be  traced  from  its 
earliest  appearance  to  its  final  extinction,  through  all  the  stages  of 
development,  and  is  consequently  well-adapted  to  illustrate  some 
principles  of  evolution. 


428 


HISTORICAL  GEOLOGY 


Graptolites  began  in  the  Cambrian  as  small,  bushy  forms  (Fig.  398  A)  which,  as  a 
rule,  lived  throughout  life  attached  to  the  sea  bottom.  Before  the  close  of  this  period 
however,  a  change  in  the  mode  of  life  occurred  which  was  to  give  the  class  entirely  dif- 
ferent habits  and  as  a  result  bring  about  important  modifications  in  the  structure. 
For  some  unknown  reason,  perhaps  to  avoid  a  new  creeping  enemy,  the  colonies  left 
the  sea  bottom.  At  first  the  branches  of  the  bush-like  colonies  hung  suspended, 


FIG-  397-—  Graptolites:  A  and  B,  Didymograptus  nitidus ;  C,  Phyllograptus  typus; 
D,  Monograptus  clintonensls;  E,  Gonlograptus  postremus;  F,  Diplograptus  pristis; 
C,  Phyllograptus  angustifolius ;  H,  Dichograptus  octobrachiatus ;  /,  Dictyonemaflabelli- 
forme;  J,  Climacograptus  bicornis ;  K,  Tetragraptus  fruticosus. 

later  they  became  horizontal,  and  still  later  the  branches  were  turned  upward.  This 
change  was  accompanied  by  a  reduction  in  the  number  of  branches.  The  irregular 
many-branched  early  forms  gave  way  to  regular,  many-branched  colonies  (Bryo- 
graptus,  Fig.  398  B},  then  to  eight-branched  (Dichograptus,  Figs.  398  C  and  397  H), 
these,  in  turn,  to  four-branched  forms  (Tetragraptus,  Figs.  398  D  and  397  .AT), 
and  these  to  two-branched  forms  (Didymograptus,  Figs.  397  A  and  398  E).  In 


THE  ORDOVICIAN   PERIOD 


429 


addition  to  the  changes  in  the  main    line  of  descent  many    aberrant    forms    came 
into  existence,  but  were  not  long-lived. 

Another  important  change  in  the  race  was  brought  about  when  the  graptolites 
became  detached  from  seaweeds  and  led  an   independent    floating   existence,  being 


FIG.  398.  —  Diagram  showing  the  evolution  of  one  branch  of  the  Graptolitoidea.  At 
first  attached  to  the  sea  floor,  they  later  became  attached  to  floating  seaweeds  and 
finally  acquired  floats.  This  change  in  their  mode  of  life  induced  important  changes 
in  structure,  one  of  which  resulted  in  a  reduction  in  the  number  of  branches. 

buoyed  up  by  "floats"  (Figs.  397  F  and  398  C,  D,  E)  to  which  they  were  attached 
by  threads. 

Towards  the  end  of  the  race  numerous  spines  appeared  on  some  species,  a  network 
of  protecting  fibers  was  developed  on  others,  and  the  colonies  became  small.  They 
were  on  the  defensive  and  soon  disappeared.  The  forms  which  survived  the  longest 
were  those  inconspicuous  ones  which  had  remained  attached  to  the  sea  bottom  from 
the  beginning  of  the  race. 

Because  of  the  many  progressive  changes  which  the  race  under- 
went, and  also  because  the  colonies  were  carried  about  by  currents 
over  the  seas  of  the  world, 
graptolites  are  excellent  fossils 
for  correlating  (determining 
identity  of  age)  widely  sepa- 
rated beds.  The  simple  forms 
are  especially  characteristic  of 
the  Upper  Cambrian  and  Lower 
Ordovician.  The  group  in  the 
main  became  extinct  in  the 
Silurian,  but  a  few  species  lived 
even  into  the  Carboniferous.1  FIG.  399.  —  Ordovician  corals :  A,  Colum- 

Stromatopora.  —  An     extinct    naj ia    halli>'     \  Streptelasma    profundum. 

.  r  (A  portion  has  been  removed  to  show  the 

order  of  organisms    known   as    interjor.) 

stromatoporoids  were  especially 

abundant  as  reef  builders  in  the  Ordovician,  Silurian,  and  Devonian. 

They  were  allied  to  the  corals  and  consisted  of  colonies  of  minute 

polyps  which  secreted   concentric   layers  of  thin   calcareous   plates 

^uedemann,  R., —  Graptolites  of  New  York,  Part  2:   N.Y.  State  Museum,  Mem.  n,  1908. 


43° 


HISTORICAL  GEOLOGY 


connected  by  vertical  rods.  Limestone  masses,  sometimes  five  by 
ten  feet  in  horizontal  extent  and  several  inches  thick,  were  built 
by  them.  The  aggregate  amount  of  limestone  built  by  the  stroma- 
toporoids  was  very  large. 

Corals.  —  This  class  was  present  in  the  Ordovician  and  was  rep- 
resented by  several  types,  among  which  were  the  simple,  horn-shaped 
cup  corals  (Fig.  399  B)  and  those  living  in  colonies  (Fig.  399  A). 
The  description  of  these  types  will  be  taken  up  under  the  Silurian 

(p.  444). 

Echinodermata 

Cystoids  (Greek,  custis,  a  bladder)  were  so  named  because  of  the 
bladder-like  shape  of  the  body.  Essentially,  the  animals  had  a  sack- 
like  or  bladder-like  body  made  up  of  calcareous  plates,  on  the  upper 


A 

FIG.  400.  —  Ordovician  cystoids  :   A,  Lepidodiscus  (Agelacrinus)  cincinnatiensis ; 
B,  Pleurocystis  filitextus ;   C,  Amygdalocystites  florealis. 

side  of  which  two  arms  were  sometimes  attached,  while  some  species 
were  armless  (Fig.  400  A-C).  The  body  was  rooted  by  a  tapering 
stem  to  the  sea  bottom.  Cystoids  first  appeared  in  the  Cambrian 
and  reached  their  climax  in  the  Ordovician  and  Silurian,  after  which 
they  suddenly  diminished  in  the  number  of  species,  although  locally 
a  few  forms  lived  in  considerable  abundance.  They  are  characteristic 
of  the  Ordovician  and  probably  became  extinct  early  in  the  Carbon- 
iferous. 

Crinoids  (Greek,  crinon,  a  lily)  are  living  in  the  present  seas  and 
still  constitute  a  vigorous  stock,  even  though  the  race  began  in  the 
Cambrian.  The  name  "  sea  lily  "  was  given  to  this  class  of  animals 
because  of  their  flower-like  appearance.  The  animal  (Fig.  401  A-D) 
consists  of  a  body  composed  of  plates,  as  in  cystoids,  and  is  attached 
to  the  sea  bottom  by  a  jointed  stem.  From  the  upper  margin  of  the 
body  (calyx)  spring  the  arms,  which  are  short  and  simple  in  some 


THE  ORDOVICIAN   PERIOD 


431 


FIG.  401.  —  Ordovician  crinoids  :   A,  Ectenocrinus  grandis;   B,  Hybocrinus  tumidus ; 
C,  Glyptocrinus  decadactylus ;   D,  Heterocrinus  (locrinus)  subcrassus. 

species  and  long  and  many-branched  in  others.  Within  the  arms  is 
the  mouth,  to  which  food  particles  are  carried  by  the  currents  set  in 
motion  by  the  arms. 

Blastoids,  Starfish  (Fig.  402),  Brittle  Stars,  and  Sea  Urchins  lived 
in  this  period,  but  as  they  were  rare  they  will  be  discussed  in  later 
chapters.  The  origin  of  the  starfish  probably 
goes  back  to  the  Proterozoic,  as  may  be  inferred 
from  the  complex  metamorphism  of  the  starfish 
larva.  The  absence  of  fossil  starfish  in  the 
Cambrian  sediments  may  mean  that  a  pre- 
servable  starfish  skeleton  was  not  evolved 
until  the  Ordovician. 

Molluscoidea 

Brachiopods.  — The  preponderance  of  hinged     ^ 

*  &.          FIG.    402. —Ordovician 

(articulate)  (tig.  403,  except  C  and  L)  species  starfish:  piaster  simplex. 
of  brachiopods  over  hingeless  (inarticulate) 

(Fig.  403  C  and  L)  is  very  marked  in  the  Ordovician.  A  con- 
spicuous feature  of  many  of  the  species  was  the  greater  thickness  of 
the  shells  and  the  ribbing  (Fig.  403,  F,  G,  H)  of  the  exterior  by 
means  of  which  the  shell  was  strengthened.  Brachiopods  were  very 
abundant  and  are  important  in  determining  the  subdivisions  of  the 
series. 

CLELAND   GEOL.  —  28 


432 


HISTORICAL  GEOLOGY 


FIG.  403. — Ordovician  brachiopods :  A,  Orthis  tricenaria;  B,  Raphinesquina 
alternata;  C,  Lingula  rectilateralis ;  D,  Plectambonites  sericeus;  E,  Trematis  ottawcznsis ; 
F,  Leptcena  rhomboidalis;  G,  Platystrophia  lynx;  H,  Rhynchotrema  capax;  7,  Dal- 
manella  testudinaria;  J,  Hebertella  borealis ;  K,  Strophomena  rugosa;  L,  Schizocrania 
filosa;  M,  Zygospira  recurvirostris. 


*®J¥ 


D  E 

FIG.  404.  —Ordovician  bryozoans  :  A,  Escharopora  subrecta;  B,  Corynotrypa  inflala 
(enlarged);  and  the  same,  C,  in  its  natural  position  and  size  on  a  brachiopod ; 
Z),  Callopora  pulchella;  E,  Constellaria  fiorida. 


THE  ORDOVICIAN  PERIOD 


.,433 


Bryozoa.  —  Fossil  bryozoans  (Greek,  bruon,  moss,  and  zoon,  animal) 
consist  of  small  branching  stems  and  lacelike  mats  (Fig.  404  A-E),  the 
skeletons  of  minute  animals  that  lived  in  colonies.  They  resemble  cer- 
tain corals  in  external  appearance,  but  are  related  to  the  brachiopods. 
They  can,  as  a  rule,  easily  be  distinguished  from  corals  by  the  smaller 
size  of  the  colonies  in  which  the  polyps  lived.  Bryozoan  fossils  are 
very  common  in  limestones  of  Ordovician  age  and  were  important 
limestone  makers.  They  are  valuable  "  index  fossils  "  in  determining 
the  age  of  Ordovician  strata,  since  they  were  abundant  not  only  in 
individuals  but  also  in  species. 


Mollusca 

Pelecypods  are  abundant  in  the  salt  and  fresh  waters  of  the  present, 
being  represented  by  the  clam,  pecten,  oyster,  and  many  others. 
They  have  bivalve 
shells  in  which  the 
two  valves  are  usu- 
ally nearly  alike  (Fig. 
405  A-E).  In  exter- 
nal form  they  differ 
from  brachiopods, 
which  they  resemble, 
in  the  lack  of  bi- 
lateral symmetry. 
Aside  from  fossils 
whose  relationships 
are  doubtful  (Fordilla 
and  Modioloides)  this 
great  class  is  almost 
unknown  previous  to 
the  Ordovician.  As  a  rule,  pelecypods  are  rather  rare  fossils  in  the 
Ordovician  rocks,  being  more  abundant  in  sandstone  and  shales  than 
in  limestones,  thus  showing  that  they  lived  best  on  sandy  and  muddy 
bottoms. 

Gastropods.  —  This  class  was  more  abundant  than  the  pelecypods, 
and  even  in  the  early  Ordovician  was  represented  by  a  considerable 
variety  of  forms  (Fig.  406  A-G)  which  closely  resemble  modern  rela- 
tives in  external  appearance. 


D  E 

FIG.  405.  —  Ordovician  pelecypods:  A,  Pterinea  de- 
mis  sa;  B,  Rhytimya  radiata;  C,  Cyrtodonta  billingsi; 
D,  Ctenodonta  nasuta;  E,  Byssonychia  radiata. 


434 


HISTORICAL  GEOLOGY 


FIG.  406.  Ordovician  gastropods:  A,  Hormotoma  gracilis ;  S,  Maclurea  logani; 
C,  Protowarthia  cancellata;  D,  Cyrtolites  ornatus;  E,  Lophospira  bicincta;  F,  Trocho- 
nema  umbilicatum;  G,  Ophileta  compacta. 

Cephalopods.  —  This  is  the  most  highly  developed  class  of  the 
mollusks.  All  Ordovician  cephalopods  (Fig.  407  A-D)  have  shells 
such  as  those  possessed  by  the  nautilus  of  to-day.  The  shell  is  divided 

into  a  number  of 
chambers  by  trans- 
verse partitions, 
called  septa,  through 
which  a  tube,  the 
siphuncle  (Fig.  407 
A  and  Z)),  extends 
from  one  end  to  the 
other,  the  animal 
living  in  the  body 
chamber  (Fig.  407  A) 
at  the  larger  end. 
The  juncture  of  the 
septa  with  the  shell 
is  called  the  suture, 
and,  as  will  be  seen 
(p.  -528),  the  shape 
of  this  line  is  of  great 
importance  in  deter- 
mining the  evolution 
of  many  genera.  The 
shape  and  size  of 
Ordovician  cephalo- 
pods varied  greatly, 


FIG.    407.  —  Ordovician    cephalopods  : 
Trocholites  ammonius;  B,  Schrcederoceras  eatoni  ; 
C,    Oncoceras    pandion;     Z),    Orthoceras    multi- 
cameratum.      (A  portion  of  the  shell  is  removed 
to  show  the  partitions  and  siphuncle.) 


THE  ORDOVICIAN   PERIOD 


435 


some  being  straight  (Fig.  407  D),  some  curved  (Fig.  407  B,  C),  and 
some  tightly  coiled  (Fig.  407  A).  The  straight  forms,  represented  by 
Orthoceras  (Greek,  orthos,  straight,  and  ceras,  a  horn),  were  most  char- 
acteristic of  the  period,  some  (Endoceras)  attaining  a  length  often  or 
more  feet  and  a  maximum  diameter  of  about  one  foot.  At  the  other 
extreme  were  some  less  than  an  inch  in  length  and  one  eighth  of  an 
inch  in  diameter.  Cephalopods  have  been  called  the  scavengers  of 
the  Ordovician,  and  they  were  probably  the  most  powerful  animals 
then  living.  The  great  diversity  of  the  Ordovician  cephalopods  is 
evidence  that  the  group  began  in  the  early  Cambrian. 

Crustacea 

Trilobites. — The  rapid  evolution  of  the  trilobites  noted  in  the  dis- 
cussion of  the  Cambrian  was  continued  in  the  Ordovician,  during  which 


FIG.  408.  —  Ordovician  crustaceans.  Trilobites:  A,  Ceraurus  pleurexanthemus ; 
B,  Bathyurus  longispinus ;  C,  Isotelus  gigas  (greatly  reduced);  Z),  Trinucleus  (Crypto- 
lithus]  tessellatus;  E,  Bumastus  trentonensis ;  F,  G,  rolled  and  straight  specimens 
of  Calymene  callicephala;  H,  /,  two  views  of  a  rolled  specimen  of  Thaleops  ovata. 
Ostracod  :  /,  Leperditia  inflata. 


436  HISTORICAL  GEOLOGY 

period  the  class  attained  its  greatest  development  (Fig.  408  A-I), 
more  than  half  of  all  the  known  genera  of  trilobites  being  represented 
at  that  time.  During  the  remainder  of  the  Paleozoic  they  gradually 
declined  until  their  extinction  was  reached  in  the  closing  stages. 
When  Cambrian  and  Ordovician  trilobites  are  compared,  it  is  seen 
that  the  latter  have  rounder  eyes,  that  the  tail  shield  (pygidium) 
is  larger,  and  that  they  have  acquired  the  ability  to  roll  themselves 
up  (Fig.  408  F,  H,  I)  and  thus  protect  the  lower  portions  of  the  body, 
many  being  found  in  this  position  which  was  apparently  taken  at  the 
approach  of  death.  The  Ordovician  trilobites  were  not  as  large  as 
some  Cambrian  species,  the  maximum  length  being  about  18  inches 
as  compared  with  about  two  feet  for  one  Cambrian  species. 

Other  Arthropods  continued  from  the  Cambrian.  Ostracods  (Fig. 
408  /),  small  bivalve  crustaceans,  flourished  during  portions  of  the 
period,  and  also  eurypterids  (p.  449). 

Fishes 

An  important  addition  to  the  fauna  of  the  Ordovician,  and  one 
which  had  a  profound  effect  on  the  evolution  of  life  in  subsequent 
ages,  was  the  fishes,  remains  of  which  have  been 
found  in  Colorado  and  Wyoming. 

PLANTS 

Seaweeds.  —  Our  knowledge  of  the  plants  of 
the  Ordovician  is  almost  as  incomplete  as  that 
of  the  Cambrian.  No  land  plants  have  been 
found  and  no  marine  plants  higher  than  sea- 
weeds (Fig.  409)  and  calcareous  algae.  The 
absence  of  land  plants  in  Ordovician  strata, 
however,  does  not  prove  that  a  land  vegetation 
was  lacking,  since  the  known  plant-bearing  strata 
are  all  of  marine  origin  and  consequently  the 
FIG.  409.  —  Ordovi-  absence  of  land  plant  fossils  would  not  be  re- 

cian     plants     (Spheno~  ILI  i          1111  111 

phycuslatifolius).  They    markable>   even  though   land   plants   had  been 

are    probably    append-    abundant. 

ages  of  floating  algae.  _ 

SUMMARY 

Progress  and  Character  of  Ordovician  Life.  —  The  life  of  the  period 
as  shown  by  the  fossils  was  fuller,  more  varied,  and  of  a  higher  grade 
than  that  of  the  Cambrian.  Trilobites,  cephalopods,  gastropods, 


THE  ORDOVICIAN  PERIOD  437 

pelecypods,  cystoids,  graptolites,  and  corals  became  diversified  and 
of  higher  types ;  and  bryozoans,  crinoids,  and  fishes  are  known  for 
the  first  time.  During  this  period  graptolites  and  cystoids  attained 
their  climax  and  were  never  again  important.  In  this  period,  too, 
the  straight  cephalopods  rapidly  developed  from  small  to  gigantic 
forms  and  into  many  species,  but  occupied  a  subordinate  place  after 
the  Silurian.  Before  the  close  of  the  period  all  of  the  great  types  of 
life  and  most  of  the  important  subdivisions  were  present. 

When  the  faunas  of  the  Ordovician  stages  of  North  America  are 
compared  with  those  of  Europe,  it  is  found  that,  although  the  genera 
are  usually  identical,  the  species  are  different  though  similar. 

Adaptation  to  environment  was  almost  as  well  established  then  as 
now.  Certain  species  lived  almost  exclusively  on  muddy  bottoms, 
certain  ones  on  sandy,  and  still  others  on  calcareous  bottoms.  There 
was  also  adaptation  to  shallow  and  deep  water. 

The  effect  of  isolation  is  noticeable  when,  for  example,  a  portion  of 
an  epicontinental  sea  was  cut  off  by  some  barrier,  such  as  a  gentle 
upfolding  of  a  portion  of  the  sea  bottom,  or  a  bar,  or  when  ocean  cur- 
rents, because  of  their  lower  or  higher  temperature,  prevented  the  life 
of  different  portions  of  the  sea  from  mingling,  the  isolation  of  the  fauna 
permitting  an  independent  development  without  interference  from 
outside.  It  consequently  sometimes  happened  that  the  faunas  of  ad- 
joining seas  differed  considerably.  When  the  barriers  were  removed, 
a  rapid  and  marked  change  in  the  fauna  was  often  quickly  brought 
about. 

The  evolution  of  the  life  of  the  period  gave  rise  to  many  new  species, 
with  the  result  that  when  the  fauna  of  the  earliest  and  latest  Ordovi- 
cian are  compared,  they  are  found  to  differ  widely.  It  is  because  of 
the  appearance  of  new  species  that  the  Ordovician  series  of  strata  have 
been  divided  into  several  stages,  which  are  usually  easily  distinguished 
by  their  contained  fossils. 

Climate  and  Duration  of  the  Ordovician.  —  Fossils  found  in  Ordo- 
vician strata  of  Arctic  lands  show  that  the  climate  there  was  not  unlike 
that  of  the  temperate  and  tropical  regions  of  the  same  time.  During 
the  Middle  Ordovician,  and  again  later  in  the  period,  reef  corals  were 
common  from  Alaska  to  Texas.  The  conclusion  is  that  climatic  zones 
did  not  exist,  but  that  the  climate  of  the  world  was  uniformly  equable 
and  less  diversified  than  now. 

The  duration  of  the  period  was  about  the  same  as  that  of  the  Cam- 
brian, perhaps  4,000,000  years. 


438 


HISTORICAL  GEOLOGY 


REFERENCES   FOR  THE  ORDOVICIAN   PERIOD 

BLACKWELDER  AND  BARROWS,  —  Elements  of  Geology,  pp.  339-348. 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  2,  pp.  304-367. 

SCHUCHERT,  CHAS.,  —  Paleogeography  of  North  America:    Bull.  Geol.  Soc.  America, 

Vol.  20,  1910,  pp.  485-489. 

SCOTT,  W.  B.,  —  An  Introduction  to  Geology,  pp.  560-577. 
ULRICH,  E.  O.,  —  Revision  of  the  Paleozoic  Systems:   Bull.  Geol.  Soc.  America,  Vol.  22, 

1911,  pp.  281—680. 


CHAPTER  XVII 
THE   SILURIAN1   PERIOD 

THIS  system  has  been  divided  into  a  number  of  subdivisions  which 
in  New  York  are  as  follows : 

Rondout  water  lime 

Cobbleskill  limestone 

Salina  shales,  salt,  water  lime 

Lockport  and  Guelph  dolomites 

Rochester  shale 

Clinton  shale,  sandstone,  limestone,  and  iron  ore 

Medina  and  Oneida  sandstone  and  conglomerate 

In  eastern  North  America  the  Silurian  strata,  for  the  most  part, 
rest  unconformably  upon  the  deformed  and  eroded  Ordovician  rocks. 
In  the  Middle  States  the  Lower  Silurian  is  usually  absent,  and  the 
Middle  Silurian  strata  rest  unconformably  upon  the  Ordovician  or 
upon  older  rocks,  showing  that  during  early  Silurian  times  the  central 
portion  of  the  continent  was  land.  In  Montana  and  Utah  the  strata 
of  the  Ordovician,  Silurian,  and  Devonian  are  apparently  conformable, 
and  their  separation  is  more  or  less  arbitrary  because  of  the  scarcity 
of  fossils. 

Geography  of  the  Silurian.  —  The  period  can,  for  convenience,  be 
divided  into  three  epochs,  (i)  During  the  first  (early  lower)  the 
epicontinental  seas  were  apparently  restricted  to  three  principal  bays 
(Fig.  410):  one  stretching  up  the  Mississippi  Valley  to  northern  Illinois; 
a  second  extending  across  Newfoundland  and  northern  New  Brunswick; 
and  a  third  occupying  the  Appalachian  trough,  and  stretching  east 
and  west  over  central  New  York  and  Ontario.  Later  in  the  Lower 
Silurian  the  seas  were,  for  a  time,  withdrawn  from  the  Appalachian 
trough  and  New  York.  (2)  The  middle  (later  lower)  of  the  period 
(Fig.  411)  saw  an  expansion  of  the  seas  over  a  large  portion  of  Canada 
to  the  Arctic  Ocean  and  over  the  United  States  east  of  the  Mississippi 
River,  and  an  extension  of  two  seas  on  the  west,  one  from  California 

1  The  name  Silurian  has  been  taken  from  the  Silures,  an  ancient  tribe  which  dwelt  in  Wales. 

439 


440 


HISTORICAL  GEOLOGY 


through  Idaho  to  Canada  and  another  from  Mexico  into  Arizona 
and  New  Mexico.  It  was  during  this  time  that  the  great  limestone 
strata  (Niagaran)  were  deposited.  (3)  The  epicontinental  seas 
were  again  restricted  in  the  Upper  Silurian  (Fig.  412),  the  most 
important  of  them  extending  from  Wisconsin  and  Illinois  through 

New  York  and  over 
the  Appalachian 
trough.  The  three 
subdivisions  of  the 
period  are  therefore 
characterized  (i)  by 
constricted  seas,  (2) 
by  expanded  seas, 
and  (3)  by  a  later 
shrinking  and  shift- 
ing of  the  seas.  It 
should  be  pointed 
out  in  this  connec- 
tion that  Silurian 
strata  do  not  cover 
all  of  the  areas  shown 
in  the  maps  (p.  403), 
but  often  lie  only  in 
widely  separated 
patches  which  appear 
to  be  remnants  of  a 
once  continuous 
formation.  The  age 
and  correlations  of 
FIG.  410.  — Map  showing  the  probable  outline  of  North  these  patches  are  de- 
America  during  a  portion  of  the  Lower  Silurian.  (Modified 
after  Schuchert.)  termmed  by  their 

contained  fossils. 

The  sediments  which  were  deposited  in  the  Appalachian  trough 
were  derived  from  the  broad  island  or  continent  of  Appalachia  (p.  406). 
Character  and  Thickness  of  the  Sediments.  —  Limestones  are  the 
common  strata  of  the  Silurian,  but  in  eastern  North^i]1  erica  con- 
glomerates, sandstones,  and  shales  predominate.  Thero  ]atter  were 
deposited  in  shallow  seas,  as  the  ripple  marks  and  cross-bedding  show. 
The  formation  and  distribution  of  these  coarse  sediments  teach  an 
important  lesson.  Along  the  western  boundaries  of  the  eastern  lands, 


THE  SILURIAN  PERIOD 


441 


gravel  and  sand  were  carried  by  rapid  streams  flowing  from  the  areas 
newly  raised  by  the  Taconic  deformation  (p.  422)  and  were  spread 
along  the  shores,   forming  wide  beaches,  the  gravel   (forming  the 
Oneida  conglomerate)  rapidly  thinning  toward  the  west.     Sand  (Me- 
dina) was  carried  farther  out  into  the  sea  by  the  currents  and  formed 
extensive    sandstone 
strata.     When  in  the 
course    of    time    the 
lands      were      worn 
down,    the     streams 
were  unable  to  carry 
such  large  quantities 
of  coarse  sediment  as 
formerly,  and  the  belt 
of  gravel    along  the 
shores    was    conse- 
quently   narrowed 
and    sand    was    de- 
posited nearer  shore 
and  upon  the  earlier 
gravels.      It    is    evi- 
dent from  the  above 
that  the  conglomer- 
ates and   sandstones 
were       contempora- 
neous. A  still  greater 
lowering  of  the  land, 
either  by  erosion  or 
by    subsidence,    fur- 
ther reduced  the  ca- 
pacity of  the  streams 
for  cutting,  and  dur- 
ing  one   epoch    lime    ooze   (Niagaran)    and    during    another   mud 
(Salina)  accumulated  on  the  sandstones  and  conglomerates.     Where 
the  conglomerates  have  been  tilted  by  later  folding  and  cut  by  ero- 
sion, their  jp^urned  edges  form  mountain  ridges. 

The  Silii  ?^h  formations  west  of  New  York  are  largely  limestone, 
in  portions  of  which  well-developed  coral  reefs  are  to  be  distinguished. 
The  falls  of  Niagara  are  due  to  the  presence  of  a  massive  layer  of 
limestone  of  Silurian  Period,  from  which  an  important  stage,  the 


FIG.  411.  —  Map  showing  the  probable  outline  of 
North  America  during  a  portion  of  the  Middle  Silurian. 
(Modified  after  Schuchert.) 


442 


HISTORICAL  GEOLOGY 


Niagaran,  received  its  name.  The  limestone  of  this  stage  is  thickest 
in  the  Mississippi  Valley,  where  it  probably  had  been  accumulating 
for  a  longer  time  than  in  New  York.  Such  a  great  thickness  of 
limestone  indicates  a  long  period  during  which  few  oscillations  in 
level  occurred,  and  when  the  lands  were  so  low  that  little  secliment 

was    carried    to    the 
sea. 

The  thickness  of 
the  Silurian  in  Mary- 
land is  about  3000 
feet ;  in  western  Ten- 
nessee, about  1500 
feet;  in  Alabama, 
about  500  feet;  in 
central  Tennessee, 
300  feet ;  in  Wiscon- 
sin, about  800  feet ; 
and  in  Nevada,  about 
1000  feet. 

Clinton  Iron  Ore. 
—  One  of  the  most 
widespread  iron  ore 
deposits  known  was 
accumulated  during 
the  Clinton  stage  of 
the  Silurian  Period. 
It  outcrops  in  one  or 
more  broken  belts 
from  New  York, 
through  Pennsyl- 
vania to  Alabama, 
and  occurs  in  beds 

at  different  horizons  in  the  formation,  sometimes  as  many  as  four  beds 
being  present  in  one  locality.  The  thickness  of  the  ore  beds  varies 
from  40  feet  to  a  fraction  of  an  inch,  but  a  bed  10  feet  thick  is  unusual. 
The  ore  is  called  "  fossil  "  and  "  pea  "  ore  because  fossil  fragments  are 
commonly  found  in  it  with  the  shell  substance  entirely  replaced  with 
hematite,  while  some  beds  are  made  up  of  rounded  grains  of  a  concre- 
tionary character.  The  ore  was  deposited  close  to  shore,  probably 
in  lagoons  and  marshes,  and  was  probably  a  chemical  precipitate, 


FIG.  412.  —  Map  showing  the  probable  outline  of 
North  America  during  a  portion  of  the  Upper  Silurian. 
(Modified  after  Schuchert.) 


THE   SILURIAN   PERIOD 


443 


the  iron  having  been  brought  to  the  sea  by  streams  which  had  leached 
it  from  the  igneous  rocks  over  which  they  flowed. 

The  presence  of  iron  ore,  limestone,  and  coal  within  short  distances 
of  each  other  near  Birmingham,  Alabama,  has  made  that  city  a  great 
center  for  iron  and  steel  industries.  Coal  is  necessary  to  reduce  the 
iron,  and  limestone  is  used  as  a  flux  to  carry  away  the  siliceous  im- 
purities. 

Deserts.  —  During  a  portion  of  the  Silurian  (Salina)  in  eastern 
North  America  the  climate  was  arid  and  desert  conditions  prevailed. 
This  is  shown  by  the  beds  of  salt  and  gypsum,  and  by  the  red  color  of 
the  shales.  In  New  York  state  325  feet  of  solid  salt  have  been  pene- 
trated by  wells.  These  salt  beds  are  lens-shaped,  and  the  conditions 
under  which  they  were  deposited  may  not  have  been  unlike  those 
to-day  in  the  region  of  the  Caspian  Sea,  the  Dead  Sea,  and  Great  Salt 
Lake,  or  back  of  bars  as  described  below.  Such  an  arid  climate  may 
have  been  produced  by  high  lands  to  the  south  and  east,  which  shut 
off  the  moist  winds  from  the  Atlantic  and  the  Gulf  of  Mexico. 

Origin  of  Rock  Salt.  —  Salt  is  primarily  formed  by  the  evaporation 
(p.  135)  of  the  salt  water  of  lakes  or  the  ocean,  and  is  accumulating  to- 
day in  certain  salt  lakes  which  have  been  greatly  concentrated.  The 
evaporation  of  inland  salt  lakes  does  not,  however,  seem  adequate  to 
produce  thick  beds  of  pure  salt  such  as  occur  in  certain  regions. 

The  theory  which  best  explains  the  origin  of  massive  salt  deposits 
assumes  that  a  body  of  ocean  water  had  been  shut  off  partly  or  com- 
pletely by  a  low  bar.  If  the  region  in  which  this  occurred  was  arid, 
the  evaporation  of  the  water  back  of  the  bar  would  exceed  that  car- 
ried in  by  the  rivers  and  that  derived  from  the  ocean.  The  lowering 
of  the  water  of  the  bay  by  evaporation  would  permit  the  ocean  water  to 
flow  in  if  the  bar  were  incomplete ;  if,  however,  the  bar  were  complete 
and  the  bay  entirely  shut  ofF  from  the  ocean,  forming  a  lake,  ocean 
water  would  enter  only  during  storms  or  at  high  tide.  In  time,  the 
concentration  of  the  water  would  be  so  great  that  common  salt  and 
other  salts  would  be  precipitated.  Under  conditions  such  as  those 
outlined  above,  pure  salt  might  accumulate  to  a  considerable  thickness 
without  the  admixture  of  mud.  Occasionally,  the  purity  of  the  salt 
might  be  broken  by  sheets  of  mud  brought  in  by  streams  swollen  by 
the  torrential  showers  of  desert  regions. 

The  Silurian  salt  of  New  York  seems  to  have  been  deposited  either 
in  extensive  salt  lakes  or  in  an  arm  of  the  sea  which  was  partially  shut 
off  from  the  sea  by  a  bar. 


444 


HISTORICAL  GEOLOGY 


Igneous  Rocks.  —  The  Silurian  was  a  period  of  quiet  as  far  as  vol- 
canism  was  concerned.  In  North  America  some  igneous  intrusions 
of  this  age  are  known,  but  they  are  not  extensive. 

Other  Continents.  —  The  Silurian  epicontinental  seas  of  Europe 
were  extensive  and  had  much  the  same  position  as  in  the  Ordovician. 
Two  distinct  seas,  one  in  the  north  and  the  other  in  the  southern 
part  of  the  continent,  were  separated  by  a  land  ridge.  The  life  of 
the  two  seas  was  unlike  in  many  particulars,  that  of  the  northern  sea 
being  typical  of  the  period  in  other  continents,  while  that  of  the  south- 
ern had  many  peculiarities  which  indicate  that  it  was  partly  inclosed. 

LIFE  OF  THE  SILURIAN 

Aside  from  a  notable  increase  in  the  number  and  variety  of  corals, 
crinoids,  brachiopods  (spine  bearers),  and  eurypterids,  and  a  decrease 
in  the  graptolites  and  straight  cephalopods,  the  life  of  the  Silurian  did 
not  differ  greatly  in  general  aspect  from  that  of  the  Ordovician. 
When,  however,  one  looks  for  identical  species  and  genera,  he  finds 
that  the  change  was  a  marked  one. 

Ccelenterata 

Corals  forged  ahead  and  became  important  in  the  Silurian.  In- 
stead of  the  few,  usually  simple  forms  of  the  Ordovician,  a  varied  and 


•SL  \v,          BCD  E 

FIG. .413.  —  Silurian  corals:  A,  chain  coral,  Haly sites  catenulatus;  B  and  C,  cup 
corals,  Streptelasma  (Enter olasma)  calicula;  D,  Syringopora  retiformis;  E,  Goniophyl- 
lum  pyramidale. 

abundant  coral  fauna,  many  of  the  corals  compound,  throve  in  the 
clear  seas  of  the  time.  Four  well-marked  types  were  abundant :  (i) 
chain  corals  (Halysites,  Fig.  413  A},  made  up  of  vertical  tubes  joined 
together  in  such  a  way  as  to  give  them  the  appearance  of  a  linked 
chain.  Since  chain  corals  began  in  the  Ordovician  and  became  extinct 
in  the  basal  Devonian,  their  presence  in  a  formation  shows  it  to  be 
either  Ordovician  or  Silurian.  (2)  Honeycomb  corals  (Favosites)  were 
composed  of  six-sided  parallel  columns,  like  a  honeycomb,  which 


THE   SILURIAN  PERIOD 


445 


were  divided  by  horizontal  partitions.  Honeycomb  corals  were  rare 
in  the  Ordovician,  but  built  coral  reefs  in  the  Silurian  and  Devonian. 
(3)  Cup  corals  (Enterolasma,  Fig.  413  B,  C,  E)  were  horn-shaped, 
with  a  depression  in  the  top.  A  peculiar  cup  coral  of  the  period 
(Goniophyllum,  Fig.  413  E)  was  provided  with  a  cover  which  con- 
sisted of  four  triangular  plates,  hinged  to  the  margins  of  the  cup.  The 
covering  was  evidently  for  protection  against  enemies,  but  since  the 
genera  which  possessed  it  have  no  living  representatives,  it  is  probable 
that  the  device  was  not  successful.  Cup  corals  occurred  in  the  Or- 
dovician and  continued  until  the  close  of  the  Paleozoic;  many  of 
these  were  separate  individuals,  while  some  were  in  hemispherical 
colonies  (Fig.  451  A,  p.  480).  (4)  Organ-pipe  corals  (Syringopora, 
Fig.  413  D)  were  similar  to  chain  corals,  but  their  cylindrical  columns 
were  not  attached  along  their  entire  length. 

Coral  reefs  date  from  the  later  Ordovician.  Before  this  time  simple 
corals  predominated,  and  even  these  were  rare.  When,  however,  com- 
pound corals  such  as  the  honeycomb  became  abundant,  the  lime- 
stone secreted  by  many  generations,  together  with  that  of  associated 
animals  such  as  brachiopods,  gradually  built  up  reefs  at  short  distances 
from  the  shores.  Si- 
lurian coral  reefs  were 
seldom  of  great  thick- 
ness. 

Other  Ccelenter- 
ates.  —  Stromatopora 
were  important  reef 
builders,  but  grapto- 
lites  no  longer  played 
an  important  role  in 
America,  and  by  the 
end  of  the  period 
were  practically  ex- 
tinct. Sponges  (Fig. 
414  )  are  common  in 
certain  beds,  the  peculiar  family  Receptaculites  (Fig.  414  B)  which 
began  in  the  Ordovician  being  not  uncommon  in  some  localities. 

Echinodermata 

Crinoids  (Fig.  415  A,  C)  became  so  numerous  in  the  Silurian  that 
their  "  joints  "  constitute  an  important  part  of  the  beds  of  certain 


FIG.  414.  —  Silurian  sponges  :  A,  Astraospongia  meniscus, 
two  views;  B,  Receptaculites  ohioensis. 


446 


HISTORICAL  GEOLOGY 


limestones.     Where  these  flowerlike  animals  were  abundant  on  the 
sea  bottom,  they  must  have  presented  an  appearance  not  unlike  that 


FIG.  415.  —  Silurian  crinoids  :    A,  Eucalyptocrinus  elrodi;   C,  Eucalyptocrinus  crassus 
(closed).     Cystoid  :   B,  Caryocrinus  ornatus.     Blastoid  :  D,  Troostocrinus  reinwardti. 

of  a  field  of  lilies.     Not  only  did  they  live  in  great  numbers,  but  the 
variety  of  forms  which  were  developed   was   large. 


continued  to  be  abun- 
dant when  conditions 
were  favorable  for 
their  growth,  but  at 
the  close  of  the  period 
they  were  no  longer  an 
important  element  of 
the  fauna. 

Molluscoidea 

Brachiopods.  —  Al- 
though    the     Silurian 

FIG.   416. -Silurian  brachiopods:    A,  Rhynochotreta    brachi°P°ds   (Fig-  4'6 
cuneata    americana;    B,    Spirifer    radiatus;    C,    Streptis 


grayi;  D,  Pentamerus  oblongus. 


A-D)    differed    almost 
entirely  from  those  of 


THE  SILURIAN  PERIOD 


447 


the  Ordovician  in  species,  the  importance  of  the  race  did  not 
diminish.  Some  improvements  in  structure  were  accomplished,  and 
new  genera  which  later 
became  important 
were  evolved.  The 

evolutional       changes    M  ,^^^HHI^^^^^^        J£H  D 

were  doubtless  directly 
or  indirectly  the  result 
of  the  changes  in  en- 
vironment, which  con- 
sisted in  shiftings  of 
the  epicontinental  seas 
and  the  consequent 
frequent  migrations  of 
faunas  and  struggles 
between  them.  FIG.  417.  —  Silurian  bryozoans  (B-E),  and  pteropod 

Bryozoa.    The    ^):    ^»  Tentaculites  gyr acanthus;    B,  Lichenalia  concen- 

11-1  ,  trie  a;   C,  a  portion  of  B  enlarged;   D,  Callopora  elegan- 

coral-hke      bryozoans  fula.  ^  a  portjon  of  D  enlarged 

(Fig.    417)    were   less 

important  reef  builders  in  the  Silurian  than  in  the  Ordovician. 

Mollusc  a 

Gastropods.  — Aside  from  an  increase  in  the  number  and  variety 
of  species  with  elevated  spines  and  in  a  somewhat  greater  abundance, 


B 

^MHHP^  ^^F.   — ^ 

FIG.  418.  —  Silurian    pelecypod :      A,    Pterinea    emacerala.     Silurian    gastropods: 
B  and  C,  two  views  of  Trematonotus  alpheus;  D,  Strophostylus  cyclostomus;  E,  Platy- 
ostoma  (Diaphorostoma)  niagarense. 
CLELAND   GEOL. —  29 


448 


HISTORICAL  GEOLOGY 


FIG,  419.  —  Silurian 
cephalopods :  Ay  Daw- 

sonoceras  americanum ; 
By  Phragmoceras  par- 
vum ;  Cy  Trochoceras 
desplainense. 


no  important  changes  in  the  gastro- 
pods (Figs.  418  B-E)  are  shown. 

Pelecypods  (Fig.  418  A)  also  con- 
tinued much  as  in  the  Ordovician. 

Cephalopods. — Curved  and  coiled 
cephalopods  (Fig.  419  B,  C)  were 
more  numerous  than  straight  forms 
(Fig.  419  A),  while  the  latter  were 
smaller  and  were  commonly  orna- 
mented by  rings  and  low,  trans- 
verse ridges.  The  body  chamber  of 
many  Silurian  cephalopods  was  con- 
stricted (Fig.  419  B),  the  constric- 
tion being  apparently  for  the  pur- 
pose of  protecting  the  soft  parts  of 
the  animal  from  enemies.  As  in 
the  Ordovician,  this  class  was  the 
most  powerful  of  the  time. 

Arthropoda 

Trilobites. — This  interesting  class 
(Fig.  420  A-D)  was  still  important, 
but  the  decline  had  already  begun 
and  it  was  numerically  decidedly 
less  prominent  than  in  the  Ordo- 


vician (Fig.  421).     Since  no  new  families  appeared,  the  general  aspect 


B 


D 


FIG.  420.  —  Silurian  trilobites  :  Arctinurus  (Lichas)  bigsbyi  (boltoni) ;  B,  Ceratocephala 
dufrenoyi;   C,  Dalmanites  limulurus ;  D,  Bumastus  (Illcenus]  ioxus. 


THE   SILURIAN  PERIOD 


449 


CAMBRIAN  ORDOVICIAN        SILURIAN  DEVONIAN  CARBONIFEROUS!  PERMIAN 

3WER     [MIDDLE.       iiPPER        LOWER       MIDDLE    I   UPPEI 


TRILOBITES— < 


FIG.  421.  — Table  showing  the  history  of  the  trilobites.  It  is  seen  that  the  class 
did  not  gradually  increase  and  then  gradually  decrease,  but  that  there  were  times 
during  which  the  species  and  individuals  greatly  increased  and  others  in  which,  for  a 
time,  there  was  a  decrease. 

of  the  class  did  not  differ  greatly  from  that  of  the  preceding  period. 
The  most  significant  change  from  the  Ordovician  was  in  the  dis- 
appearance of  Ordovician  genera. 

Eurypterids.  —  The  arthropods  (Greek,  arthron,  joint,  and  pous, 
foot),  the  branch  of  which  the  crustaceans  and  insects  are  members, 
reached  their  greatest  size  in  the  eurypterids  (Fig.  422).  Some  of  the 
Silurian  forms  at- 
tained a  length  of  one 
and  a  half  feet,  while 
in  the  Devonian  there 
were  giants  eight  feet 
long.  They,  together 
with  the  giant  cephal- 
opods,  were  prob- 
ably the  terrors  of 
the  sea  until  the  fish 
obtained  the  mas- 
tery. They  had  elon- 
gated bodies  covered 
with  a  leathery  or 
horny  integument. 
On  the  under  side 
were  six  pairs  of  legs, 
of  which  the  first  had 
largeorsmall  pincers. 
Eurypterids  are  re- 
lated to  the  horse- 
shoe crabs  (Limulus). 

The  presence  of  gills  and  their  association  with  cephalopods  and 
trilobites  in  the  Ordovician  show  that  they  lived  in  water  and  were 
for  the  most  part  mud  crawlers,  although  some  were  good  swimmers. 


FIG.  422.  —  Silurian  eurypterid  :  Dolichopterus  macro- 
cheirus.      (After  J.  M.  Clarke.) 


450 


HISTORICAL  GEOLOGY 


They  were  at  first  marine  animals,  but  late  in  the  Paleozoic  became 
adapted    to   brackish   and    possibly  to   fresh-water  conditions,   and 
there  is  evidence  for  the  belief  that  some  even 
lived  in  lagoons  where  the  water  was  more 
salty  than  that  of  the  sea. 

Scorpions  (Fig.  423)  first  appear  in  the 
Silurian,  but  probably  lived  in  water  and  got 
their  oxygen  there,  not  on  land  as  do  their 
modern  relatives,  the  ability  to  breathe  in  air 
having  been  acquired  later  in  the  Paleozoic. 

Fishes 

Fragmentary  fish  remains  have  been  found 
in   Silurian   rocks   (Fig.   424  A,  B)   of  both 
Europe  and  America.     The  fact  that  fishes 
were  abundant  and  of  considerable  variety  in 
the  Devonian  is  presumptive  evidence  that  a 
somewhat  varied    fish   fauna  existed   during 
FIG.  423T— Restoration  of    the  dosing  days  of  the  Silurian, 
a  Silurian  scorpion. 

SUMMARY 

Life  on  the  Land.  —  It  is  probable  that  the  lands  of  the  period  were 
clothed  with  plants,  but  if  so,  little  evidence  is  afforded  either  from 


FIG.  424.  —  Restorations  of  Silurian  fishes  (ostracoderms) :  Ay  Thelodus; 
B,  Pteraspis. 

the  remains  of  plants  or  of  land  animals.     The  highly  developed  land 
plants  of  the  Devonian  (p.  467),  however,  are  indirect  evidence  of  the 


THE   SILURIAN   PERIOD  451 

existence  of  land  plants  in  the  preceding  period.  Nevertheless,  the  ab- 
sence of  the  sediments  of  fresh-water  lakes  in  America,  where  land 
fossils  are  likely  to  be  preserved,  leaves  us  without  evidence,  although 
it  does  not  prove  that  there  were  no  land  plants  or  animals. 

Migration.  —  The  presence  of  thirty  or  more  identical  species  in  the 
Silurian  strata  of  Iowa  and  northwestern  Europe  indicates  that  migra- 
tion between  the  two  continents  took  place.  This  presumption  is 
strengthened  by  the  discovery  of  a  peculiar  genus  of  coral  (Fig.  413  E, 
p.  444)  whose  quadrangular  opening  was  protected  by  a  calca- 
reous covering.  Since  the  interior  seas  of  North  America  had  no  free 
communication  on  the  east,  it  is  thought  that  the  migration  took 
place  along  a  belt  of  shallow  water  which  extended  through  Canada, 
Alaska,  and  perhaps  the  Arctic  region. 

Climate  and  Duration.  —  The  climate  of  this  period  seems  to  have 
been  uniform  over  the  entire  world,  as  during  the  preceding  periods, 
there  being  no  positive  evidence  of  the  existence  of  climatic  zones. 
The  presence  of  salt  and  gypsum  beds,  locally  40  to  80  feet  in  thick- 
ness, in  the  Silurian  strata  (Salina)  of  New  York  and  Ohio  is  evidence 
that  desert  conditions  prevailed  during  a  portion  of  the  period,  prob- 
ably over  a  considerable  area. 

The  Silurian  Period  was  probably  not  more  than  one  half  as  long 
as  the  Ordovician. 

Close  of  the  Silurian.  —  The  change  from  the  Silurian  to  the 
Devonian  in  eastern  North  America  is  even  less  clearly  marked  than 
that  between  the  Ordovician  and  the  Silurian,  the  formations  of  one 
often  passing  into  the  other  without  an  unconformity.  In  portions  of 
Great  Britain  an  unconformity  separates  the  two  systems,  but  in 
other  parts  of  Europe  there  is  no  break  in  the  sedimentation.  The 
separation  in  such  cases  is  based  upon  the  fauna. 

REFERENCES  FOR  THE  SILURIAN  PERIOD 

BLACKWELDER  AND  BARROWS,  —  Elements  of  Geology,  pp.  349-357. 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  2,  pp.  368-417. 

SCHUCHERT,  CHAS.,  —  Paleogeography  of  North  America:    Bull.  Geol.  Soc.  America, 

Vol.  20,  1910,  pp.  489-491. 

SCOTT,  W.  B.,  —  An  Introduction  to  Geology,  pp.  578-589. 
ULRICH,  E.  O.,  —  Revision  of  the  Paleozoic  Systems:   Bull.  Geol.  Soc.  America,  Vol.  22, 

I9XI. 


CHAPTER  XVIII 
THE  DEVONIAN1  PERIOD 

THE  passage  from  the  Silurian  to  the  Devonian  in  eastern  North 
America  was  without  any  physical  break,  the  transition  from  one  to 
the  other  being  marked  by  no  great  unconformity.  In  fact,  so  grad- 
ual was  the  change  that  much  controversy  has  arisen  as  to  the  exact 
limits  of  the  two  systems.  Not  only  was  the  change  in  the  lithologi- 
cal  character  and  structure  of  the  strata  slight,  but  the  life  at  th<* 
close  of  the  Silurian  and  the  beginning  of  the  Devonian  was  vei^ 
similar.  Wherever  the  Devonian  seas  spread  over  a  wider  or  dif- 
ferent area  from  that  of  the  Silurian  the  sediments  were  laid  down  un- 
conformably  on  older  rocks.  Unconformities  of  this  sort  occur  in  Iowa 
and  elsewhere,  but  they  are  inconspicuous  and  are  sometimes  with 
difficulty  recognized  as  unconformities,  since  the  underlying  rocks 
are  horizontal  and  their  surfaces  had  not  been  greatly  roughened  by 
erosion.  The  lack  of  great  relief  as  shown  in  these  unconformities 
affords  an  evident  explanation  for  the  fineness  of  the  sediments  in 
the  Western  Interior  in  the  closing  stages  of  the  Silurian,  as  well  as  of 
those  of  the  early  days  of  the  Devonian. 

Subdivisions  of  the  Devonian.  —  The  subdivisions  of  this  period 
in  New  York  state  are  given  below,  both  because  it  was  in  this  state 
that  they  were  first  studied  with  care  in  North  America,  and  also 
because  the  system  is  best  developed  there.  • 

Catskill  and  Chemung  sandstones 
Portage  shale  and  sandstones 
Genesee  shale 
Tully  limestone 
Hamilton  shale 
Marcellus  shale 
Onondaga  limestone 
Oriskany  sandstone 
Helderberg  limestone 

1  The  Devonian  received  its  name  from  the  shire  of  Devon,  England,  where  a  great  series  of 
strata  of  this  period  occur. 

452 


THE   DEVONIAN   PERIOD 


453 


Geography.  —  The  close  of  the  Silurian  found  few  epicontinental 
seas  in  North  America.  In  the  east  (Fig.  425)  the  Appalachian 
trough,  portions  of  New  York,  and  certain  areas  in  the  Maritime 
Provinces  of  Canada  were  covered  with  seas,  and  a  bay  extended  from 
the  Gulf  of  Mexico  toward  the  north  along  the  valley  of  the  Missis- 
sippi. In  the  west  an  arm  of  the  sea  extended  from  the  Pacific  across 
the  site  of  the  Sierra 
Nevada  into  Utah. 
The  outlines  of  these 
seas  were  not  con- 
stant but  changed 
from  stage  to  stage. 
Later  in  the  period 
(Fig.  426)  the  seas 
spread  widely  over 
.  ie  continent,  calling 
to  mind  the  sub- 
mergent  condition  of 
the  Middle  Ordo- 
vician  and  Middle 
Silurian. 

In  New  York  state 
the  formations  of  the 
first  half  of  the  De- 
vonian are  for  the 
most  part  limestones 
with  occasional  shales 
and  sandstones,  but 
in  the  later  half  of 
the  period  shales  and 
sandstones  predomi- 
nate. The  shales  and 
sandstones  were 
brought  into  the  sea  by  streams  from  the  Taconics  of  Massachusetts, 
and  probably  from  land  areas  which  existed  to  the  north  in  Canada. 
The  Devonian  strata  cover  a  greater  area  at  the  surface  in  New 
York  than  any  other  rocks,  and  their  combined  thickness  is  more 
than  4000  feet  (Fig.  427).  They  are  much  thicker  in  Pennsylvania, 
but  thinner  in  the  Mississippi  Valley,  and  are  said  to  be  8000  feet 
thick  in  portions  of  Nevada. 


FIG.  425.  —  Map  showing  the  probable  outline  of 
North  America  during  a  portion  of  the  Lower  Devonian. 
(Modified  after  Schuchert.) 


454 


HISTORICAL  GEOLOGY 


The  Devonian  in  New 
York.  —  There  are  three 
Devonian  formations  in 
New  York  which  deserve 
especial  mention:  the 
Oriskany,  the  Onondaga, 
and  the  Catskill.  The 
Oriskany  is  a  sandstone 
formation  made,  for  the 
most  part,  of  clean  beach 
sands,  which  in  New  York 
is  from  a  foot  to  several 
feet  thick  and  in  the 
Middle  Atlantic  States  is 
several  hundred  feet  thick. 
The  formation  indicates  a 
raising  of  the  land  or  an 
increase  in  the  rainfall,  or 
a  combination  of  the  two, 
since  stronger  currents  are 
necessary  to  supply  coarse 
waste. 

The  Onondaga  lime- 
stone with  its  wealth  of 
corals  and  brachiopods  in- 
dicates warm,  clear  seas 
of  long  duration  sur- 
rounded by  low  lands. 

The  Catskill  formation, 
thousands  of  feet  thick, 
extends  from  Virginia  to 
the  Catskill  Mountains  in 
New  York  and  is  a  great  delta  deposit,  made  of  alternate  layers  of  sandstone  and 
shale,  sometimes  the  one  and  sometimes  the  other  predominating.  Upon  this  assump- 
tion the  whole  constitutes  a  delta.  The  form  of  the  plain  was  probably  somewhat 


FIG.  426.  —  Map  showing  the  probable  outline  of 
North  America  during  a  portion  of  the  Middle  Devonian. 
Solid  black  shows  continental  deposits.  (Modified  after 
Schuchert.) 


Devonian  Lv.-l  Silurian  f-_-_-|  Ordovician 

FIG.  427.  —  A  Section  in  New  York  state  showing  the  relation  of  the  Ordovician, 
Silurian,  and  Devonian.     (After  W.  J.  Miller.) 

similar  to  that  of  the  high  plains  region  of  the  western  interior  of  North  America 
(p.  588)  (Barrell).1  While  the  Catskill  delta  was  being  built  up,  muds  were  accu- 
mulating in  the  shallow  seas  (Portage  and  Chemung). 

1  Barrell,  J.,  —  The  Upper  Devonian  Delta  of  the  Appalachian  Geosyncline:  Am.  Jour.  Sci., 
Vol.  36,  1913,  pp.  420-472;   and  Vol.  37,  1914,  pp.  87-109,  225-253. 


THE   DEVONIAN  PERIOD  455 

Continent  of  Appalachia.  —  The  continent  of  Appalachia,  situated 
east  of  the  present  Appalachian  Mountains,  which  during  the  pre- 
ceding periods  of  the  Paleozoic  was  supplying  the  streams  with  sedi- 
ment for  the  Appalachian  geosyncline,  was  extensive  at  this  time 
and  was  probably  a  broad,  mountainous  upland  whose  eastern  bound- 
ary may  have  been  beyond  the  present  eastern  limit  of  the  continen- 
tal shelf.  This  conclusion  is  justified  when  the  volume  of  sediments 
laid  down  in  the  Appalachian  trough  is  computed.  Such  a  computation 
shows  that  the  crest  of  Appalachia  would  have  had  to  be  lowered  from 
five  to  seven  miles  to  supply  the  Upper  Devonian  sediments,  if  it  had 
not  extended  beyond  the  continental  shelf.  (Barrell.)  It  seems  likely, 
therefore,  that  Appalachia  extended  from  the  edge  of  the  Appalachian 
trough  eastward  over  the  present  site  of  the  continental  shelf  and  prob- 
ably fifty  miles  beyond.  The  broad  Appalachian  continent  probably 
never  reached  Alpine  heights,  but  was  rather  slowly  raised  as  the 
Appalachian  trough  sank.  The  sediments  of  the  trough  are  those 
formed  from  igneous  rocks  of  the  land  which  had  been  subjected  to 
chemical  decay,  and  are  not  such  as  would  have  resulted  from  the 
mechanical  disintegration  of  frost  or  changes  in  temperature.  The 
sediments,  moreover,  are  seldom  coarse,  showing  that  the  streams  did 
not  flow  from  a  high,  mountainous  region  in  proximity  to  the  sea. 

Igneous  Rocks.  —  In  Maine,  New  Brunswick,  and  Nova  Scotia, 
granite  intrusions  and  volcanic  extrusions  took  place  during  the 
Devonian.  The  city  of  Montreal  lies  at  the  foot  of  a  volcano,  and  there 
are  other  volcanoes  to  the  southeast.  This  was  the  first  premonitory 
indication  of  the  movements  which  were  later  to  form  the  great  Appa- 
lachian Mountains.  When  North  America  as  a  whole  is  considered, 
the  Devonian  Period  closed  with  almost  no  deformation. 

Devonian  Oil  and  Gas.  —  A  discussion  of  the  Devonian  would  be 
incomplete  without  mention  of  the  important  oil  and  gas-bearing 
strata  of  West  Virginia,  Pennsylvania,  and  southwestern  New  York. 
The  oil  and  gas  are  more  likely  to  be  found  at  or  near  the  crests  of  low 
anticlines  (p.  425)  than  in  any  other  situation. 

Devonian  of  Other  Continents.  —  Epicontinental  seas  were  wide- 
spread in  Europe  and  Asia  during  the  Devonian,  and  smaller  seas 
covered  portions  of  Africa,  South  America,  and  Australia.  The 
Devonian  of  England  is  of  unusual  interest  because  of  the  develop- 
ment of  a  continental  deposit  of  red  sandstone,  called  the  "  Old  Red 
Sandstone."  It  appears  to  have  been  laid  down  under  desert  condi- 
tions, although  no  gypsum  or  salt  beds  prove  this  contention.  In 


456 


HISTORICAL  GEOLOGY 


addition  to  the  Old  Red  Sandstone  there  are  marine  deposits  contain- 
ing abundant  fossils.  Volcanic  action  in  Europe  during  the  Devonian 
is  proved  by  thick  volcanic  accumulations  in  Great  Britain  and  west 
central  Europe. 

LIFE  OF  THE  DEVONIAN 

The  invertebrate  life  of  this  period  was,  in  general  aspect,  like 
that  of  the  Silurian,  but  there  were  many  changes  in  genera  and  an 
almost  total  change  in  species.  The  contrast  between  the  inverte- 
brate life  of  the  Silurian  and  the  Devonian  was  about  as  marked  as  that 
between  the  Ordovician  and  the  Silurian.  As  in  the  foregoing  periods, 
certain  species  were  characteristic  not  only  of  the  period  as  a  whole 
but  of  each  of  its  stages. 

Ccelenterata 

Corals  (Fig.  428  A—C)  were  present  in  great  numbers  and  species, 
being  almost  or  quite  as  abundant  as  in  the  Silurian.  Cup  corals 

(Tetracoralla,  Fig.  428 
Ay  C),  honeycomb  corals 
(Favosites),  and  organ- 
pipe  corals  (Syringo- 
pora)  flourished  when 
conditions  were  favor- 
able, but  chain  corals 
(Halysites)  had  be- 
come extinct  in  the  be- 
ginning of  the  period. 

The  coral-reef  character 
of  some  of  the  limestones 
of  the  period  is  splendidly 
exhibited  at  the  falls  of 
the  Ohio,  Louisville,  Ken- 
tucky, "where  the  corals 
are  crowded  together  in 
great  numbers,  some  stand- 
ing as  they  grew,  others  lying  in  fragments,  as  they  were  broken  and  heaped  by  the 
waves,  branching  forms  of  large  and  small  size  mingling  with  massive  kinds  of  hemi- 
spherical and  other  shapes."  "Some  of  the  cup  corals  are  6  or  7  inches  across  at 
the  top,  indicating  a  coral  animal  6  or  8  inches  in  diameter."  "Hemispherical  com- 
pound corals  occur  5  or  6  feet  in  diameter."  "The  various  coral  polyps  of  the  era 
had,  beyond  doubt,  bright  and  varied  colorings,  like  those  of  the  existing  tropics, 
and  the  reefs  were  therefore  an  almost  interminable  flower  garden."  (Dana.) 


FIG.  428.  —  Devonian  cup  and  compound  corals : 
Ay  Heliophyllum  halli;  B,  Pleurodictyum  stylopora;  C, 
Acervularia  davidsoni. 


THE   DEVONIAN  PERIOD 


457 


Corals  are  not  equally  abundant  in  all  Devonian  formations;  they 
are  rare  in  shales  and  sandstones,  but  are  usually  common  in  lime- 
stones. This  is  not  remarkable,  since  corals  do  not  thrive  in  muddy 
waters. 

Echinodermata 


were 


Crinoids    (Fig.    429   B)    and    starfish    (Fig.   429   A)   were    much 
more    abundant    than     in     previous     periods,    but    cystoids 
rarer  than  in  the  Silurian. 

Blastoids  (Greek,  blastos, 
bud)  were  locally  abundant. 
These  echinoderms  (Fig.  429 
C),  as  the  name  implies,  were 
oval,  with  five  petal-like  divi- 
sions resembling  a  flower 
bud.  They  were  armless 
and  were  attached  to  the  sea 
bottom  by  a  jointed  stem. 
Beginning  in  the  Ordovician, 
they  culminated  in  the  Mis- 
sissippian  (p.  480),  after 
which  they  occurred  sparsely 
and  disappeared  with  the 
Paleozoic. 

The    starfish   of   Devonian 
times  had  already  acquired  the  habits  of  feeding  which  they  possess 
to-day.1 

Molluscoidea  and  Mollusc  a 

Brachiopods  (Fig.  430  A-P)  were  never  more  abundant  in  in- 
dividuals and  species  than  during  portions  of  this  period,  and 
many  characteristic  species  were  present.  Long-hinged  spirifers 
were  especially  abundant  and  highly  developed  throughout  the 
Devonian. 

Bryozoans  (Fig.  431  A-D)  were  locally  abundant. 

Pelecypods  (Fig.  432  A-E)  flourished  where  the  bottoms  were 
muddy  and  other  conditions  favorable. 

Gastropods  (Fig.  433  A-C)  were  subordinate  in  numbers  to  the 
pelecypods  but  were  not  uncommon. 


FIG.  429.  —  Devonian  starfish,  crinoid,  and 
blastoid  :  A,  Palaaster  eucharis;  B,  Melocrinus 
milwaukeensis  ;  C,  Nucleocrinus  verneuili. 


1  Clarke,  J.  M.,  —  Early  Adaptation  in    the  Feeding  Habits  of  Starfish:   Acad.  Nat.  Sci., 
Philadelphia,  Vol.  15,  1912,  pp.  113-118. 


FIG.  430.  —  Devonian  brachiopods :  ^,  Hipparionyx  proximus;  B,  Spirifer  acumi- 
natus;  C,  Stropheodonta  demissa;  D,  Spirifer  mucronatus;  E,  Rhipidomella  oblata; 
F,  Camarotoechia  endlichi;  G,  Tropidoleptus  carinatus ;  H,  A  try  pa  reticularis;  /,  Gypi- 
dula  gale ata;  J,  Spirifer  disjunctus;  K,  Eatonia  medialis;  L,  Chonetes  coronatus; 
M,  Productella  spinulicosta ;  N  and  0,  two  views  of  Hypothyris  cuboides;  P,  Rens- 
selceria  ovoides. 


FIG.  431.  —  Devonian  bryozoans :  A,  Fistulipora  micropora  surrounding  a  crinoid 
stem;  B,  a  portion  of  the  same  greatly  enlarged  to  show  the  arrangement  of  the  cells; 
C,  branches  of  Cystodictya  hamiltonensis ;  Z),  Polypora  lilcea. 


THE   DEVONIAN   PERIOD 


459 


FIG.  432.  —  Devonian  pelecypods:    A,  Pterinea  flabellum ;    B,  Modiomorpha  concen- 
tric a;  Cy  Conocardium  ohioense ;  D,  Buchiola  retrostriata ;  E,  Palceoneilo  constricta. 


Cephalopods.  —  A  rather  incon- 
spicuous member  of  this  class,  the 
goniatite  (Greek,  gonia,  angle),  but 
one  whose  modified  descendants  were 
to  become  the  most  prominent  inver- 
tebrates of  the  Mesozoic,  began  in 
the  Devonian.  The  important  char- 
acteristic of  this  coiled  cephalopod 
was  the  angled  and  lobed  suture  line 
(Fig.  434  A,  B),  i.e.,  instead  of  smooth 
partitions  (septa,  p.  434)  which  joined 
the  outer  shell  in  straight  lines  or  in 


FIG.  433.  —  Devonian  gastropods: 
y  Strophostylus  expansus;  B,  Platy- 


simple  curves,  the  septa  were  crumpled    ceras  *"•*»•*;  C,  Loxonema  noe. 
at  the  edges  at  the  juncture  with  the  outer  shell,  forming  angled 


FIG.  434.  —  Devonian  cephalopods  :   A,  Manticoceras  oxy;   B,  Tornoceras  mithras. 


460 


HISTORICAL  GEOLOGY 


sutures.  The  straight  (Orthoceras)  and  coiled  (Gomphoceras)  ceph- 
alopods  with  simple  sutures  continued  throughout  the  period  but 
were  much  less  common  in  the  later  portion. 

Arthropoda 

Trilobites.  —  During  the  earlier  stages  of  the  Devonian  more  than 
50  species  of  trilobites  (Fig.  435  A-C)  are  known  to  have  existed, 


FIG.  435.  —  Devonian  trilobites  :  A,  Lichas  (Gaspelichas]  forillonia  (cephalon) ; 
B,  Dipleura  dekayi;  C,  Phacops  rana. 

but  the  numbers  rapidly  decreased  during  the  later  stages.  In  the 
earlier  portions  of  the  period  especially,  a  number  of  highly  orna- 
mented, spinous  forms  (Fig.  435  A)  lived,  but  later  these  extrava- 
gant species  largely  disappeared,  and  those  of  simpler  outlines 
remained.  The  decline  of  the  trilobites  during 
the  Devonian  was  very  marked,  and  at  its  close 
they  were  on  the  verge  of  extinction,  although 
a  few  survived  until  the  close  of  the  Paleozoic. 
Other  crustaceans  (Fig.  436)  also  lived  in  con- 
siderable abundance. 

Barnacles,  which  are  retrograde  crustaceans 
that  have  given  up  a  free-moving  existence  for 
a  stationary  one  in  which  they  are  protected 
by  a  calcareous  covering,  began  in  the  Ordo- 
vician,  but  the  common  acorn  barnacle  began  in 
this  period. 

Eurypterids  attained   their  greatest  size  during 
FIG.  436.  —  Echino-      .       _*f       .  '    ..  ,      * 

cans  punctata,  a  De-    tne  LJevoman,  one  species  reaching  a  length  or 

vonian  crustacean.          almost  eight  feet  (Fig.  437). 


THE   DEVONIAN  PERIOD  461 

Insects.  —  No  undoubted  re- 
mains of  insects  have  been  found 
in  strata  of  this  or  earlier  periods, 
although  their  discovery  may  be 
expected  at  any  time. 

Fishes 

The  appropriateness  of  the 
term  Age  of  Fishes  as  applied 
to  the  Devonian  Period  is  evi- 
dent when  the  importance  of 
this  great  class,  not  only  in  the 
life  of  that  time  but  as  the  prob- 
able progenitors  of  all  subse- 
quent vertebrate  life,  is  con- 
sidered. Fish  were  the  rulers  of 
the  Devonian  seas  and  rivers, 
perhaps  even  to  a  greater  degree 
than  were  the  trilobites  in  the  FIG.  437.  —  Stylonurus,  a  gigantic  De- 
Cambrian  and  the  cephalopods  vonian  eurypterid,  some  of  which  were  eight 
intheOrdovician.  The  fact  that  feet  long'  (After  Clarke  and  Ruedemann.) 
fish  were  abundant  during  the  period  does  not  imply  that  other  forms 
of  life  were  less  abundant  than  in  previous  periods.  For  example, 
brachiopods  are  exceedingly  common  fossils  in  almost  all  Devonian 
strata,  while  in  many  of  the  rocks  of  this  age  fish  fossils  are  extremely 
rare  and  a  search  of  many  days  may  not  be  rewarded  by  even  a 
fragment. 

Ostracoderms.  —  One  of  the  strangest  classes  of  Devonian  animals 
was  the  ostracoderm  (Greek,  ostrakon,  shell,  and  derma,  skin).  These 
were  fishlike  in  shape  but  were  probably  not  even  closely  related 
to  fishes.  The  description  of  one  well-known  member  of  the  class 
(Cephalaspis,  Greek,  cephale,  head,  and  aspis,  shield)  gives  a  general 
notion  of  this  group  (Fig.  438  A).  The  most  striking  feature  was  the 
crescent-shaped  plate  which  covered  the  head  and  fore  part  of  the  body. 
Besides  the  protection  afforded  by  this  head  shield,  the  tail  was  covered 
with  rhomboidal  scales.  The  eyes  were  situated  close  together  on  the 
top  of  the  head.  The  lower  jaw,  if  it  ever  existed,  has  not  been  found. 
Although  many  hundreds  of  the  bony  parts  have  been  found,  no  in- 
ternal skeleton  has  been  discovered,  and  it  is  therefore  probable  either 
that  none  existed  or  that  it  was  cartilagenous  and  was  consequently 


462 


HISTORICAL  GEOLOGY 


incapable  of  fossilization.  In  another  genus  (Bothriolepis)  a  pair  of 
appendages  encased  in  bony  plates,  somewhat  as  are  the  appendages 
of  a  lobster,  extended  from  the  sides  of  the  head.  Ostracoderms 
seldom  attained  a  size  greater  than  6  or  7  inches. 

Certain  inferences  as  to  the  habits  of  ostracoderms  can  be  drawn 
from  their  structure.  They  probably  lived  on  the  sea  bottom  as  did 
the  trilobites,  either  burrowing  in  the  mud  above  which  only  their 
eyes  and  their  dorsal  shield  showed,  or  because  of  their  dull  coloring 
crawling  over  it  inconspicuously.  The  fact  that  they  were  protected 
implies  that  they  had  to  contend  with  enemies  more  powerful  than 
themselves.  They  lived  in  large  numbers  in  certain  localities,  as  is 


FIG.  438.  —  Devonian  ostracoderms:  A,  Cephalaspis,  about  six  inches  long; 
By  Bothriolepis,  about  seven  inches  long. 

shown  by  the  great  abundance  of  their  shields  which  form  thin  beds 
in  some  places  and  are  said  to  be  hardened  by  the  oil  from  their  re- 
mains. 

Ostracoderms  began  in  the  Ordovician,  reached  their  climax  in  the 
Devonian,  and  became  extinct  at  its  close. 

Sharks.  —  Sharks  lived  in  the  Devonian  in  considerable  numbers, 
but  since  their  skeletons  were  cartilagenous  the  fossil  evidence  of 
their  existence  consists  largely  of  teeth,  spines  (which  probably  stood 
in  front  of  the  dorsal  fin),  and  small  bony  denticles  which  were  doubt- 
less embedded  in  the  skin.  The  best  known  and  most  simple  in  struc- 
ture of  these  ancient  sharks  (Cladoselache,  Fig.  439  A]  varied  from 
two  to  six  feet  in  length.  It  had  a  short,  blunt  snout  with  the  mouth 
situated  on  the  lower  side  but  farther  front  than  in  the  modern  shark. 


THE   DEVONIAN   PERIOD 


463 


The  teeth  occurred  in  clusters  (Fig.  440)  and  were  arranged  in  six 
or  seven  rows  one  behind  the  other.  The  fins  were  very  simple,  con- 
sisting of  a  flap  of  skin  strengthened  by  straight  rods  of  cartilage. 
It  was  very  unlike  modern  sharks  in  the  contour  of  its  body. 


FIG.  439.  —  A,  shark,  Cladoselache,  which  sometimes  reached  a  length  of  six  feet  (see 
Fig.  440) ;   B,  the  Port  Jackson  shark,  Cestracion,  a  modern  shark  of  ancient  type. 

Other  sharks,  now  represented  by  the  Port  Jackson  shark  of  Aus- 
tralian waters  (Cestracion,  Fig.  439  J?),  were  abundant  in  the  later 
Paleozoic,  judging  from  the  number  of  their  spines  and  pavement 
teeth.  The  teeth  of  these  sharks  have  been  called  "  cobblestone  " 
pavement  teeth  because  of  their  resemblance  in  shape  and  arrange- 
ment in  the  jaw  to  a  pavement.  Such  teeth  would  be  of  use  in  crush- 
ing thin-shelled  crustaceans 
and  shellfish,  but  could  not 
have  been  used  for  rending. 
It  is  evident,  therefore,  that 
their  possessors  probably  lived 

on  muddy  bottoms   and  fed  FIG   ^  _  Teeth  rf  cladost!ache. 

on    brachiopods,    pelecypods, 

or  crustaceans.  The  structure  of  the  tooth  plate  of  sharks  "  is  very 
much  like  that  of  a  shark's  skin,  and  it  is  the  teeth  of  these  and  other 
sharks  that  best  illustrate  the  fact  that  teeth  are  really  modifications 
of  the  skin  and  do  not  belong  in  the  same  category  as  bones." 

CLELAND   GEOL.  —  30 


464 


HISTORICAL  GEOLOGY 


Lungfish.1  —  (i)  Armored  Lungfish. — The  most  formidable  and 
remarkable  fish  of  the  Devonian,  as  far  as  appearance  and  size 
is  concerned,  were  related  to  the  rare  lungfish  of  to-day,  although 
they  probably  did  not  possess  lungs.  One  of  the  most  remarkable 


B 

FIG.  441.  —  Armored  lungfishes:  A,  Dinichthys,  the  giant  fish  of  the  Devonian, 
some  of  which  attained  a  total  length  of  more  than  ten  feet,  with  head  three  feet  long; 
B,  Coccosteus,  a  fish  of  much  smaller  size. 

of  the  Devonian  lungfish  was  the  Dinichthys  (Greek,  deinos,  terrible, 
and  ichthus,  a  fish)  (Fig.  441^)  which  grew,  in  one  species,  to  be  25 
feet  long,  and  resembled  an  overgrown  catfish,  in  external  form.  The 
head,  which  in  one  species  was  six  feet  broad,  and  the  front  of  the  body 
were  protected  by  thick  bony  plates,  although  the  posterior  portions 
seem  to  have  been  quite  naked  unless  covered  by  a  leather-like  skin, 


FIG.  442.  —  An  unarmored  lungfish,  Scaumenacia. 

as  is  perhaps  indicated  by  certain  marks  upon  the  exterior  of  the  bony 
plates.  Their  powerful  jaws  were  adapted  for  tearing  and  cutting, 
and  their  shape  formerly  led  to  the  belief  that  these  fish  were  fierce, 

1  There  is  much  doubt  as  to  the  relationship  of  the  armored  lungfish,  some  holding  that  they 
should  be  placed  with  the  higher  ostracophores  (Bothriolepis,  etc.,  in  the  group  Placodermata), 
retaining  for  the  rest  of  the  ostracoderms  (Cephalaspis)  the  name  Enostracophori. 


THE   DEVONIAN   PERIOD 


465 


predaceous  creatures,  but  it  is  more  probable  that  they  lived  on  the 
ocean  bottom  and  subsisted  largely  on  shellfish,  using  their  powerful 
jaws  for  crushing.  With  their  heavy  armor  and  clumsy  shape  they 
were  probably  sluggish  in  their  movements. 

(2)  Unarmored  Lungfish.  —  Another  abundant  group  of  lung- 
fishes  whose  descendants  have  succeeded  in  living  to  the  present 
were  unhampered  by  armor  but  were  covered  with  thin  scales  (Fig. 
442).  A  modern  representative  (Ceratodus)  lives  in  Australian 
waters,  and  two  other  genera  are  known,  one  in  Australia  and  one  in 
South  America. 

Ganoids.  —  (i)  Fringed- finned  Ganoids  (Crossopterygians}.  —  Evo- 
lutionally,  this  is  the  most  important  of  the  Devonian  fishes,  since  it 


FIG.  44 j.  —  A  fringe-firmed  ganoid,  Holoptychius.     Some  of  these  were  four  feet  long. 

possessed  so  many  characters  in  common  with  early  amphibians 
(p.  485)  that  it  is  probable  that  the  latter  arose  from  this  order. 
The  fringe-finned  ganoids  had  conical  teeth  generally  fluted,  were 
covered  with  scales  which  were  rhomboidal  in  some  species  and 
rounded  in  others,  and  had  limblike  fins  (Fig.  443)  which  were  jointed 
to  the  skeleton  within  the  body. 

(2)  Another  order  of  ganoids  (Actinopteri)  may,  for  convenience, 
be  called  typical  ganoids  to  distinguish  them  from  the  fringe-finned 
ganoids.  These  fishes,  for  the  most  part,  had  thick,  rhomboidal 
scales,  such  as  those  of  their  modern  representative,  the  gar  pike 


FIG.  444.  —  A  typical  ganoid,  the  modern  gar  pike. 


466 


HISTORICAL  GEOLOGY 


(Fig.  444).     Typical  ganoids  were  the  most  abundant  and  character- 
istic fish  of  the  Triassic  and  Jurassic. 

Teleosts  or  Bony  Fish.  —  The  typical  fish  of  to-day,  such  as  the 
trout,  perch,  cod,  and  mackerel,  were  absent  and  do  not  appear  until 
the  Mesozoic. 

Comparison  of  Devonian  and  Modern  Fish.  —  The  teeth  of  Devo- 
nian and  Carboniferous  fishes  were  adapted  for  crushing  and  few  had 

,          ,  ^^      the  sharp,  rending  teeth 

possessed  by  fish  to-day. 
The    fishes    of   the    De- 
vonian    and     Carbonif- 
erous  were,    as    a   class, 
massive    and    clumsy   as 
compared  with  those  of 
the    present,    and    their 
bodies  were  probably  less 
flexible.       In     Devonian 
fishes  the  backbone  runs 
through  to  the  end  of  the 
tail,  and  the  fin  is  formed 
by  vertical  rays  extend- 
.ing  from  above  and  below 
(Fig.  445  -D).     In   some, 
the  resulting  fin  is  sym- 
metrical, but   in   others, 
as  in  the  modern  shark, 
it  is  unsymmetrical,  the 
FIG.  445.  —  Evolution  of  the  tail  fin  of  fishes,    backbone     turning     up- 
(After  Dean.)     A,  embryonic  tail   fin;    C,  uneven-    w_rJQ   w:tu     an    lineniial 
lobed  tail  fin  of  the  Port  Jackson  shark;  D,  uneven-    WafdSr  **"    *"    l    'eqUal 
lobed   tail   fin   of  a   Devonian   shark,   Cladoselache ;    lobe    formed    of   rays    on 
y  even-lobed   tail   fin  of  the  fringe-finned  ganoid ;    the      under     side.        The 

tails  of  all  Devonian 
fishes  were  either  sym- 
metrical or  unsymmetrical  (Fig.  445  C,  Z>),  but  none  had  the  homo- 
cereal  tail  (Fig.  445  E)  of  modern  bony  fish  (Teleosts),  in  which  the 
backbone  ends  in  a  broad  plate  from  which  diverging  rays  spread 
to  form  a  symmetrical  tail  of  a  type  very  different  from  that  of 
Devonian  fish. 

It  is  interesting  in  this  connection  to  note  that  the  heavily  armored 
fish  were  the  first  to  become  extinct.  They  were  admirably  suited 


and  E,  the  tail  fin  of  a  modern  Teleost,  such  as  the 
trout. 


THE  DEVONIAN  PERIOD  467 

for  certain  conditions,  being  protected  from  their  enemies  by  heavy 
armor,  but  when  the  environment  and  food  changed  their  very  weight 
and  size  were  of  disadvantage  (p.  550),  and  they  failed  to  survive. 

Why  the  Vertebrate  Type  was  "  Fit."  —  The  reason  for  the  estab- 
lishment of  the  vertebrate  type  of  animals  and  their  rapid  rise  when 
once  they  appeared  is  evident  when  their  structure  is  considered.  An 
internal  skeleton  offers  an  excellent  attachment  for  muscles,  and  at 
the  same  time  permits  a  great  flexibility  of  the  body.  As  flexibility 
is  necessary  for  rapid  movement  through  the  water,  such  animals 
as  possessed  it  were  better  able  both  to  escape  their  enemies-  and  to 
secure  their  prey.  Moreover,  it  permitted  of  greater  size  than  as  a 
whole  appears  to  have  been  possible  in  other  classes.  The  position 
and  arrangement  of  the  nervous  system  appears  also  to  have  been 
especially  advantageous. 

Plants 

In  the  Devonian,  for  the  first  time  in  the  history  of  the  world,  land 
plants  are  known  to  have  been  abundant.  Discoveries  of  Silurian 
ferns  and  club  mosses  have  been  announced,  but  they  are  still  open 
to  doubt.  The  plants  of  the  Devonian  were  of  about  the  same  general 
level  of  organization  as  some  of  those  of  the  present  day,  although 
very  different  in  appearance ;  changes  have  occurred,  as  will  be  pointed 
out  from  time  to  time,  but  the  plants  of  this  early  period  were,  never- 
theless, so  highly  developed  as  to  prove  an  enormous  antiquity. 
"  There  are  probably  no  biologists  now  living  who  oppose  the  doctrine 
of  evolution  in  toto,  but  if  there  were,  they  might  draw  a  telling, 
though  falacious  argument  from  the  high  organization  of  the  Devo- 
nian flora."  1  (Scott.) 

At  this  time  horsetails,  ferns,  club  mosses,  gymnosperms  (of  which 
the  cypress,  yew,  and  pines  are  members),  and  the  extinct  spheno- 
phylls  and  seed  ferns  (pteridosperms)  are  known  to  have  existed. 
The  discussion  of  the  Devonian  flora  will  be  taken  up  in  the  descrip- 
tion of  the  Carboniferous  (p.  491),  since  in  this  later  period  it  reached 
the  climax  of  its  development. 

SUMMARY 

Migration  and  Evolution.  —  The  faunas  of  the  various  stages  of 
the  Devonian  often  differ  so  widely  from  one  another  as  to  suggest 

1  By  the  term  flora  is  meant  all  the  plants  that  grow  in  a  given  region  or  belong  to  a  given 
period. 


468  HISTORICAL  GEOLOGY 

that  at  certain  times  evolution  proceeded  at  an  unusually  rapid  rate. 
The  difference  in  the  faunas  of  succeeding  stages  is  due  to  several 
causes.  At  the  beginning  of  the  period  there  were  a  number  of  em- 
bayments  so  isolated  that  the  evolution  of  the  faunas  of  each  pro- 
ceeded independently,  until  each  possessed  many  characteristic  and 
peculiar  species.  As  the  seas  spread  over  the  land  later  in  the  period 
these  embayments  were,  one  after  another,  joined  together,  and  as 
quickly  as  a  waterway  opened  species  from  each  embayment  spread 
to  the  others,  and  a  struggle  for  existence  resulted  which  produced 
rapid  and  marked  changes  in  the  life,  exterminating  many  species. 
The  conflict  thus  brought  about  also  caused  the  rapid  rise  of  new 
forms  not  found  in  any  of  the  original  faunas. 

The  changes  in  the  physical  conditions  were  another  cause  of  rapid 
evolution.  As  a  result  of  the  extension  of  the  epicontinental  seas, 
new  food  was  doubtless  introduced  and  currents  were  developed 
which  may  have  brought  about  changes  in  temperature. 

It  should  not  be  forgotten  in  this  connection,  however,  that  the 
differences  in  the  faunas  of  beds  of  nearly  the  same  age  may  be  due 
entirely  to  the  fact  that  one  bed  was  deposited,  for  example,  in  shallow 
water  and  consequently  had  a  shallow  water  fauna,  and  another  in 
deep  water  and  had  a  deep  water  fauna.  The  life  of  two  such  beds 
may,  consequently,  differ  more  widely  than  those  of  very  different 
ages  which  were  deposited  under  similar  conditions. 

Climate  and  Duration.  —  Little  more  can  be  said  of  the  climate  of 
the  Devonian  than  of  the  Cambrian  and  Ordovician,  and  the  evidence, 
as  in  the  latter,  points  to  a  uniformly  warm  climate  over  the  entire 
world.  In  certain  places  deserts  existed  as  now,  while  in  others  ex- 
tensive swamps  were  present. 

The  period  was  probably  little  more  than  half  as  long  as  the  Ordo- 
vician. 

REFERENCES  FOR  THE  DEVONIAN  PERIOD 

BLACKWELDER  AND  BARROWS,  —  Elements  of  Geology,  pp.  358-368. 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  2,  pp.  418-495. 

SCHUCHERT,  CHAS.,  —  Paleogeography  of  North  America:    Bull.  Geol.  Soc.  America, 

Vol.  20,  1910,  pp.  491-493. 

SCOTT,  W.  B.,  —  An  Introduction^  Geology,  pp.  590-608. 
ULRICH,  E.  O.,  —  Revision  of  the  Paleozoic  Systems:  Bull.  Geol.  Soc.  America,  Vol.  22, 

1911. 


CHAPTER  XIX 
THE  CARBONIFEROUS  PERIODS 

THE  Carboniferous  formerly  included  the  Lower  Carboniferous 
(Mississippian),  the  Upper  Carboniferous  (Pennsylvanian),  and  the 
Permian.1  American  geologists  have  been  led  to  the  conclusion 
that  each  of  these  three  subdivisions  is  of  a  rank  equal  to  that  of  the 
Ordovician,  Silurian,  or  Devonian,  and  should  be  called  a  period.  In 
this  study  it  seems  advisable  to  discuss  the  life  of  the  three  periods 
together  (the  Lower  and  Upper  Carboniferous,  and  Permian)  since 
by  so  doing  the  sequence  of  life  changes  can  best  be  followed. 

MISSISSIPPIAN  OR  LOWER  CARBONIFEROUS 

The  epicontinental  seas  (Fig.  446)  of  the  early  portion  of  this  period 
were  about  as  extensive  as  in  the  Devonian  and  occupied  much  the 
same  regions.  As  a  result,  over  large  areas  the  transition  between 
the  Devonian  and  Mississippian  is  not  indicated  by  abrupt  changes. 
Towards  the  close  of  the  period  (Fig.  447),  the  seas  again  became 
constricted. 

The  sediments  brought  into  the  Appalachian  trough  from  the  con- 
tinent of  Appalachia  were  for  the  most  part  coarse  sands  and  muds. 
Sun  cracks,  ripple  marks,  the  footprints  of  amphibians,  and  other 
evidences  indicate  an  arid  or  semi-arid  climate,  and  that  the  sediments 
(Pocono  and  Mauch  Chunk)2  were  portions  of  a  great  delta  or  alluvial 
plain  built  by  shifting  streams  which  flowed  over  it.  The  Mississip- 
pian conglomerates  are  important  mountain  makers  in  the  Appala- 
chians. 

In  the  central  and  western  states  the  Mississippian  sediments  are 

1  The  term  Carboniferous  was  given  because  of  the  large  quantities  of  coal  (carbon)  in  the  rocks 
of  the  period.     The  subdivisions  —  Mississippian  and  Pennsylvanian  —  were  named  because 
of  the  great  development  of  the  rocks  of  the  periods  in  the  Mississippi  Valley  and  in  Pennsyl- 
vania respectively.     The  term  Permian  was  given  because  of  the  wide  extent  in  the  province  of 
Perm  in  Russia. 

2  Barrell,  J.,  —  Origin  and  Significance  of  the  Mauch  Chunk  Shale,  Bull.  Geol.  Surv.,  Vol. 
18,  1907,  pp.  449-476. 

469 


470 


HISTORICAL  GEOLOGY 


finer  than  in  the  East,  and  limestone  becomes  increasingly  abundant 

until,  west  of  Ohio,  it  constitutes  the  greater  mass  of  the  sediments. 

The  presence  of  gypsum  and  salt  in  portions  of  the  strata  of  this 

age  in  Michigan  shows  that  the  climate,  for  a  time  at  least,  was  dry, 


§ 


FIG.  446.  —  Map  showing  the  probable  outline  of  North  America  during  a  portion 
of  the  Lower  Mississippian.  Continental  deposits  are  shown  in  solid  black.  (Modified 
after  Schuchert.) 

and  that  the  sea  or  seas  of  this  region  were  isolated.  The  Mississip- 
pian gypsum  of  Nova  Scotia  implies  a  similar  condition  for  that  re- 
gion, and,  as  has  been  seen,  an  arid  or  semiarid  climate  was  present 
over  the  lands  contiguous  to  the  Appalachian  trough. 

The  Mississippian  seas  spread  over  a  large  area  of  the  Cordilleras 
of  the  West,  from  Mexico  to  the  Arctic,  the  strata  of  this  period  being 
several  thousand  feet  thick  in  certain  places. 

Close  of  the  Mississippian.  —  The  extensive  seas  of  the  early 
Mississippian  were  gradually  drained,  so  that  before  the  close  of  the 


THE   CARBONIFEROUS   PERIODS 


471 


period  the  eastern  portion  of  North  America  was  land.     In  the  west 
the  seas  also  seem  to  have  been  withdrawn. 

Other  Continents.  —  Mississippian  seas  spread  over  a  large  area 
in  England,  Ireland,  and  Europe,  and  the  sediments  which  had  been 


FIG.  447.  —  Map  showing  the  probable  outline  of  North  America  during  a  portion 
of  the  Upper  Mississippian.  The  areas  in  black  show  where  continental  deposits 
were  laid  down.  (Modified  after  Schuchert.) 


accumulating  in  them  were  locally  folded  at  the  close  of  the  period. 
The  term  Paleozoic  Alps  which  has  been  applied  to  this  folded  re- 
gion assumes  that  the  present  folded  areas  are  the  "  roots  of  former 
mountain  ranges."  Whether  erosion  kept  pace  with  elevation  or 
not  cannot  be  stated.  Strata  which  are  believed  to  be  of  this  age 
occur  in  northern  and  southern  Africa,  in  western  and  central  Asia  and 
China,  in  Australia  and  New  Zealand,  and  in  Argentina  and  Chile. 
Coal  occurs  in  the  strata  of  this  age  in  China,  Russia,  England,  and 
elsewhere. 


472 


HISTORICAL  GEOLOGY 


PENNSYLVANIAN  OR  UPPER  CARBONIFEROUS 

The  Pennsylvanian  system  is  generally  separated  from  the  Mississip- 
pian  by  an  unconformity,  the  Mississippian  strata  in  some  parts  of  the 
central  United  States  having  been  gently  folded,  faulted,  and  eroded 
before  the  deposition  of  the  Pennsylvanian  sediments.  A  few  seas 

persisted  in  Utah 
and  Arizona  in  which 
sedimentation  con- 
tinued throughout 
the  period  without 
interruption,  but 
such  areas  are  rare. 
During  the  emergent 
condition  of  the 
continent  the  sur- 
face rocks  were 
weathered,  leaving 
a  residual  layer  of 
insoluble  quartz  and 
clay.  In  the  east 
this  easily  remov- 
able material  was 
carried  into  a  long, 
narrow  sea,  formed 
by  the  down-warp- 
ing of  the  eastern 
part  of  the  old  Ap- 
palachian trough, 
FIG.  448.  — Map  showing  the  probable  outline  of  where  it  was  worked 
North  America  during  a  portion  of  the  Upper  Penn-  over  and  sorted  by 
sylvanian.  Continental  deposits  are  shown  in  solid  i  r 

black.     (Modified  after  Schuchert.)  the     seas     tO     torm 

the       conglomerate 

(Pottsville)  of  the  basal  Pennsylvanian.  As  this  trough  was 
weighted  down  by  the  sediments  carried  into  it  by  streams,  it 
sank  intermittently.  For  long  intervals  this  area  was  slightly  above 
the  sea  level,  and  the  sediments  which  then  accumulated  were 
continental  and  not  marine ;  at  other  times  the  sea  encroached  on 
the  land  and  formed  immense,  shallow  seas  which,  upon  being  further 
shallowed  by  sediment  from  the  land,  formed  vast,  fresh,  and 


THE  CARBONIFEROUS   PERIODS 


473 


SANDSTONE 
COAL 

"CLAY 


SANDSTONE 
CONGLOMERATE 


brackish  water  swamps  in  which  were  accumulated  the  great  coal 
beds  of  the  Carboniferous.      When  the  sinking  kept  pace  with  the 
accumulation  of  the  vegetable  matter,  for  many 
years,  deposits  of  peat  100  or  more  feet  in  thick- 
ness were  sometimes  accumulated.     These  when 
compressed  to  coal  formed  workable  coal  beds. 
The  accumulation  of  coal  began  in  the  Lower 
Pennsylvanian  (Pottsville),  but  it  was  in  the 
upper  half  of  the  period  that  its  formation  took 
place  on  a  large  scale. 

While  coal  was  accumulating  in  large  quan- 
tities (never  perhaps  more  than  per  two  cent,  of 
the  total  thickness  of  the  deposit  (Fig.  449)  in 
any  one  place)  in  Pennsylvania,  West  Virginia, 
Ohio,  Tennessee,  Illinois,  and  Iowa,  marine 
conditions  prevailed  in  the  west  and  southwest, 
and  limestones  and  shales  were  deposited  with 
no  coal.  The  red  Pennsylvanian  sandstone  of 
South  Dakota  and  the  red  conglomerate  of 
Colorado  were  probably  deposited  on  land  by 
streams  in  an  arid  climate.  Marine  sediments 
to  a  depth  of  several  thousand  feet  were  de- 
posited over  the  site  of  the  Sierra  Nevada 
Mountains. 

The  thickness  of  the  Pennsylvanian  system 
varies  from  4000  to  5000  feet  in  the  Appa- 
lachians, to  1000  feet  in  Kansas  and  Nebraska. 

In  Texas  it  is  said  to  be  5000  feet  thick  and  in        pIG  Section 

Nevada  about  10,000  feet  thick.     The  probable  of  coal-bearing  strata  in 

distribution  of  the  seas  and  swamps  of  portions   Pennsylvania,     showing 

„  ,  ,  the   relative   amount   or 

of  the  Pennsylvaman  is  shown  in  the  accom-   coaj  and  barren  rock  in 

panying  map  (Fig.  448).  a  rich  field. 


CONCEALED 
IMPURE    COAL 


SANDY    SHALE 


SANDY    SHALE 
COAL 

COAL 
•CLAY 

SANDY  SHALE 
COAL 


COAL  FIELDS  OF  NORTH  AMERICA 

Productive  Coal  Fields.  —  (i)  Eastern  Canadian  and  New  Eng- 
land Fields.  —  Important  coal  deposits  occur  in  Nova  Scotia  and  New 
Brunswick  on  both  sides  of  the  Bay  of  Fundy.  Metamorphic  coal 
is  also  found  in  Rhode  Island,  but  it  is  so  graphitic  as  to  be  of  little 
value  at  present. 


474 


HISTORICAL  GEOLOGY 


(2)  Appalachian  Field.  —  The  great  coal  field  (Fig.  450)  of  the 
world  is  that  which  underlies  an  area  of  about  50,000  square  miles  in 
central  and  western  Pennsylvania,  western  Maryland  and  Virginia, 
West  Virginia,  and  eastern  Ohio,  Kentucky,  and  Tennessee.  In 


FIG.  450.  —  Map  showing  the  distribution  and  extent  of  the  Carboniferous  coal 
fields  (black),  and  more  recent  coal  fields  (lines). 

this  should  be  included  the  anthracite  field,  confined  to  an  area  of  484 
square  miles  in  eastern  Pennsylvania. 

(3)  Michigan  Coal  Field.  —  This  field  covers  an  area  of  only  1 1,000 
square  miles  and  was  probably  formed  in  an  isolated  basin.     It  is  not 
of  great  value  as  compared  with  the  Appalachian  field,  since  it  is  deeply 
buried  and  the  coal  beds  are  usually  comparatively  thin. 

(4)  The  Indiana- Illinois  Field  covers  an  area  of  about  58,000  square 
miles,  of  which  30,000  square  miles  are  underlain  by  workable  coal. 

(5)  The  Iowa-Missouri- Texas  Field  extends  from  northern  Iowa  to 
central  Texas  and  covers  an  area  of  about  94,000  square  miles,  being 
about  800  miles  from  north  to  south.     The  Indiana-Illinois  and  Iowa- 
Texas  fields  were  probably  once  continuous,  but  are  now  separated 
by  the  Mississippi  Valley. 

The  Pennsylvanian  strata  dip  beneath  much  younger  strata  when 
traced  westward.  In  the  mountains  of  the  Great  Basin  region,  they 
are  found  to  consist  of  marine  deposits  and  contain  no  coal.  The 
coal  fields  of  Wyoming  and  Colorado  are  of  a  later  date. 


THE   CARBONIFEROUS   PERIODS  475 

SUMMARY  OF  THE  PENNSYLVANIAN 

Iron  and  Oil.  —  Beds  of  iron  ore  occur  associated  with  coal.  Such 
beds  are  sometimes  continuous,  but  the  ore  is  often  in  the  form  of 
nodules.  The  origin  of  these  beds  of  iron  ore  is  probably  the  same 
as  that  of  the  "  bog  iron  ore  "  which  is  accumulating  in  the  swamps 
and  lakes  of  the  present.  Surface  waters  containing  carbon  dioxide 
dissolved  iron  from  the  soil  and  rocks ;  the  dissolved  mineral  was  then 
carried  to  swamps  and  lakes  by  the  streams,  and  there  precipitated, 
either  as  iron  carbonate  or  iron  hydroxide. 

Oil  and  gas  occur  in  some  of  the  sandstones  of  the  Pennsylvanian 
system  in  Illinois,  Kansas,  and  Oklahoma. 

Duration.  —  The  exact  length  of  the  Pennsylvanian  Period  is  as 
doubtful  as  that  of  the  preceding  periods,  but  is  usually  stated  as 
being  about  2,000,000  years.  In  estimating  the  duration  of  former 
periods  it  has  been  necessary  to  depend  upon  the  rate  of  sedimenta- 
tion, but  in  the  Pennsylvanian  an  additional  basis  is  afforded  by  the 
coal.  This  measure  is,  however,  inaccurate,  since  the  rate  of  accu- 
mulation is  not!  definitely  known.  The  aggregate  thickness  of  the 
coal  in  a  single  section  of  the  Carboniferous  is  often  150  feet,  and  sec- 
tions are  known  where  the  total  thickness  of  the  coal  beds  is  250  feet. 
If  the  vigorous  vegetation  of  a  fertile  region  in  North  America  to-day 
were  accumulated  for  1000  years  without  loss  and  compressed  to  the 
density  of  coal,  it  would  form  a  layer  only  seven  inches  thick,  but  since 
in  the  making  of  coal  it  is  probable  that  four  fifths  of  the  vegetation 
disappears  as  carbon  dioxide  (CO2),  methane  (CH4),  and  other  gases, 
the  rate  of  accumulation  would  be  only  one  and  one  half  inches  in 
1000  years.  It  is  readily  seen,  therefore,  that  1,000,000  or  2,000,000 
years  may  have  been  required  for  the  accumulation  of  the  Pennsyl- 
vanian coal. 

Other  Continents.  —  The  Pennsylvanian  was  the  greatest  coal- 
producing  period  of  the  world.  Workable  beds  occur  in  Great  Britain 
and  Ireland  and  in  all  of  the  principal  countries  of  Europe  except 
Norway,  Sweden,  Denmark,  and  Italy.  The  Pennsylvanian  strata 
in  China,  Asia  Minor,  and  eastern  Siberia  contain  coal  beds,  many 
of  which  are  of  great  value ;  in  China,  especially,  coal  beds  of  great 
thickness  and  excellent  quality  have  been  reported.  The  Carbonif- 
erous strata  of  Africa,  Australia,  and  South  America  are  seldom  coal 
bearing,  but  in  some  areas  valuable  deposits  occur  and  much  coal  is 
mined. 


476  HISTORICAL  GEOLOGY 

PERMIAN 

In  North  America  the  Permian  is  a  continuation  of  the  Pennsyl- 
vanian  and  is  a  period  in  which  the  far-reaching  seas  of  the  latter  were 
withdrawn.  Where  the  two  systems  occur  in  the  same  section  in 
North  America  they  are  almost  always  conformable.  It  is,  however, 
more  important  as  the  transition  period  between  the  Paleozoic  and 
the  Mesozoic.  In  the  eastern  United  States  the  comparatively  small 
areas  of  Permian  rocks  are  separated  from  the  underlying  Pennsyl- 
vanian  on  the  basis  of  their  plant  remains,  which  are  more  closely  re- 
lated to  European  Permian  plants  than  to  those  of  the  underlying 
Pennsylvanian.  They  consist  of  about  1000  feet  of  sandstone,  shale, 
and  limestone,  and  a  few  beds  of  coal. 

In  Nova  Scotia,  New  Brunswick,  and  Prince  Edward  Island,  strata 
composed  of  red  shale  and  sandstone  are  believed  to  have  been 
deposited  in  an  inclosed  basin  during  the  Permian. 

During  the  early  portion  of  the  period  a  shallow  sea  extended  from 
the  Gulf  of  Mexico  through  Texas  into  Kansas  and  Nebraska,  as  is 
shown  by  the  presence  of  marine  fossils  in  the  rocks.  Later  in  the 
period  the  sea  withdrew,  leaving  a  great  region  dotted  here  and  there 
with  salt  lakes  which  left  beds  of  gypsum  and  salt,  upon  drying.  The 
aridity  of  the  climate  of  this  area  is  shown  not  only  by  the  presence 
of  the  salt  and  gypsum,  but  by  the  sun-cracked  and  ripple-marked  red 
sandstones  and  soft  red  shales  or  "  red  beds."  It  was  a  region  not 
unlike  the  Great  Basin  of  Utah  of  to-day.  These  desert  conditions 
continued  into  the  Triassic,  and  it  is,  consequently,  difficult  and  in 
many  cases  impossible.to  determine  the  dividing  line  between  the 
two  systems.  Portions  of  the  Pacific  border  were  covered  with  seas 
in  which  marine  life  abounded. 

Before  the  close  of  the  period  the  epicontinental  seas  had,  with 
one  or  two  exceptions,  withdrawn  from  the  continent. 

Permian  Glaciation.  — One  of  the  surprising  features  of  the  Permian 
is  the  evidence  of  widespread  glaciation  during  the  period.  The  lo- 
cation of  the  glaciated  areas  is  also  remarkable.  They  occur  on  both 
sides  of  the  equator  and  within  18  to  21  degrees  of  it;  that  is,  they 
extend  slightly  within  the  torrid  zone.  The  limits  of  these  ancient 
glaciers  are  not  definitely  known,  since  the  evidence  has  been  largely 
obliterated,  but  the  proof  at  hand  implies  an  area  greater  than  that 
during  the  "  Great  Ice  Age." 

The  proof  of  this  ancient  glaciation  is  conclusive  and  consists  of 


THE   CARBONIFEROUS   PERIODS  477 

bowlder  clay,  often  containing  smoothed  and  striated  bowlders  which 
in  places  rest  upon  a  striated  and  polished  pavement  of  older  rocks. 

The  glaciated  areas  were  not  in  the  polar  regions  nor,  in  many 
places,  at  a  high  altitude,  as  is  shown  by  the  relation  of  the  glacial 
deposits  to  strata  containing  marine  fossils.  In  Australia,  for  exam- 
ple, the  glacial  formations  are  interbedded  with  marine  sediments. 
Moreover,  coal  beds  occur  in  the  formation. 

In  particular,  the  Permian  glacial  deposits  occur  in  the  following 
countries.  In  India  ancient  bowlder  clay  rests,  in  places,  directly 
upon  a  striated,  roche-moutonnee  surface,  and  some  of  the  glaciated 
areas  extend  nearly  to  sea  level.  This  is  remarkable  when  taken  in 
connection  with  the  fact  that  the  glaciated  area  is  within  the  tropics. 
In  South  Africa  the  bowlders  of  the  glacial  deposit  are  often  striated 
and  rest  on  a  striated  rock  pavement.  In  Australia  several  bowlder 
beds  point  to  several  advances  of  the  ice  or  to  several  periods  of  gla- 
ciation.  Thin  glacial  moraines  of  Lower  Permian  age  resting  upon  a 
glaciated  surface  of  Upper  Carboniferous  rocks  have  been  discovered 
in  Germany,  and  Permian  bowlder  beds  in  England  have  been  inter- 
preted as  of  glacial  origin.  Bowlder  clay  of  glacial  origin  occurs  in 
Permian  strata  in  Brazil  and  Argentina,  and  conglomerates  near  Bos- 
ton, Massachusetts,  believed  to  be  in  part  of  glacial  origin,  are  thought 
to  be  either  of  Pennsylvanian  or  Permian  age. 

The  exact  age  of  the  glacial  deposits  of  India,  South  Africa,  and 
Australia  is  somewhat  in  doubt.  They  are  usually  called  Permo- 
Carboniferous  and  occurred  either  at  the  close  of  the  Pennsylvanian 
or  in  the  Lower  Permian.  The  development  of  annual  rings  in  Upper 
Permian  trees  of  certain  regions  has  given  rise  to  the  belief  in  warm 
summers  and  cold  winters  for  at  least  a  few  thousand  years  near  the 
close  of  the  period. 

Permian  Deserts.  —  Over  large  areas  of  the  earth's  surface  deserts 
existed  in  the  Lower  Permian,  as  the  ripple-marked  and  sun-cracked 
red  sandstones  and  shale  and  the  interbedded  salt  and  gypsum  tes- 
tify. Central  and  western  Europe,  England,  and  western  North 
America  are  known  to  have  been  so  affected. 

Igneous  Activity.  —  Numerous  volcanoes  broke  out  in  England  dur- 
ing the  period,  and  the  rocks  were  broken  by  earthquake  shocks,  as 
is  shown  by  earthquake  fissures  filled  with  what  appears  to  be  Permian 
sandstone. 

Appalachian  Deformation.  —  In  the  discussion  of  the  geography 
of  the  various  periods  of  the  Paleozoic,  attention  has  been  called  re- 


478  HISTORICAL  GEOLOGY 

peatedly  (i)  to  the  continent  of  Appalachia,  a  broad  upland,  sometimes 
high  and  sometimes  low,  and  (2)  to  the  Appalachian  trough  west  of 
it  in  which  much  of  its  waste  was  poured.  Two  points  have  been  em- 
phasized :  (i)  that  the  trough  or  geosyncline  sank  as  it  was  weighted 
with  sediments,  and  (2)  that,  as  Appalachia  was  worn  down  by  the 
streams,  a  compensating  rise  took  place.  With  the  exception  of  com- 
paratively short  periods  of  emergence,  sediments  were  accumulating 
in  the  Appalachian  trough  from  the  beginning  of  the  Cambrian  until 
the  Permian,  during  which  time  more  than  25,000  feet  of  sediments 
were  laid  down.  One  of  the  most  important  upward  movements  of 
the  trough  occurred  near  the  close  of  the  Ordovician,  apparently  at 
about  the  time  the  Taconic  deformation  (p.  422)  was  taking  place. 
Others  occurred  in  the  Silurian  and  between  the  Mississippian  and 
Pennsylvanian  periods.  With  these  and  other  minor  exceptions 
the  great  Appalachian  trough  was  the  site  of  deposition  during  the 
long  periods  of  the  Paleozoic,  and  a  thickness  of  more  than  five  miles 
of  sediment  accumulated. 

Towards  the  close  of  the  Carboniferous  the  most  striking  event  in 
the  geological  history  of  eastern  North  America  was  consummated. 
At  this  time  the  sediments  of  the  Appalachian  trough  yielded  to  the 
strain  that  had  long  been  accumulating  and  folded  into  a  great  moun- 
tain system  (Fig.  351,  p.  361),  the  axes  of  the  folds  extending  in  a 
northeast-southwest  direction,  one  range  reaching  from  Nova  Scotia 
to  Rhode  Island,  another  from  New  York  to  Alabama,  and  a  third  in 
Arkansas  forming  the  Ouachita  Mountains. 

The  probable  cause  of  the  yielding  of  this  particular  portion  of  the 
crust  to  lateral  pressure  was  the  fact  that  the  geosyncline  was  a  zone 
of  weakness  "  just  as  the  bend  in  a  crooked  stick  determines  the  point 
at  which  it  will  break  when  pressure  is  applied  at  the  ends."  The 
rocks  in  all  portions  of  the  trough  were  not  equally  deformed  :  those 
in  Pennsylvania  and  West  Virginia  have  been,  for  the  most  part,  com- 
pressed into  gentle  folds,  while  those  in  the  southern  Appalachians 
in  Tennessee  and  elsewhere  were  broken  by  so  many  thrust  faults 
that  the  reconstruction  of  the  region  is  often  difficult.  The  intensity 
of  the  folding  diminished  from  east  to  west.  In  eastern  Pennsylvania, 
for  example,  the  folds  are  more  compressed  and  faults  are  more 
common  than  in  the  central  part  of  the  state,  while  in  the  western 
portion  the  rocks  were  little  disturbed  and  are  almost  horizontal. 
The  greater  deformation  on  the  eastern  side  of  the  trough  is  also  seen 
in  the  character  of  the  coal  in  eastern  and  western  Pennsylvania. 


THE  CARBONIFEROUS   PERIODS  479 

In  the  former,  it  is  metamorphosed  to  anthracite,  while  in  the  latter 
it  is  bituminous.  The  effect  of  lateral  pressure  on  competent  strata 
(p.  257),  such  as  quartzites  and  limestones,  and  on  incompetent,  such 
as  shales,  is  well  shown.  Where  the  former  were  thick  the  strata 
were  either  thrown  into  great  folds  or  when  broken  were  thrust 
over  the  adjacent  rocks.  The  effect  on  shales  is  in  marked  contrast, 
for  they  were  crumpled  into  minute  folds  and  crushed. 

Not  only  were  the  sediments  of  the  Appalachian  trough  folded,  but 
the  rocks  of  the  continent  of  Appalachia  were  also  deformed  as  they 
had  indeed  been  a  number  of  times  before.  As  a  result,  those  por- 
tions of  this  old  land  which  are  at  present  exposed  at  the  surface 
are  extremely  complex. 

Although  the  Permian  was  the  period  during  which  the  principal 
folding  occurred,  some  deformation  had  previously  taken  place.  In 
the  Ordovician  folding  occurred,  and  in  the  Middle  Devonian  moun- 
tains were  formed  in  Maine,  New  Brunswick,  and  Nova  Scotia.  "  The 
Appalachian  revolution  began  in  the  Middle  Devonian,  the  first 
mountain  bulwarks  being  thrown  upon  the  eastern  side  of  the  Appala- 
chian system  and  to  the  north."  1 

Age  of  the  Deformation.  —  The  time  at  which  the  Appalachian  def- 
ormation took  place  is  known  from  the  usual  evidence.  The  young- 
est rocks  which  are  infolded  are  Pennsylvanian.  Upon  the  upturned 
edges  of  these  the  Triassic  rocks  rest  unconformably  in  certain 
places.  Since  the  Permian  strata  are  absent  in  eastern  Pennsylvania, 
it  is  probable  that  the  deformation  took  place  during  the  latter  and 
that  it  continued  into  the  Triassic,  since  the  oldest  rocks  of  that  period 
seem  to  be  everywhere  lacking. 

Other  Continents.  —  The  western  half  of  Europe  was  part  of  a 
large  continent  that  extended  from  Russia  far  into  the  Atlantic. 
In  the  southern  part  of  this  continent  lakes  and  swampy  depressions 
existed  in  the  Lower  Permian,  in  which  rank  vegetation  grew  and  large 
and  small  amphibians  and  primitive  reptiles  dwelt.  These  swampy 
areas  were  drained  by  an  elevation  in  the  Upper  Permian  which  con- 
verted them  into  broad  plains  separated  by  hills  and  mountains. 
The  climatic  effect  of  these  changes  was  marked,  some  portions  of 
the  region  becoming  very  humid  and  others,  from  which  the  moist 
winds  were  shut  off  by  the  mountains,  becoming  arid.  Volcanic 
activity  was  prevalent  during  a  portion  of  the  period.  The  epoch 

1  Barrell,  J.,  —  The  Upper  Devonian  Delta  of  the  Appalachian  Geosyncline:  Am.  Jour.  Sci. 
Vol.  37,  1914,  PP.  225-253. 

CLELAND  GEOL. — 3! 


480 


HISTORICAL  GEOLOGY 


of  elevation  was  followed  by  one  of  subsidence  and  later  by  disturb- 
ances which  cut  off  a  great  lake,  like  the  Caspian  Sea,  and  salt  and 
gypsum  were  deposited  as  it  dried  up.  During  the  Upper  Permian 
the  thickest  known  salt  deposits  were  accumulated,  one  of  which, 
near  Berlin,  has  been  penetrated  4000  feet. 

Permian  strata  cover  large  areas  in  southern  Asia,  in  Australia, 
in  southern  Africa,  and  in  South  America,  the  unique  features  of 
which  are  the  extensive  glacial  deposits  (p.  505). 


INVERTEBRATES  OF  THE  CARBONIFEROUS 

Protozoans.  —  During  the  Carboniferous,  for  the  first  time  in  the 
Paleozoic,  Foraminifera  became  abundant  and  varied.  Certain  genera 

built  up  limestone 
deposits,  occasionally 
of  considerable  thick- 
ness. One  of  the 
most  characteristic 
forms  of  the  Missis- 
sippian (Fusulina) 
was  like  a  grain  of 
wheat  in  form  and 
size  (Fig.  455  H,  7, 
p.  482).. 

FIG.  451.  —  Carboniferous  corals:  A,  Lithostrotion  Coelenterates  and 
canadense  (Mississippian) ;  B,  Hapsipkyllum  calcareforme  Echinoderms.  —  Cup 
(Mississippian);  C,  Lophophyllum  profundum  (Pennsyl-  /T?-  j  r\  A 

vanian).  vr  !g-    45 J    A-^)     and 

honeycomb  corals  con- 
tinued to  be  important  in  the  Carboniferous,  and  contributed 
largely  to  the  formation  of  thick  limestone  strata.  Blastoids 
(Fig-  452  F>  G)  were  so  abundant  in  the  Mississippian  that  some 
beds  are  largely  made  up  of  them,  but  their  extinction  was  reached 
before  the  end  of  the  Pennsylvanian.  Where  favorable  conditions 
existed,  crinoids  (Fig.  452  A-E)  were  unusually  abundant ;  especially 
was  this  true  in  the  Mississippian.  Sea  urchins  (echinoids)  were 
more  abundant  and  larger  than  ever  before,  but  were  subordinate 
to  the  crinoids  in  numbers. 

Molluscoids.  —  Bryozoans  lived  in  considerable  numbers.  Among 
many  less  striking  forms  was  one  genus  with  a  peculiar  habit  of  growth 
about  an  axis  which  gave  it  a  screwlike  shape,  hence  the  name  Archi- 


THE  CARBONIFEROUS   PERIODS 


481 


FIG.  452.  —  Mississippian  echinoderms.  Crinoids  :  A,  Actinocrinus  multiradiatus ; 
B,  Platycrinus  discoideus ;  C,  Onychocrinus  exsculptus;  D,  Batocrinus  (Dizygocrinus) 
rotundus  (with  arms  removed) ;  E,  the  same  as  D  with  arms.  Blastoids :  F,  Pentre- 
mites  robustus ;  G,  Pentremites  pyriformis.  Brittle  Stars:  H,  Ony chaster  flexilis. 

medes  (Fig.  453).  The  Carboniferous  was  rich  in  brachiopods  (Fig. 
454  A-M),  but  before  its  close  the  leading  Paleozoic  genera  had  dis- 
appeared. One  characteristic  genus  (Productus) 
(Fig.  454  A,  G)  had  one  large,  convex,  spinose 
valve  and  one  concave  one. 

Mollusks. — Gastropods  (Fig.  455  A-G)  and 
pelecypods  (Fig.  456  A—F)  continued  much  as  in 
the  Devonian.  The  cephalopods  (Fig.  457  A-E), 
on  the  other  hand,  showed  considerable  advance 
in  the  complexity  of  the  suture  lines  (p.  530).  The 
angle-sutured  goniatites  were  common,  and,  be- 
fore the  close  of  the  Permian,  ammonites  with 
their  complex  sutures  appeared.  Some  of  the 
straight,  simple  orthoceratites  continued  through- 
out the  Paleozoic  into  the  Triassic. 

Arthropods. — Trilobites  (Fig.  458)  and  euryp- 
terids  continued  into  the  Carboniferous,  but  be- 
came extinct  at  its  close. 

Insects.  —  The    earliest    insects    of  which    any 
fossils  have  as  yet  been  found  lived  in  the  Car- 
boniferous (Fig.  459  A,  B),  and  from  that  period        FIG.  453-  —  Car- 
v      &    73     |  ..      ,        r™  bomferous  bryozoan : 

about  1000   species   have   been  described.      1  hey    Archimedes   northern 
appear  to  have  been  more  generalized  than  those    (Mississippian). 


482 


HISTORICAL  GEOLOGY 


M 


FIG.  454.  —  Carboniferous  brachiopods  :  A,  Productus  costatus  (Pennsylvanian) ; 
B,  Athyris  lamellosa  (Mississippian) ;  C,  Spirifer  cameratus  (Pennsylvanian) ;  D,  Rhipi- 
domella  burlingtonensis  (Mississippian) ;  E,  Seminula  argentea  (Pennsylvanian) ;  F, 
Derbya  crassa  (Pennsylvanian) ;  G,  Productus  burlingtonensis  (Mississippian) ;  H, 
Spiriferina  spinosa  (Pennsylvanian) ;  /,  Seminula  subquadrata  (Mississippian) ;  /,  Eu- 
metria  marcyi  (Mississippian) ;  K,  L,  Dielasma  bovidens  (Pennsylvanian) ;  M,  Hustedia 
mormoni  (Pennsylvanian). 


FIG.  455.  —  Carboniferous  gastropods  :  A,  Plastostoma  broadheadi  (Mississippian) ; 
B,  C,  two  views  of  Bellerophon  sublcevis  (Mississippian) ;  D,  Pleurotomaria  nodulostnata 
(Mississippian) ;  E,  Worthenia  tabulata  (Pennsylvanian) ;  F,  Naticopsis  altonensis 
(Pennsylvanian);  G,  Bellerophon  percarinatus  (Pennsylvanian).  Foraminifera :  //, 
Fusulina  secalica  (Mississippian) ;  /,  section  of  Fusulina,  showing  structure. 


THE  CARBONIFEROUS   PERIODS 


483 


FIG.  456.  —  Carboniferous  pelecypods  :  A,  Grammy sia  hannibalensis  (Mississip- 
pian) ;  B,  Pleurophorus  tropidophorus  (Pennsylvanian) ;  C,  Allorisma  terminate  (Penn- 
sylvanian) ;  Z),  Aviculopecten  occidentalis  (Pennsylvanian) ;  Ey  Myalina  recurvirostris 
(Pennsylvanian) ;  F,  Monopteria  longispina  (Pennsylvanian.) 

of  subsequent  periods ;  that  is,  they  were  simple  in  structure  and 
united  characteristic  features  of  two  or  more  distinct  groups,  and 
were  therefore  probably  the  ancestors  of  those  insects  whose  char- 
acters they  combine.  One  extinct  order  (Paleodictyoptera,  old, 
netted  wing)  is  especially  interesting  because  it  is  believed  to  be 
the  stock  from  which  all  insects  were  descended.  Carboniferous 


FIG.  457.  —  Carboniferous  cephalopods :    A,  Medlicottia  copei;    B,  Aganides  rota- 
torius;   C,  Waagenoceras  cumminsi;   D,  Temnocheilus  forbesianus;   E,  Muensteroceras 


484 


HISTORICAL  GEOLOGY 


insect  groups  seem  widely  different  when  the 
external  form  only  is  considered,  but  a  more  care- 
ful study  shows  that  the  differentiation  had  little 
depth.  The  wings  were  all  membranous,  none 
having  yet  been  developed  for  protective  cover- 
ing, as  in  the  beetle.  The  number  of  wings  in 
every  case  was  four,  none  having  been  dropped 
at  that  time. 

Two  groups  were  especially  prominent,  the  cock- 
roaches (Fig.  459  B)   of  which  there  were  large 
numbers,   some    being   as   large  as   a   ringer,   and 
many  species ;    and  the  dragon  flies  (Fig.  459  A),  which  reached  the 
great  size  of  almost  two  and  a  half  feet  across  the  wings.     The  ab- 


FIG.  458.  —  Car- 
boniferous trilobite : 
Phillipsia  major. 


FIG.  459.  —  Carboniferous  insects  :  dragon  fly,  A,  Meganeura  monyi  (some  of  these 
were  two  and  a  half  feet  across  the  wings) ;  By  Adeloblatta  columbiana;  C,  spider, 
Arthrolycosa  antiqua. 


THE   CARBONIFEROUS   PERIODS 


485 


sence  of  all  sucking  insects,  such  as  bees,  wasps,  and  butterflies,  whose 
food  consists  of  the  nectar  of  flowers  procured  by  a  sucking  apparatus, 
and  in  fact  of  all  insects  which  depend  upon  flowers  for  food  is  not 
surprising,  since  no  flowering  plants  were  in  existence  at  this  time. 

The  active  dragon  fly  rather  than  the  sluggish  amphibian  (p.  489) 
might  seem  "  a  far  superior  type  of  being,  a  far  more  promising  can- 
didate for  the  position  of  ancestor  of  the  intelligent  life  which  was  to 
appear  in  the  dim  future."  "  But  the  insect  had  fulfilled  the  mechan- 
ical possibilities  of  which  his  structural  organization  was  capable." 
(Matthew.) 

VERTEBRATES  OF  THE  CARBONIFEROUS 

Fishes.  —  During  the  Mississippian  sharks  existed  in  an  abundance 
which  has  not  been  equaled  before  or  since,  as  is  shown  by  the  large 
number  of  species,  of  which  nearly  300  have  been  de- 
scribed. Before  the  close  of  the  Carboniferous,  how- 
ever, the  number  had  become  very  small,  only  about 
20  species  being  known  in  the  Permian.  As  in  the 
Devonian,  the  sharks  are  known  from  their  fin  spines 
and  teeth  (Fig.  460),  the  latter  being  of  the  crushing 
type.  The  sharks  were,  for  the  most  part,  small, 
being  seldom  more  than  five  feet  long,  while  none 
attained  the  size  of  their  modern  relatives. 

Only  sharks  and  typical  ganoids  (Actinopteri)  were 
important,  the  armored  lungfish  and  fringe-finned 
ganoids  having  reached  their  climax  in  the  Devonian. 

Amphibians.  —  Next  to  the  appearance  of  the  fishes 
the  most  important  event  in  the  history  of  vertebrates 

was  the  rise  of  amphibians,  of  which  the  salamanders, 

,  r  ,  FIG.  460.— 

newts,  and  rrogs  are  modern  representatives,  since  the  ^  j)acjc   sp{ne) 

ancestors  of  reptiles,  of  mammals,  and  even  of  man  tooth,  and  scale 
himself  are  to  be  found  among  them.  Amphibians  of  a  Mississip- 

11  -11          ••«•**  i  i      r          i         Pian  shark. 

resemble  reptiles,  but  differ  from  them  in  the  fact  that 

in  the  earlier  period  of  life  they  breathe  in  water  by  means  of  gills,1 
like  fishes  ;  and  it  is  only  in  the  later  period  of  life  that  they  breathe 
air  by  means  of  lungs,  like  reptiles.  The  most  important  difference 
between  fishes  and  amphibians  is  in  the  organs  of  locomotion,  fishes 
having  fins  and  amphibians  legs,  in  the  adult  stage. 


1  Some  amphibians  never  lose  their  gills. 


486 


HISTORICAL  GEOLOGY 


It  is  hard  to  point  out  briefly  all  the  differences  between  amphibians 
and  reptiles,  and  it  is  indeed  sometimes  extremely  difficult,  if  not  im- 
possible, to  tell  from  the  skeleton  alone  of  extinct  forms  to  which 
class  a  specimen  belongs.  One  important  difference,  however,  is 
to  be  found  in  the  articulation  of  the  skull  to  the  backbone,  which  in 
amphibians  is  by  means  of  two  knuckle-like  projections  of  bone 
(condyles)  and  in  reptiles  and  birds  by  one.  In  fishes  there  is  no 
movable  articulation. 

All  Carboniferous  amphibians  belonged  to  the  Stegocephali  (Fig. 
461)  (Greek,  siege,  roof,  and  cephale,  head),  in  which  the  skull  was 


FIG.  461.  —  A  Permo-Carboniferous  landscape.  The  characteristic  vegetation  ;  two 
figures  of  the  amphibian  Eryops  upon  the  land,  each  about  seven  feet  long,  are  shown ; 
a  reptile  (Limnoscelis)  in  the  water;  and  a  gigantic  dragon  fly  in  the  air.  (After 
Prof.  S.  W.  Williston.) 

covered  with  bony  plates,  and  the  teeth  were  conical  with  walls  that 
were  sometimes  highly  infolded  (Fig.  462).  The  limbs  of  most  were 
weak  and  adapted  more  for  crawling  than  for  carrying  the  body  well 
above  the  ground. 

Carboniferous  amphibians  varied  greatly  in  size  and  shape,  some 
attaining  a  length  of  almost  eight  feet.  One  characteristic  genus, 
Eryops  (Figs.  463  and  461),  had  a  rather  large,  broad,  flat  head,  and 


THE  CARBONIFEROUS   PERIODS 


487 


unevenly  spaced,  conical  (labyrinthine)  teeth,  no  neck,  and  a  thickset 

body  with  broad,  five-toed  feet  that  were  probably  webbed.     The  tail 

was  flattened  vertically.     Even  with  its 

roofed  skull,   Eryops  would    not    look 

unlike  an  overgrown  modern  Japanese 

giant  salamander,  since   the   bones  of 

the  skull  were  covered  with  skin.     It 

was  able  to  crawl  clumsily  and  slowly 

over  the  land,  but  must  have  been  far 

more  at  home  in  the  water.     That  this 

clumsy,  small-brained  beast  should  be 

one  of  the  highest  types  of  living  beings 

of  its  time  may  help  us  to  realize  how 

remote  the   period   was,  and   to  what 

an    extent    vertebrate    life    has    been 

evolved  since  then.  FlG'  f2'  ~  Cross  f  c;tion  °' Tthe 

tooth    of    a    Stegocephahan.     Note 

Some    of    the    Carboniferous    StegO-    complicated  labyrinthine  structure, 
cephali  were   armored   and   some  had 

no  protection;  some  had  skulls  nearly  two  feet  long,  while  the 
skulls  of  others  were  not  larger  than  one's  thumb-nail ;  some  had 
stout  limbs,  while  the  limbs  of  others  were  atrophied  and  the  body 
elongated  and  snakelike. 


FIG.  463.  —  Top  view  of  the  skeleton  of  Eryops,  a  Permian  Stegocephalian.     The  head 
is  covered  with  the  thick,  bony  plates  characteristic  of  the  order. 

The  commonest  amphibian  of  the  Carboniferous,  Branchiosaurus 
(Fig.  464),  did  not  differ  greatly  in  appearance  and  habits  from  those 
of  to-day.  The  teeth  were  small  and  conical,  the  eyes  were  protected 
by  a  movable  ring  of  bony  plates,  and  the  lower  surface  of  the  body 


488 


HISTORICAL  GEOLOGY 


was  covered  with  thin  scales.  The  presence  of  gills  in  immature 
specimens  shows  that  they  lived  in  the  water  at  least  a  portion  of  their 
life. 

Certain  Permian  amphibians  (Lysorphus)  resembled  a  modern  sala- 
mander (Amphiuma)  in  size,  shape,  and  habits  so  strongly  that  it 

seems  actually  to  have 
been  related  to  it.  So 
abundant  are  the 
skeletons  of  these  am- 
phibians in  certain  lo- 
calities in  the  Permian 
strata  of  Texas  that 
hundreds  have  been 
found  embedded  in 


FIG.  464.  —  Branchiosaurus,  a  Carboniferous  am- 
phibian of  small  size  occurring  abundantly.  The 
"roofed"  head,  the  rings  of  bone  about  the  eyes,  and 
the  scaly  covering  of  the  lower  side  are  shown. 


nodules. 

Amphibians  are 
known  from  their  foot- 
prints to  have  lived  in 
theMississippian.  The 
knowledge  which  can  be  gained  of  the  animal  and  of  the  conditions 
under  which  he  lived  is  well  illustrated  in  the  footprints  preserved 
in  one  layer  of  Mississippian  (Mauch  Chunk)  shale  near  Pottsville, 
Pennsylvania. 

"There  is  a  succession  of  six  steps,  along  a  surface  little  over  five  feet  long;  each 
step  is  a  double  one,  as  the  hind  feet  trod  nearly  in  the  impressions  of  the  fore  feet. 
The  prints  were  hand-like;  that  of  the  fore  foot  five-fingered  and  four  inches  broad; 
that  of  the  hind  foot  somewhat  smaller  and  four-fingered.  That  the  Amphibian  was 
therefore  large  is  also  evident  from  the  length  of  the  stride,  which  was  thirteen  inches, 
and  the  breadth  between  the  outer  edges  of  the  footprints  eight  inches.  There  is 
also  a  distinct  impression  of  a  tail  an  inch  or  more  wide.  The  slab  is  crossed  by  a 
few  distinct  ripple  marks  (eight  or  nine  inches  apart)  which  are  partially  obliterated 
by  the  tread.  The  whole  surface,  including  the  footprints,  is  covered  throughout 
with  raindrop  impressions. 

"We  thus  learn  that  in  the  region  about  Pottsville  a  mud  flat  was  left  by  the  re- 
treating waters,  perhaps  those  of  an  ebbing  tide,  covered  with  ripple  marks;  that 
the  ripples  were  still  fresh  when  a  large  Amphibian  crossed  the  flat ;  that  a  brief  shower 
of  rain  followed,  dotting  with  its  drops  the  half-dried  mud  ;  that  the  waters  again  flowed 
over  the  flat,  making  new  deposits  of  detritus,  and  so  buried  the  records."  (Dana.) 

Origin  of  Amphibians.  —  Several  lines  of  evidence  show  that  am- 
phibians may  have  been  descended  from  the  fringe-finned  ganoids 
(crossopterygians) :  (i)  the  teeth  of  both  are  often  labyrinthine 


THE  CARBONIFEROUS   PERIODS  489 

(Fig.  462) ;  (2)  the  bones  of  their  skulls  are  similar  in  position  and 
arrangement;  (3)  some  primitive  amphibians  have  rings  of  bony 
plates  (sclerotic  plates)  about  the  eyes ;  (4)  the  structure  of  the  fin 
of  the  fringe-finned  ganoids  was  such  that  a  leg  might  have  been 
formed  from  it  by  modification. 

Rise  of  Amphibians.  —  The  rise  of  amphibians  was  a  momentous 
step  in  the  evolution  of  life,  but  it  was  not  a  surprising  one.  Fishes 
were  the  predominant  race  of  the  Devonian,  and  it  was  to  be  expected 
that  the  structure  of  some  of  them  would  in  time  become  modified 
to  take  advantage  of  the  realm  of  the  lands  where  food  was  either 
more  easily  obtainable  or  of  a  more  nutritious  quality  than  in  the  seas, 
or  where  the  competition  was  less  keen.  The  extensive  swamps  of  the 
Devonian  and  Carboniferous  and  the  shiftings  of  the  seas  have  been 
assigned  as  the  immediate  causes  of  the  rise  of  the  amphibians  from 
fishes.  It  seems  more  probable,  however,  that  during  every  period 
of  the  Paleozoic,  swamps  and  shallow  water  were  present,  in  which 
amphibians  would  have  been  evolved  had  fishes  been  present  which 
were  so  constructed  that  by  slight  modifications  they  could  become 
adapted  to  land  conditions.  How  this  was  accomplished  is  shown  in 
the  development  of  the  individual  amphibian,  which  in  the  tadpole 
stage  is  physiologically  a  fish,  but  which  later  breathes  by  means  of  a 
simple  sack-like  lung  instead  of  gills.  It  is  important  to  note  that 
the  Carboniferous  amphibians,  more  than  their  modern  relatives, 
possessed  characters  closely  allying  them  to  the  fishes,  and  that  their 
fishlike  characters  were  more  like  those  of  Devonian  than  of  modern 
fishes. 

Some  doubtful  amphibian  tracks  have  been  found  in  the  Devonian, 
but  no  bones  earlier  than  the  Carboniferous  have  been  discovered. 
However,  the  well-developed  limbs  of  the  earliest  species  indicate  a 
line  of  ancestors  that  lived  in  the  Devonian.  Amphibians  attained 
their  greatest  importance  in  the  Carboniferous  (Pennsylvanian  and 
Permian)  and  have  taken  a  very  subordinate  place  since  the  Triassic. 
Even  before  the  close  of  the  Pennsylvanian  they  had  begun  to  give 
place  to  the  reptiles. 

Reptiles.  — The  reptiles  of  the  Permian  were  even  more  varied  than 
the  amphibians  and  have  been  placed  in  three  groups.  The  first  group 
(cotylosaurs)  includes  the  most  primitive  reptiles  known,  its  mem- 
bers differing  from  other  reptiles,  among  other  particulars,  in  having 
a  roofed-over  skull.  All  the  members  of  this  class  that  are  known 
had  very  short  necks;  short,  stout  limbs;  rather  heavy  bodies,  and 


490 


HISTORICAL  GEOLOGY 


FIG.  465.  —  The  head  of  a  Permian 
reptile,  Labidoiaurus. 


usually  long  tails  (Fig.  461).  Some  had  long,  sharp,  curved  teeth 
(Labidosaurus,  Fig.  465)  arranged  in  two  or  more  rows,  which  were 
probably  used  for  prodding  in  the  mud  for  soft-bodied  invertebrates, 
and  for  crushing  ;  some  had  conical  teeth  in  front  and  crushing  teeth 
behind  ;  and  some  had  a  mouth  filled  everywhere  on  jaws  and  palate 

with  short,  stumpy  teeth,  suitable 
only  for  crushing  shellfish;  while 
others  had  slender  teeth  indicat- 
ing insectivorous  habits.  In  some, 
the  claws  terminated  in  flattened 
nails,  while  in  others  the  claws 
were  sharp  and  curved.  The 
habits  of  this  group  varied  some- 
what, some  being  more  terrestrial  than  others,  but  all  probably 
lived  in  swampy  places  and  about  lagoons. 

A  second  group  of  Carboniferous  reptiles  (pelycosaurs)  had  lighter 
skulls  than  those  of  the  first  group  ;  larger  necks  ;  longer,  better- 
formed  legs  and  feet,  and  usually  longer  tails.  The  best  known  of 
these  (Dimetrodon,  Fig.  466)  was  about  10  feet  in  length  and  was 
especially  characterized  by  a  finlike  crest  on  the  back  formed  by 
spines  of  its  vertebrae.  The  skull  had  strong,  sharp,  carnivorous 
teeth,  indicating 
that  the  creature 
was  a  fierce,  pre- 
daceous  animal. 
The  use  of  the 
crest  on  the  back 
is  unknown,  and 
aside  from  its 
presence  the  ani- 
mal was  very 
primitive  in 
structure.  Some 

members  of  this 

group     (Varano- 

saurus,  Fig.  467)  were  swift-running  reptiles,  living  in  the  forests, 

hiding  under  logs,  and   feeding  on  the  numerous  cockroaches  and 

other  insects. 

A  third  group,  although  inconspicuous  in  the  Permian,  were  of 
great  importance  both  because  of  their  great  development  in    the 


'  ~  Dimetrodon,  a  spiny  but  primitive  reptile  from  the 
Permian  of  Texas.     (Modified  after  Jaekel.) 


THE   CARBONIFEROUS   PERIODS 


491 


Triassic  and  also  because  they  were  the  probable  ancestors  of  the  mam- 
mals. They  were  small  reptiles  with  short  but  strong  legs  and  large 
heads.  The  mammalian  characters  are  to  be  seen  in  the  skull,  the 


FIG.  467.  —  Faranosaurus,  a  Permian  reptile  forty-four  inches  long,  on  a  Sigillaria 
log.     (After  Prof.  S.  W.  Williston.) 

teeth,  and  other  parts  of  the  skeleton.  This  important  group  is  de- 
scribed more  fully  in  the  following  chapter  (p.  536). 

Rise  of  Reptiles.  —  The  rise  of  reptiles  from  amphibians  was  a  logi- 
cal sequence.  On  account  of  their  aquatic  larval  life,  amphibians 
were  restricted  to  the  vicinity  of  the  water,  while  reptiles,  because  of 
the  development  of  a  firm  eggshell  and  the  omission  of  the  aquatic 
stage  of  the  young,  were  able  to  populate  the  dry  lands  and  thus  to 
take  advantage  of  many  kinds  of  food.  Amphibians  may  indeed  be 
considered  as  transitional  forms  by  which  reptiles  were  evolved  from 
fishes. 

The  rapid  development  of  this  class  after  it  was  once  well-started 
was  probably  due  to  its  higher  organization,  to  the  lack  of  competition 
in  the  new  surroundings,  and  to  the  abundance  of  food.  It  has  been 
suggested  that  the  purification  of  the  air  as  a  result  of  the  withdrawal 
of  carbon  dioxide  in  the  formation  of  coal  produced  an  atmosphere 
which  was  more  favorable  for  the  development  of  air-breathing  ani- 
mals, but  the  amount  of  carbon  dioxide  withdrawn  does  not  seem  to 
have  been  sufficient  to  have  made  a  great  difference. 

CARBONIFEROUS  PLANTS 

The  forests  of  the  Carboniferous  were  very  different  in  appearance 
from  those  of  to-day.  None  of  the  trees  common  at  present  in  the  for- 
ests and  swamps  were  in  existence,  flowering  plants  were  conspicuous 


492 


HISTORICAL  GEOLOGY 


by  their  absence,  and  even  grasses  and  mosses  were  lacking.  The 
living  relatives  of  the  Carboniferous  trees  are  for  the  most  part  lowly, 
inconspicuous  plants. 

The  land  plants  of  the  Carboniferous  belonged  to  six  great  groups : 
(i)  ancestral  ferns,  (2)  seed  ferns  (pteridosperms),  (3)  club  mosses 
(lycopods),  (4)  an  ancient  extinct  group,  the  sphenophylls,  (5)  the 
horsetails  (Calamites),  and  (6)  the  Cordaites  and  possibly  some 
conifers. 

(i)  Ancestral  Ferns  and  (2)  Seed  Ferns.  —  The  shale  overlying  coal 
beds  is  often  full  of  the  fronds  of  fernlike  plants  (Fig.  468),  and  so 
perfect  is  the  preservation  of  some  of  them  that  the  finest  venation  is 

shown.  The  beauty  of  such  fossils 
is  especially  striking  when  the 
shale  is  light  in  color,  since  under 
such  conditions  the  delicate  out- 
line of  the  black  fossil  frond  is 
brought  out  with  great  distinct- 
ness. It  was  formerly  thought 
that  these  fernlike  leaves  were 
the  remains  of  ferns,  but  a  more 
careful  study  and  further  dis- 
coveries have  shown  that  few  are 
true  ferns;  on  the  contrary,  the 
great  majority  belong  to  an  ex- 
tinct family,  the  seed  ferns  or 
pteridosperms. 

The   important   difference   be- 
tween these  families  lies   in  the 
reproduction,  the  true  fern  pro- 
ducing spores  and  the  pteridosperms,  seeds. 

Ancestral  Ferns.  —  The  most  important  family  of  Paleozoic  ferns 
(Marattiaceae)  has  descendants  living  to-day,  but  the  Paleozoic 
members  of  the  family  were  tree  ferns,  reaching  in  some  species  a 
height  of  upwards  of  60  feet.  This  great  family  has  dwindled  to  a 
few  genera  which  are  now  confined  to  the  tropics ;  one  of  which,  how- 
ever, the  elephant  fern,  sends  up  huge  fronds  to  a  height  of  10  or  12 
feet.  In  addition  to  the  tree  ferns  there  were  doubtless  low,  herba- 
ceous ferns,  living  under  the  same  conditions  as  the  ferns  of  to-day. 

Seed  Ferns  (Pteridosperms).  — The  members  of  this  extinct  group, 
if  living  to-day,  would  probably  be  called  ferns  by  the  casual 


FIG.  468.  —  A  Paleozoic  fernlike  plant, 

Pecopteris. 


THE   CARBONIFEROUS   PERIODS 


493 


observer,  but  a  careful  examination  would  show  that  instead  of  hav- 
ing small  sporangia  on  the  back  of  the  fronds  they  bore  seeds,  some- 
times as  large  as  hazelnuts,  which  were  surrounded  by  a  thick,  fleshy 
outer  coat.  The  pteridosperm  group 
(Fig.  469)  is  interesting  as  a  con- 
necting type,  since  it  is  a  link  be- 
tween the  ferns,  on  the  one  hand, 
and  the  cycads,  which  were  the 
dominant  plants  in  the  Mesozoic, 
on  the  other.  Whether  it  stands  as 
a  connecting  link  between  the  ferns 


FIG.  469.  —  Lyginodendron.  Restora- 
tion showing  the  stem,  roots,  and  foliage; 
a,  seeds  ;  b,  disks  and  pollen  sacks.  (After 
Mrs.  D.  H.  Scott.) 


FIG.  470.  —  Restoration  of  Lepido- 
dendron,  showing  the  position  and 
character  of  the  leaves,  the  fruit,  and 
the  diamond-shaped  markings  on  the 
trunk.  (See  also  Fig.  471.)  Compare 
the  branching  of  Lepidodendron  with 
that  of  Sigillaria. 


and  the  great  groups  of  higher  plants,  or  whether  it  leads  to  the  cy- 
cads and  stops  there,  cannot  as  yet  be  affirmed. 

(3)  Club  Mosses  (Lycopods). — The  conspicuous  trees  of  the 
Carboniferous  were  gigantic  club  mosses,  some  of  which  grew  to  a 
height  of  more  than  100  feet.  One  of  these,  the  Lepidodendron 
(Greek,  lepis,  scale,  and  dendron,  tree)  was  freely  branched  (Fig.  470) 


494 


HISTORICAL  GEOLOGY 


and  had,  consequently,  somewhat  the  general  outline  of  our  forest 
trees,  but  with  that  the  resemblance  ended.  The  leaves  were  numer- 
ous and  slender  and  were  commonly  arranged  in  oblique  rows 
about  the  trunk  and  branches.  When  shed,  their  bases  left  diamond- 
shaped  scars  which  gave  a  characteristic  appearance  to  the  bark 
(Fig.  471).  The  largest  Lepidodendron  trunk  yet  described  was 
found  to  be  114  feet  long  up  to  the  point 
where  it  began  to  branch;  the  diameter 
of  the  base  was  about  three  feet,  while 
at  a  height  of  114  feet  it  was  one  foot, 
showing  that  it  was  a  tree  of  slender 
proportions.  Other  species,  however,  . 
were  of  a  somewhat  sturdier  build. 

The  Sigillaria  (Latin,  sigillum,  seal) 
differed  externally  from  the  Lepidoden- 
dron, especially  in  two  particulars;  it 


FIG.  471.  —  Impression  of  the  bark  of 
the  Lepidodendron,  showing  the  leaf  bases 
and  characteristic  diamond-shaped  scars. 
(See  Fig.  470.) 


FIG.  472.  —  Restoration  of 
Sigillaria,  showing  the  position 
and  character  of  the  leaves  and 
the  fruit,  and  the  peculiar  bark. 
(See  Fig.  473.)  Some  of  them 
grew  to  a  height  of  a  hundred 
feet,  with  a  diameter  of  six  feet. 


branched  sparingly  (Fig.  472)  and  the  leaves  were  arranged  in 
vertical  rows  (Fig.  473),  the  leaf  scars  on  the  bark  giving  rise  to  the 
name  "  seal  tree."  In  some  species  the  leaves  were  a  yard  long. 
Sigillarian  trunks  almost  as  slender  and  high  as  those  of  the  Lepi- 
dodendron have  been  found,  but,  as  a  rule,  they  were  shorter  and 


THE  CARBONIFEROUS   PERIODS 


495 


stouter,  one  specimen  six  feet 
in  diameter  and  18  feet  high 
having  been  described. 

The  fruit  of  both  the  Lepido- 
dendron  and  Sigillaria  was  in 
the  form  of  well-defined  cones 
that  were  usually  borne  at  the 
ends  of  the  smaller  branches.  In 
the  clay  underlying  coal  seams 
the  "  roots  "  or  underground 
stems  of  the  lycopods  often 
occur  and  are  called  Stigmaria. 

The  trunks  of  the  lycopods 
consisted  of  a  hard,  woody  rind 
and  a  soft,  cellular  interior  which 
quickly  decayed.  As  a  result 
of  this  structure  the  fossil  trunks 
seldom  show  their  original  cy-  ,  FlG'  473-  — Bark  of  Sigillaria,  showing 
••  i  •  •  >  ,  ,,  the  vertical  arrangement  or  the  leaves  and 

lindncal  form,  but  are  usually    the  fluted  surface. 

flattened  into  thin  sheets. 

The  great  lycopods  of  the  Carboniferous  are  now  represented  by 
the  insignificant  ground  pine  and  Selaginella. 

The  coal  beds  of  the  Carboniferous  are 
largely  composed  of  Lepidodendron  and  Sigil- 
laria remains.  Some  coal  of  this  period 
(cannel),  however,  is  made  up  chiefly  of 
spores  of  Carboniferous  plants. 

(4)    Sphenophylls.  —  This    extinct    group    is 
interesting  because  it  suggests  a  common  an- 
cestor for   the  lycopods  and  horsetails  (Equi- 
setales).     The  plant  had  a  slender,  ribbed  stem, 
seldom    more   than    a    quarter   of   an    inch   in 
diameter,  which    bore    delicate,  wedge-shaped 
FIG.  474.  —  Stem  and    leaves   (Fig.   474)    attached    in  whorls   to   the 
leaves of  Sphenophyllum,    stem    b      their   ends      Sometimes    the    leaves 
a  slender  plant,  the  stem  .         ,  ,  .  ,  ,  i     •  IM 

seldom    exceeding    two    were  deeply  cut,  making  them  almost  hairhke 

fifths    of    an    inch    in  in  appearance.     These  plants  probably  had  a 

diameter.    The  spheno-  tram-ng  habit,  or  perhaps  supported  themselves 
phyllums   probably  sup-  .  J^  ,  i     11      u 

ported     themselves    by  on  stronger   plants.     Sphenophylls  bore  cones 

limbing.  somewhat  like  those  of  the  Calamites. 

CLELAND   GEOL. — 32 


496 


HISTORICAL  GEOLOGY 


(5)  Horsetails  (Calamites).  —  Except  for  their  greater  size,  the 
horsetails  of  the  Carboniferous  had  much  the  appearance  of  the  lowly 
horsetail  or  scouring  rush  of  to-day  and  were,  indeed,  related  to  it. 
The  tree  horsetails  of  the  Paleozoic  are  called  Calamites  (Fig.  475), 
from  the  most  important  genus  of  that  time.  They  were  trees  which 
reached  a  height  of  60  or  more  feet  and  were  almost  as  conspicuous  as 

their  Lepidodendron  and  Sigillaria 
neighbors.  Their  habit  of  growth  ap- 
pears to  have  been  similar  on  a  glorified 
scale  to  that  of  some  of  their  living 
relatives.  Some  were  simple  shafts, 
while  others  were  probably  gracefully 
branched  trees  with  many  boughs. 

The  bark  (Fig.  476  A]  of  the  Cala- 
mites had  characters  which  readily 
distinguished  it  from  other  trees  of  its 
time.  The  stems  were  ribbed,  the  ribs 
ending  at  each  "  node  "  and  a  new  set 
continuing  beyond  the  node  to  the 
next,  when  an  alternate  set  again  ap- 
peared. The  leaves  (Fig.  476  B)  were 
simple  and  were  attached  to  the  nodes 
in  whorls.  Conelike  fruits  were  borne 
at  the  ends  of  the  twigs  and  contained 
spores  of  one  kind. 

Calamites  began  in  the  Devonian 
and  went  out  with  the  Paleozoic,  but 
the  horsetails  of  the  Mesozoic  were 
transitional,  both  in  size  and  in  struc- 
ture, to  the  modern  horsetails. 

(6)  Cordaites  and  Other  Gymno- 
sperms.  —  The  trees  of  this  group  were 
the  most  highly  developed  of  the  Paleozoic  forests.  They  were  large 
trees  (Fig.  477)  which  sometimes  grew  to  be  100  feet  in  height  and 
were  easily  distinguished  from  the  other  trees  of  the  time  by  the 
large,  sword-shaped  leaves  which  were  borne  in  a  crown  on  the  top  of 
the  main  trunk.  Some  specimens  of  Cordaites  leaves  exceed  three  feet 
in  length  and  closely  resemble  the  leaves  of  such  plants  as  the  lily  and 
Indian  corn,  although  not  related  to  them.  The  fructifications,  which 
were  borne  in  a  poorly  developed  cone,  were  of  two  kinds,  male  and 


FIG.  475.  —  Calamites.  They 
seem  to  have  had  habits  similar  to 
those  of  the  horsetails  of  to-day. 
(See  Fig.  476  A  and  B.) 


THE   CARBONIFEROUS   PERIODS 


497 


female,  the  latter  having  a  fleshy  cover  somewhat  like  a  plum.  Cor- 
daites  were  on  a  level  with  the  seed  ferns  as  regards  seeds,  but  in  the 
structure  of  the  wood  and  in  other  respects  they  were  more  highly 
organized.  Cordaites  became  extinct  before  the  close  of  the  Paleozoic, 
unless  the  ginkgo  (p.  567)  or  maidenhair  tree  is  a  descendant. 


FIG.  476.  —  A,  portion  of  trunk  of  a  Catamites.  The  nodes  and  vertical  stria- 
tions  are  characteristic.  Some  grew  to  be  sixty  to  ninety  feet  high.  B,  Annularia, 
showing  the  characteristic  arrangement  of  the  leaves  of  the  Calamites. 

True  conifers  possibly  appeared  before  the  close  of  the  Paleozoic, 
as  the  presence  of  the  genus  Walchia  (Fig.  478)  in  the  Permian 
appears  to  show. 

Conditions  under  which  the  Coal  Plants  Grew.  —  The  most  abun- 
dant plants  of  the  Carboniferous  swamps,  the  Calamites,  Lepido- 
dendron,  and  Sigillaria,  had  narrow  leaves  with  a  small  surface  ex- 
posed to  the  sun.  At  the  present  time  plants  with  leaves  of  this 
character  grow  in  the  bright  sunlight,  while  the  leaves  of  shade  plants 
are  large,  the  greater  size  being  necessary  in  order  that  they  may  be 
acted  upon  by  a  larger  amount  of  light.  Some  of  the  coal  plants,  such 
as  the  true  ferns  and  seed  ferns,  had  fairly  large  leaves,  but  they  were 
not  of  unusual  size,  indicating  that  they  were  only  partially  shaded  by 
the  small-leaved  Calamites  and  Sigillaria.  From  this  evidence  it 
has  been  held  that  the  Carboniferous  plants  did  not  live  in  a  misty 
atmosphere  through  which  the  sun's  rays  penetrated  with  difficulty, 
but  in  one  which  was  not  unlike  that  of  the  present. 


498 


HISTORICAL  GEOLOGY 


FIG.  477.  —  Cordaites.  Large  trees 
with  long,  narrow  leaves  sometimes 
a  yard  in  length.  They  are  allied  to 
the  conifers  as  well  as  to  other  orders 
of  plants. 


The  character  of  the  foliage  of  the 
coal-making  plants  may,  however, 
have  been  an  adaptation  to  con- 
ditions which  more  than  counterbal- 
anced the  effect  of  bright  sunlight. 
Their  roots  were  those  of  water 
plants,  and  their  leaves  were  not 
only  narrow  but  were  supplied  with 
various  devices  for  preventing  the 
loss  of  water  by  rapid  transpira- 
tion. "  If  the  water  they  grew 
in  had  been  fresh,  they  would 
not  have  had  such  leaves,  for 
there  would  have  been  no  need 
for  them  to  economize  their  water 
(which  is  physiologically  usable 
in  only  small  quantities  in  the 
plant),  but  as  we  see  in  bogs  and 
brackish  water  to-day,  plants  only 


FIG.   478.  —  Stem  and  leaves  of  Walchia,  a 
characteristic  Permian  conifer. 


partly  submerged    protect  their   leaves   from  transpiring   largely." 
(Stopes.) 

The  evidence  at  hand  (p.  472)  points  to  the  existence  of  extensive 
swamp  areas  which  slowly  sank  as  the  half-decayed  vegetation  accu- 
mulated on  them,  and  which  were  so  near  sea  level  that  a  slight 
sinking  killed  the  vegetation  growing  there  and  buried  them  under 
sand,  clay,  or  lime  ooze.  It  is  probable,  therefore,  that  the  coal 
plants  (Calamites,  Lepidodendron,  Sigillaria)  of  the  Carboniferous 
lived  not  only  in  fresh  but  even  grew  out  in  the  brackish  water  of 
the  shallow  interior  seas. 


THE   CARBONIFEROUS   PERIODS  499 

COAL 

Coal  occurs  in  very  thin  beds  in  the  Devonian  and  in  thicker  beds 
in  the  Mississippian,  but  it  is  in  the  Pennsylvanian  or  Coal  Measures 
that  it  occurs  for  the  first  time  in  beds  or  seams  thick  enough  to  be  of 
commercial  value.  The  thickness  and  purity  of  the  coal  beds  of  this 
period  are  such  as  to  make  it  the  most  important  of  all  coal-bearing 
systems. 

Mode  of  Occurrence.  —  The  total  thickness  of  the  Pennsylvanian 
or  Coal  Measures  is  4000  to  5000  feet  in  the  Appalachian  Mountains 
and  18,000  feet  in  Arkansas,  but  of  this  great  accumulation  of  sedi- 
ment seldom  more  than  two  per  cent,  is  coal,  the  remainder  being 
sandstone,  shale,  limestone,  and  iron  ore.  In  the  section  shown  in 
Fig.  449,  p.  473,  it  is  apparent  that  there  is  no  regular  order  of 
succession  of  the  beds,  except  that  often  a  bed  of  fire  clay  immediately 
underlies  a  coal  seam.  It  is  also  usual  to  find  shale  immediately 
overlying  the  coal,  although  this  does  not  invariably  happen.  In 
different  portions  of  the  same  field  the  same  order  is  usually  found,  but 
in  separate  basins  the  order  may  vary  greatly  and  is  probably  never 
the  same  in  all  particulars. 

Origin  of  Coal.  —  Coal  is  of  vegetable  origin,  as  is  proved  (i)  by 
a  microscopic  examination  which,  even  in  dense  anthracite,  shows 
the  cellular  structure  of  plant  tissue,  and  (2)  by  stumps  of  trees  with 
their  roots  penetrating  the  underclay  which  sometimes  underlies  the 
coal  seam.  In  South  Wales,  for  example,  there  are  100  coal  seams 
in  which  such  stumps  are  embedded.  In  Nova  Scotia,  of  76  coal 
seams  20  have  upright  stumps  with  spreading  roots  penetrating  the 
clay ;  in  the  United  States  few  such  occurrences  are  known.  (3)  Leaves 
are  often  beautifully  preserved  in  the  shale  immediately  overlying  the 
coal.  (4)  Fire  clay  often,  although  not  invariably,  underlies  coal 
beds.  The  character  which  a  fire  clay  possesses  of  withstanding  in- 
tense heat  is  due  to  the  absence  of  alkalies,  such  as  potash  and  soda, 
whose  withdrawal  was  brought  about  by  the  plants  whose  roots  re- 
moved the  soluble  salts  which  they  required  for  food  or  which  were 
removed  by  the  leaching  action  of  the  water  in  the  lakes  or  lagoons. 

It  is  evident,  therefore,  that  coal  is  compressed  bituminized  or  min- 
eralized vegetable  matter. 

Necessary  Conditions  for  Coal  Formation,  (i)  How  Vegetable 
Tissue  Accumulated.  —  It  is  generally  believed  that  coal  originated, 
for  the  most  part,  from  vegetation  that  grew  in  swampy  or  marshy 


5oo  HISTORICAL  GEOLOGY 

places,  although  evidence  Jias  been  advanced  recently  which  shows 
that  much  coal  was  formed  from  organic  matter,  spores,  wood,  and 
leaves,  carried  into  swamps  and  lakes.  'It  is  a  matter  of  common  ob- 
servation that  wood  decays  much  less  rapidly  below  water  than  above 
it.  This  is  shown  by  piles  and  posts  which  may  be  entirely  rotted 
away  where  exposed  to  the  air,  while  they  are  well  preserved  where 
continually  soaked  with  water.  The  reason  is  to  be  found  in  the 
fact  that  vegetation  in  the  open  air  is  readily  attacked  by  destroying 
fungi,  the  carbon  is  oxidized  to  carbon  dioxide  and  the  hydrogen  to 
water,  and  as  these  are  volatile  the  entire  substance  of  the  plant  may 
disappear;  while  in  water  the  oxidation  proceeds  much  less  rapidly 
and  completely,  and  wood-destroying  organisms  cannot  flourish  in 
water.  Of  the  vegetation  of  luxuriant  forests  only  thin  layers  of 
humus  remain,  and  the  abundant  vegetation  of  dry,  fertile  plains  fails 
to  accumulate,  although  the  slow-growing  bog  moss  (sphagnum)  of 
cold  regions  may  accumulate  to  form  thick  beds  of  peat.1 

(2)  How  it  was  Kept  from  Decay.  —  The  chemical  changes  which 
take  place  in  vegetable  tissue  (which  has  a  composition  approximately 
of  C6Hi0O6)  when  deposited  in  water,  result  in  the  formation  of  marsh 
gas  (CH4),  carbon  dioxide  (CO2),  and  other  gases.  The  effect  of 
these  changes  consists  in  (i)  a  reduction  in  volume,  (2)  a  reduction 
in  the  volatile  constituents,  (3)  a  reduction  in  the  amount  of  water, 
and  (4)  a  relative  increase  in  the  percentage  of  carbon,  since  although 
the  greater  part  of  the  hydrogen  and  oxygen  are  removed,  the  carbon 
is  only  moderately  reduced. 

The  proof  in  support  of  the  assumption  that  the  great  coal  deposits 
were  developed  in  swamps,  the  vegetation  accumulating  where  it 

"  The  peat-bog  hypothesis,  or  growth  in  situ  (autocthonous)  hypothesis,  at  the  present 
moment  has  won  the  adhesion  of  the  majority  of  the  geologists,  although  it  encounters  the  serious 
difficulty  that  peat  bogs  are  not  found  in  the  parts  of  the  earth  which  at  the  present  time 
present  the  nearest  approach  to  the  conditions  of  climate  obtaining  in  the  great  coal-forming 
epochs.  The  lacustrine  or  transport  hypothesis,  which  is  better  applicable  to  the  conditions 
of  warmth  which  are  generally  conceded  to  have  existed  in  the  most  active  periods  of  coal 
formation,  has  had  few  adherents  in  recent  years  outside  of  France.  It  is,  however,  the 
hypothesis  which  harmonizes  best  with  the  structures  found  in  coals  as  the  result  of  micro- 
scopic examination.  The  bottom  of  every  lake  is  filled  with  countless  pollen  grains  or 
spores.  As  the  bottom  becomes  shallower,  water  lilies  and  other  water  plants  make  their 
appearance  and  add  their  remains  to  the  lacustrine  accumulations.  Finally  the  coarser 
debris  of  the  land  plants  is  added  to  the  heap  and  not  long  afterwards  mosses,  grasses, 
sedges,  heaths,  and  ultimately  forest  trees,  may  flourish,  where  once  was  open  water." 
Jeffrey,  E.  C.,  —  On  the  Composition  and  Qualities  of  Coal :  Economic  Geology,  Vol.  9, 
1914,  pp.  730-742. 

It  is  believed  by  this  investigator  that  practically  all  coal  is  floated  material  and  has  not 
originated  from  plant  remains  in  situ,  and  microscopic  evidence  is  stated  by  him  to  place  this 
as  reasonable  beyond  question. 


THE  CARBONIFEROUS  PERIODS  501 

grew,  is  to  be  found  (i)  in  the  basin-shaped  seams  which  are  often 
thickest  in  the  center  and  thin  out  to  blacK  shale  at  the  edges ;  (2)  in 
the  remains  of  aquatic  animals  in  the  midst  of  the  coal;  (3)  in  the 
roots  of  trees  embedded  in  the  underclay  in  the  position  in  which  they 
grew ;  and  (4)  in  the  purity  of  the  coal.  If  the  coal  was  formed  from 
vegetation  that  had  drifted  together,  it  would  contain  sand  or  mud  in 
appreciable  amounts.  It  is  not  unusual,  however,  to  find  coal  with 
no  more  impurities  (ash)  than  the  wood  from  which  it  was  derived 
would  have  contained.  The  nearly  uniform  thickness  of  the  coal 
beds  over  hundreds  of  square  miles  is  also  offered  as  an  objection  to 
the  theory  that  the  vegetable  matter  was  drifted  together. 

(3 )  How  it  was  Changed  to  Coal  and  what  Varieties  Resulted.  —  The 
principal  varieties  of  coal  are  peat,  the  partially  decayed  vegetation 
of  swamps ;  lignite  or  brown  coal ;  bituminous  or  soft  coal ;  and  an- 
thracite or  hard  coal.  All  have  been  derived  from  peat,  lignite  being 
the  second  stage,  bituminous  the  third,  and  anthracite  the  fourth. 
The  last  stage  is  graphite,  in  which  all  the  volatile  constituents  have 
disappeared  and  pure  carbon  only  remains.  Anthracite  coal  occurs 
in  regions  where  the  strata  have  been  much  folded  and  faulted.  It 
is  therefore  generally  believed 1  that  the  heat  and  pressure  of  dynamic 
action  are  essential  processes  in  coalifaction  or  bituminization.  It 
has  also  been  suggested  that  in  regions  of  great  folding  the  fractures 
which  have  been  produced  facilitate  the  escape  of  gases  from  coal  and 
thus  hasten  the  process.  In  Rhode  Island  dynamic  metamorphism 
has  been  so  intense  that  the  coal  has  gone  beyond  the  anthracite  stage 
and  contains  so  much  graphite  as  to  be  of  little  value.  In  Colorado 
and  elsewhere  bituminous  coal  has  been  converted  to  anthracite 
where  cut  by  dikes,  and  in  Mexico  coal  has  been  baked  to  graphite  by 
heat.  Such  graphite  is  of  great  value  in  the  manufacture  of  lead 
pencils.  Some  varieties  of  coal  result  from  the  kind  of  vegetation 
of  which  it  is  composed.  Cannel  coal,  for  example,  is  made  almost 
wholly  of  the  spores  of  Carboniferous  plants. 

Conditions  Favoring  Coal  Formation  in  the  Pennsylvanian.  —  To 
understand  the  great  accumulation  of  coal  during  the  Pennsylvanian 
one  must  picture  to  himself  the  conditions  at  that  time.  The  land 
appears  to  have  been  low,  and  sluggish  streams  meandered  through 
extensive  fresh-water  marshes.  The  great  inland  seas,  shut  off  on 
the  east  by  the  continent  of  Appalachia,  were  bordered  by  wide 

1  Prof.  E.  C.  Jeffrey  offers  evidence  to  show  that  the  varieties  of  coal  depend  largely  upon 
their  composition. 


502  HISTORICAL  GEOLOGY 

fresh  and  salt  water  marshes  in  which  vegetation  flourished.  Similar 
conditions  are  to  be  seen  to-day  in  the  Dismal  Swamp  of  Virginia, 
on  the  coast  of  New  Jersey,  the  Carolinas,  and  Florida. 

Since  less  than  five  per  cent,  of  the  Coal  Measures  (Pennsylvanian) 
consist  of  coal,  the  greater  part  of  the  system  being  composed  of 
sandstones,  shales,  clays,  and  in  some  localities  limestones,  it  is  evi- 
dent that  subsidence  accompanied  deposition.  The  submergence  was 
not  continuous,  however,  but  was  interrupted  by  many  halts,  with  oc- 
casional slight  elevations.  When  the  sea  bottom  was  built  up  suffi- 
ciently, plants  grew  on  it,  and  salt  water  marshes,  which  eventually 
became  fresh,  appeared.  In  the  course  of  years  the  trees  fell,  and 
upon  their  fallen  trunks  others  grew  up.  In  the  process  of  time  their 
remains  made  thick  beds  of  peat.  A  too  rapid  subsidence  inundated 
the  swamps,  killing  the  vegetation,  and  the  peat  was  then  covered  with 
sediment.  If  the  water  was  far  from  shore,  beyond  the  reach  of  mud 
and  sand,  limestones  were  deposited  ;  if  close  to  shore,  mud  and  sand 
were  laid  down.  When  the  downward  movement  ceased,  the  bottom 
of  the  sea  was  built  up  until  it  again  became  shallow  enough  to  permit 
plants  to  grow  on  it.  The  order  of  deposition  shown  in  Figure  449 
(p.  473)  is  thus  explained.  As  has  been  stated,  an  elevation  some- 
times occurred,  as  is  shown  by  unconformities.  Some  of  the  uncon- 
formities, however,  were  produced  merely  by  the  shifting  of  the  stream 
channels  in  the  swamps. 

The  number  of  coal  beds  in  any  vertical  section  varies  greatly: 
in  Pennsylvania  and  Nova  Scotia  as  many  as  30  are  known,  while  in 
Illinois  there  are  often  less  than  10.  Some  of  these  beds  are  workable, 
but  many  are  not. 

Extent  and  Structure  of  Coal  Beds.  —  Individual  coal  swamps 
of  the  Pennsylvanian  were  very  extensive.  The  Pittsburgh  coal  bed, 
one  of  the  greatest  in  the  world,  extends  over  an  area  of  at  least  12,000 
square  miles  in  Pennsylvania,  Ohio,  and  West  Virginia.  The  extent 
of  some  modern  peat  bogs  compares  favorably  with  those  of  the  Penn- 
sylvanian, but  their  thickness  is  much  less.  One  extends  across 
Holland  and  Belgium  into  France,  and  the  Alaskan  tundra  has  a  much 
greater  continuous  area  than  the  largest  of  those  known  in  the  past. 
All  coal  beds  are  not  of  great  extent,  some  corresponding  to  the  small 
peat  bogs  of  to-day :  one  basin  200  yards  in  diameter  was  found  to 
have  two  coal  beds,  one  two  and  the  other  16  feet  in  thickness;  and 
another  one  115  yards  in  diameter  was  found  to  have  a  coal  seam 
eight  feet  in  maximum  thickness.  A  given  thickness  of  coal  repre- 


THE  CARBONIFEROUS   PERIODS  503 

sents  only  about  five  per  cent,  of  the  original  thickness  of  the  peat 
bed,  consequently  a  coal  bed  16  feet  thick  represents  a  peat  bed 
about  320  feet  in  thickness.  None  of  the  peat  bogs  of  the  present 
have  the  great  thickness  of  peat  necessary  to  make  great  beds  of 
coal. 

The  commercial  importance  of  Great  Britain  and  much  of  the  re- 
markable development  of  the  United  States  are  due  to  the  presence 
of  abundant  and  accessible  supplies  of  coal. 

Climate  during  the  Deposition  of  Coal.  —  Since  coal  occurs  not 
only  in  the  temperate  zones  and  in  the  tropics  but  even  in  the  polar 
regions,  it  has  been  assumed  that  the  climate  of  the  Pennsylvanian 
was  uniform  throughout  the  world.  This  is  further  borne  out  by 
a  study  of  the  structure  of  the  wood,  which  shows  no  rings  of  growth 
such  as  are  developed  in  plants  living  in  a  climate  in  which  there 
are  dry  and  wet  or  cold  and  warm  seasons.  The  question  as  to  the 
temperature  and  the  amount  of  moisture  has  given  rise  to  some  dis- 
cussion. It  has  been  generally  assumed  that  the  large  size  of  many  of 
the  trees  and  the  accumulation  of  their  remains  in  swamps  are  proofs 
of  a  warm,  humid  climate.  The  thickness  of  the  coal  seams  has  also 
been  considered  confirmatory  evidence.  Several  objections  have 
been  offered  to  this  belief,  however,  (i)  At  present  the  great  accu- 
mulations of  peat  are  in  cold,  temperate  climates.  (2)  Peat  is  rarely 
formed  of  rapid-growing  plants  but  chiefly  of  the  remains  of  such 
plants  as  sphagnum  moss.  (3)  It  has  been  pointed  out  that,  as  a 
whole,  the  leaves  of  Carboniferous  plants  bear  a  resemblance  to  those 
of  living  plants  that  are  adapted  to  dry  (xerophytic)  conditions  (p. 
497),  being  narrow  and  possessing  devices  to  prevent  the  rapid  evapo- 
ration of  water.  In  the  tropics  peat  accumulates  more  from  tree 
trunks  and  leaves  which  have  been  floated  into  lakes  and  marshes,  and 
little  from  moss  or  trees  that  grew  in  situ. 

When  all  the  evidence  is  considered,  there  seems  little  doubt  that 
the  climate  of  the  regions  in  which  coal  accumulated  was  moist  and 
warm,  although  not  tropical. 

REFERENCES  FOR  COAL 

JEFFREY,  E.  C.,—  The  Mode  of  Origin  of  Coal:  Jour.  Geol.,  Vol.  23,  pp.  213-230. 
SAVAGE,  T.  E.,  — On  the  Conditions  under  which  the  Vegetable  Matter  of  the  Illinois 

Coal  Beds  Accumulated:   Jour.  Geol.,  Vol.  22,  pp.  7S°-765- 
STEVENSON,  J.  J.,  —  Formation  of  Coal  Beds. 
TONGE,  JAMES,  —  Coal. 


504 


HISTORICAL  GEOLOGY 


PROBLEMS  OF  THE  PERMIAN 


No  other  period  of  the  earth's  history  offers  so  many  unsolved 
problems  as  the  Permian.  These  problems  have  to  do  with  the  cli- 
mate and  the  life  of  the  period. 

(i)  Why  was  the  Permian  so  fatal  to  marine  life  ?  During  this 
time  the  invertebrate  life,  for  the  most  part,  either  became  extinct  or 
was  much  modified  in  important  structural  features.  The  impov- 
erishment of  the  life  is  shown  in  the  estimate  that  the  number  of 
Permian  species  was  only  two  per  cent,  of  that  of  the  combined 
Mississippian  and  Pennsylvanian.  One  factor  in  the  extinction  of 
such  a  large  percentage  of  species  was  doubtless  the  emergence  of 
the  continent  and  the  consequent  withdrawal  of  most  of  the  epicon- 
tinental  seas.  This  drove  the  life  of  the  warm,  shallow  seas  into 
the  coastal  waters  of  the  ocean,  where  it  was  not  only  obliged  to  com- 
pete with  other  species  but  was  compelled  to  live  under  conditions  to 
which  it  was  not  accustomed.  Such  a  radical  change  in  environment 
was,  doubtless,  fatal  to  many  species,  and  it  is  consequently  not 
surprising  that  a  large  number  disappeared.  The  faunas  in  the 
restricted  epicontinental  seas  were  crowded,  and  competition  was 
severe.  Where  such  seas  persisted,  as  in  India  and  California,  the 
change  in  the  fauna  was  gradual  and  no  satisfactory  dividing  line  can 
be  drawn. 

A  further  result  of  the  elevation  of  the  continents  (or  withdrawal 
of  the  seas)  was  probably  the  changing  of  the  position  of  the  ocean 
currents,  which  were  forced  to  take  new  courses.  The  extinction  of 
some  of  the  plant  food  upon  which  the  animals  of  the  epicontinental 
seas  ultimately  depended  would  have  had  a  marked  effect  on  the  life. 
These  and  other  causes  may  have  combined  to  bring  about  the  sweep- 
ing changes  in  the  invertebrate  life  at  the  close  of  the  Paleozoic. 

The  land  vertebrates  did  not  suffer  as  did  the  invertebrates.  This 
may  have  been  due  (i)  to  the  greater  land  area  over  which  they  could 
spread  and  the  greater  variety  of  conditions  open  to  them  ;  (2)  to  the 
greater  variety,  or  more  suitable  varieties,  of  plants  and  insects  which 
they  could  use  for  food  ;  or  (3)  to  their  better  organization.  During 
this  time,  so  fatal  to  the  marine  fauna,  the  amphibians  continued  in 
abundance  and  the  reptiles  became  supreme. 

The  plant  life  also  suffered  a  great  change  :  the  important  Carbon- 
iferous groups  became  extinct,  or  nearly  so,  and  their  places  were 
taken  by  plants  of  a  more  modern  type.  Whether  this  was  due  to  the 


THE   CARBONIFEROUS   PERIODS 

draining  of  the  swamps,  with  the  resulting  death  of  the  swamp  vege- 
tation, or  to  the  spreading  of  upland  trees  which  existed  in  the  Penn- 
sylvanian  but  of  which  nothing  is  now  known,  cannot  be  stated. 
Once  well-established,  however,  the  more  highly  organized  upland 
plants  probably  became  in  time  suited  to  swamp  conditions  and 
occupied  the  places  formerly  held  by  the  lycopods  and  horsetails. 

(2)  Why  was  the  Permian  a  period  of  glaciation,  and  in  particular, 
why  were  the  areas  affected  not  only  near  the  equator  but  near  sea 
level  ? 

Various  explanations  have  been  offered,  but  none  has  a  general  acceptance.  One 
is  based  on  the  assumption  that  the  carbon  dioxide  contents  of  the  air  was  decreased. 
The  depletion  of  carbon  dioxide  istelieved,  by  the  adherents  of  this  theory,  to  have 
been  the  result  of  a  combination  of  causes,  indirectly  because  of  the  elevation  of  the 
continents  and  an  increase  in  the  land  area.  As  a  result  of  a  greater  land  surface  being 
exposed  to  the  agents  of  the  weather,  the  rocks  upon  weathering  extracted  much  car- 
bon dioxide  from  the  air.  The  depletion  of  this  gas  was  further  hastened  by  the  oceans, 
which  are  believed  by  the  supporters  of  this  theory  to  have  absorbed  great  quantities 
of  carbon  dioxide.  The  air  was  also  freed  of  its  carbon  dioxide  during  the  accumula- 
tion of  coal.  One  doubtful  element  in  the  theory  is  the  efficacy  of  carbon  dioxide  in 
retaining  heat. 

Another  solution  of  the  problem  is  found  in  the  amount  of  water  vapor  in  the  at- 
mosphere, since  water  .vapor  is  known  to  act  as  a  blanket  in  retaining  heat.  The  en- 
larging of  the  land  area  decreased  the  amount  of  water  vapor  in  the  atmosphere  and 
thinned  the  thermal  blanket. 

(3)  Why  was  the  climate  generally  arid  in  the  northern  hemisphere 
during  the  Lower  Permian  ?     The  great  extent  of  land  and  the  nar- 
rowing of  the  oceans  undoubtedly  had  a  marked  effect  in  producing 
an  arid  climate,  since  less  moisture  would  have  been  evaporated  and  it 
would  have  been  precipitated  over  a  wider  area. 

(4)  Why  was  the  great  Appalachian  system  raised  at  this  time  ? 
It  is  usually  stated  that  strains  had  been  accumulating  in  the  earth's 
crust  throughout  the  Paleozoic  and  that  these  strains  were  relieved 
by  the  folding  of  the  Appalachian  trough  at  its  close,  but  this  does  not 
fully  answer  the  question. 

SUMMARY  OF  THE  PALEOZOIC  ERA 

The  Building  of  the  Continents.  —  The  continent  of  North  America 
was  probably  covered  by  seas  in  every  part  during  some  portion  of  the 
Paleozoic,  but  two  areas  seem  to  have  been  especially  free  from  epi- 
continental  seas  during  all  but  perhaps  a  small  part  of  the  era.  These 
areas  lie  in  the  Laurentian  region  of  eastern  Canada  and  in  the  south- 


506 


HISTORICAL  GEOLOGY 


eastern  United  States,  where  the  continent  of  Appalachia  formerly 
stood  and  of  which  the  Piedmont  Plateau  is  a  part.  During  this 
era  the  seas  varied  greatly  in  extent  and  in  position.  In  only  a  few 
areas,  notably  in  the  Appalachian  trough  and  in  the  Great  Basin 
region,  did  they  persist  through  the  greater  part  of  the  Paleozoic. 

Evolution  and  Extinction  of  Life.  —  A  study  of  the  accompanying 
table  (Fig.  479)  brings  out  some  important  points  concerning  the  life 


CAMBRIAN  ORDOVICIAN  SILURIAN  DEVONIAN        CARBONIFEROUS 


FIG.  479. 


Table  showing  the  distribution  and  relative  abundance  of  the  life 
of  the  Paleozoic. 


of  the  era.  It  is  seen  that  certain  classes  began,  or  at  least  are  first 
known,  in  the  earlier  periods,  culminated  in  the  later  periods,  and 
then  after  several  periods  of  struggle  became  extinct.  If  one  were  to 
make  a  careful  study  of  each  of  these  classes  it  would  be  found  that 
the  genera  of  each  class  had  a  shorter  life  than  the  class  as  a  whole, 
and  that  the  species  had  a  still  briefer  one.  It  would  also  be  found 
that  striking  evolutional  changes  took  place  during  their  life  histories. 
It  is  also  to  be  seen  that  some  classes  gradually  increased  in  impor- 
tance throughout  the  era,  while  others  were  inconspicuous  in  the 


THE   CARBONIFEROUS   PERIODS  507 

Paleozoic,  but  if  a  Mesozoic  table  were  examined  it  would  be  found 
that  these  inconspicuous  forms  became  prominent  in  the  latter. 

Climate.  —  Our  knowledge  of  the  climate  of  the  Paleozoic  is  not 
extensive.  As  a  whole,  the  evidence  points  to  uniform  conditions 
and  no  well-marked  climatic  zones.  There  were,  however,  glaciers 
in  certain  regions  in  the  Cambrian  and  Permian,  and  perhaps  in  other 
periods.  Certain  areas  were  arid  during  portions  of  the  Paleozoic, 
as  their  red  sediments,  gypsum,  and  salt  show,  the  aridity  having  prob- 
ably been  caused  in  most  regions  by  elevated  land  areas  which  shut 
off  the  moist  winds  of  the  oceans. 

... 

REFERENCES  FOR  THE  CARBONIFEROUS  PERIODS 

BLACKWELDER  and  BARROWS,  —  Elements  of  Geology,  pp.  369-396. 

CHAMBERLIN  and  SALISBURY,  —  Geology,  Vol.  2,  pp.  496-677. 

SCHUCHERT,  CHAS.,  —  P ale o geography  of  North  America:    Bull.  Geol.  Soc.  America, 

Vol.  20,  1910,  pp.  494-498. 

SCOTT,  W.  B.,  —  An  Introduction  to  Geology,  pp.  609-654. 
ULRICH,  E.  O.,  —  Revision  of  the  Paleozoic  Systems:  Bull.  Geol.  Soc.  America,  Vol.  22, 

1911. 


CHAPTER  XX 


MESOZOIC   ERA:  THE  AGE   OF  REPTILES 

THE  Mesozoic  is  divided  into  four  periods,  as  given  below : 


Cretaceous  Periods 


Upper  Cretaceous 


Lower  Cretaceous 
(Comanchean) 


The  word  comes  from  the  Latin 
creta,  meaning  chalk,  because  of 
the  great,  thickness  of  the  chalk 
of  this  period  in  England  and 
France. 


Jurassic  Period  So  named  because  of  the  fine 

development  of  the  strata  of  this 
period  in  the  Jura  Mountains. 

Triassic  Period  So  named  because  of  the  three- 

fold development  in  Germany 
where  the  strata  were  first  care- 
fully studied. 

PHYSICAL  GEOGRAPHY  DURING  THE  MESOZOIC 

TRIASSIC 

Atlantic  and  Gulf  Coasts.  —  A  number  of  points  seem  to  be  well 
established  concerning  the  distribution  of  land  and  water  in  North 
America  during  the  Triassic  (Fig.  480).  (i)  The  complete  absence, 
so  far  as  known,  of  marine  sediments  from  the  eastern  half  of  the 
continent  indicates  that  the  coast  line  was  farther  east  than  now 
and  that  during  the  entire  period  the  lands  were  being  reduced  by 
erosion.  Indeed,  it  is  possible  that  not  only  Newfoundland  but  even 
Greenland  and  Iceland  were  united  to  the  continent.  (2)  The  presence 
of  Triassic  rocks  in  long,  narrow  bands  roughly  parallel  to  the  Atlantic 
coast,  that  stretch  from  Nova  Scotia  to  North  Carolina,  the  longest  of 
which  extends  from  the  Hudson  River  across  New  Jersey,  southeastern 
Pennsylvania,  through  Maryland  and  Virginia,  formerly  gave  rise 
to  the  opinion  that  these  deposits  were  formed  in  tidal  estuaries  whose 
waters  for  the  most  part  were  brackish  or  nearly  fresh.  It  seems 
more  probable,  however,  that  the  deposits  were  not  formed  in  con- 

508 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


509 


tinuous  water  bodies 
but  in  river  basins 
analogous  to  the 
Great  Valley  of  Cali- 
fornia. These  conti- 
nental deposits  were 
formed  by  the  con- 
fluence of  alluvial 
fans  (p.  124)  made 
by  streams  flowing 
from  higher  land  at 
the  margin  of  the 
area ;  deposits  formed 
by  rivers  meandering 
over  the  lowland ; 
lake  deposits  in  places 
where  the  drainage 
was  obstructed,  as  in 
Tulare  Lake,  Cali- 
fornia ;  and  it  is  pos- 
sible that  parts  of  the 
area  were  covered  by 
tidal  waters  and  that 
in  such  places  estua-  FIG.  480.  —  Map  showing  the  probable  outline  of  North 
•  i  •  1-j  America  during  a  portion  of  the  Upper  Triassic.  The 

nne  deposits  were  laid   continental  deposits  are  shown  in  solid  black     (Modified 

down.     Since  the  re-  after  Schuchert.) 

gion  was  in  an  arid  or 

semiarid  condition,  deposits  of  wind-blown  sand  were  doubtless  laid 

down  on  land,   some  of  which   probably  constitute  a  part  of  the 

Triassic  sandstone. 

These  basins  were  separated  from  the  Appalachian  Mountains  on 
the  west  by  ridges  of  crystalline  rocks  (Fig.  481).     The  presence  of 


FIG.  481.  —  Section  through  the  Connecticut  valley  and  adjacent  region  in 
Massachusetts.     Js  is  Triassic  sandstone.     (After  Emerson.) 

high  land  between  the  basins  and  the  Appalachians  is  shown  by  the 
composition  of  the  sediments  laid  down  in  the  depressions,  which  were 


HISTORICAL  GEOLOGY 


not  derived  from  sedimentary  rocks,  as  would  have  been  the  case  if 
the  Appalachians  had  drained  eastward  through  them,  but  are  gra- 
nitic and  were  derived  from  metamorphic  and  igneous  rocks.  The 
present  thickness  of  these  deposits  is  very  great,  it  being  estimated 
that  some  of  those  of  Pennsylvania  and  Connecticut  are  several  thou- 
sand feet  thick.  The  liability  to  error  in  estimating  the  thickness  of 


FIG.  482.  —  Section  across  the  Connecticut  valley,  showing  the  thick  strata  of  sand- 
stone (dotted)  and  the  lava  beds  (solid  black).  The  dotted  line  shows  the  present 
outline  of  the  surface.  The  complex  structure  of  the  underlying  rock  and  the  rock  of 
the  highlands  is  well  shown.  (After  Barrell.) 

these  deposits,  because  of  the  concealed  faults,  is  so  great  that  no  fig- 
ures can  be  considered  more  than  provisional.  The  sediments  north 
of  Virginia  are  usually  red  sandstones  and  shales,  with  occasional 
thin  beds  of  black  shale  and  limestone. 

In  Virginia  and  North  Carolina  coal  conditions  prevailed,  but, 
with  the  exception  of  these  and  the  abundant  footprints  of  the  Con- 
necticut valley,  fossils  are  rare.  Fish  and  plant  remains  in  thin  beds 


FIG.  483.  —  Section  across  the  Connecticut  valley,  showing  the  same  region  as  in 
Fig.  482  after  faulting  had  occurred,  and  after  erosion  had  worn  the  region  to  a  pene- 
plain. (After  Barrell.) 

are,  however,  occasionally  found.  Since  no  marine  fossils  have  been 
discovered  in  any  of  the  Triassic  deposits  of  the  east,  the  exact  age 
of  these  deposits  is  somewhat  in  doubt,  but  the  evidence  points  to 
the  Upper  Triassic  as  the  time  at  which  they  were  laid  down.  This 
formation  is  known  as  the  Newark  because  of  its  development  near 
the  city  of  that  name  in  New  Jersey. 

During  the  deposition  of  these  sediments  lava  flows  of  considerable 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES  511 

extent  and  thickness  were  poured  out ;  dikes  and  sills  were  intruded ; 
and  in  a  few  places  volcanoes  were  in  eruption.  The  evidence  of  this 
igneous  activity  is  especially  well  shown  in  the  Connecticut  valley 
(Fig.  482),  where  in  certain  localities  there  are  three  distinct  lava 
flows.  The  lava  forming  the  Palisades  of  the  Hudson  (Fig.  323,  p.  326), 
which  varies  in  thickness  from  300  to  850  feet  and  stretches  for  70 
miles  from  north  to  south,  is  an  intrusion  (Fig.  322,  p.  326).  The 
faulting  and  subsequent  erosion  (Fig.  483)  of  the  Triassic  sediments 
and  lavas  of  Massachusetts,  Connecticut,  and  New  Jersey  have  re- 
sulted in  hills  and  mountains  of  (for  these  regions)  unusual  shape. 

Western  Interior.  —  In  the  western  interior  of  North  America 
(map,  Fig.  480),  the  deposits  are  also  for  the  most  part  red  and,  as 
a  rule,  devoid  of  fossils.  Some  of  the  sediments  were  deposited  in 
fresh-water  lakes,  others  in  salt-water  lakes,  and  some  are  probably 
of  eolian  origin. 

Pacific  Coast.  —  In  the  early  part  of  the  period,  the  Pacific 
coast  line,  with  the  exception  of  a  comparatively  narrow  bay  that 
stretched  west  from  California  to  Wyoming,  was  probably  farther 
west  than  now.  Later  in  the  period  the  sea  spread  over  the  land 
until  it  covered  a  large  area  in  Alaska,  Canada,  British  Columbia, 
Washington,  Oregon,  Nevada,  and  California,  and  smaller  areas  in 
Mexico.  The  fossils  from  the  Triassic  deposits  of  the  west  are  in 
many  cases  very  abundant. 

Triassic  in  Other  Continents.  —  Triassic  rocks  have  a  wide  distri- 
bution in  Europe,  where  both  marine  and  continental  deposits  occur; 
the  marine  being,  in  general,  in  the  south  of  Europe  and  the  conti- 
nental in  the  northern  and  central  portions.  The  Triassic  in  Germany 
has  a  threefold  division  (hence  the  name  Triassic).  The  lowest  con- 
sists of  deposits  which  were  laid  down  in  fresh  and  salt  lakes,  not  unlike 
those  of  the  Triassic  of  the  western  interior  of  North  America,  and  to 
some  extent  are  of  eolian  origin,  as  the  dune  structure  of  some  of 
the  sandstone  shows.  Sun  cracks,  raindrop  impressions,  and  tracks  of 
animals  also  occur.  The  relation  between  the  fertility  or  barrenness 
of  a  soil  and  the  rock  from  which  it  was  derived  is  well  illustrated  by 
this  formation  in  Germany.  Since  the  rocks  are  composed  of  quartz 
sand  containing  little  plant  food,  the  region  underlain  by  them  is  not 
cultivated,  but  has  been  allowed  to  remain  forested,  and  hence  this 
formation  has  been  called  the  "  forest  formation." 

During  the  Middle  Triassic  (Muschelkalk)  an  inland  sea  connected 
with  the  ocean  spread  over  a  large  part  of  the  area  of  the  earlier 

CLELAND   GEOL.  —  33 


512 


HISTORICAL  GEOLOGY 


formation.  This  sea  was  later  shut  off  from  the  ocean  and  soon  be- 
came a  salt  lake,  as  the  deposits  of  gypsum  and  salt  show.  In  the 
Upper  Triassic  (Keuper),  marine  beds,  thin  coal  seams,  gypsum  and 
salt  deposits,  and  in  the  last  stage  (Rhaetic)  marine  deposits  again 
show  a  few  of  the  fluctuations  of  the  period. 


JURASSIC 

Atlantic  and  Gulf  Coasts.  —  In  eastern  North  America  the  emer- 
gence of  the  continent  seems  to  have  continued  from  the  Triassic,  no 

trace  of  marine  sedi- 
ments being  known 
except  in  Mexico 
where  the  Gulf  of 
Mexico  extended 
west  of  its  present 
position.  Probably 
near  the  close  of  the 
Triassic  the  conti- 
nent was  warped  in 
such  a  way  that  the 
Triassic  sandstones 
and  shales  of  the 
Connecticut  valley 
were  tilted  to  the 
east,  and  those  of 
New  Jersey  and  far- 
ther south  to  the 
west.  The  Jurassic, 
like  the  Triassic,  ap- 
pears to  have  been  a 
period  of  continued 
erosion  in  the  eastern 
half  of  the  continent. 
Western  Interior. 
—  No  early  Jurassic 
rocks  are  known  with 

certainty  to  occur  in  the  western  interior,  but  later  in  the  period  an 
arm  of  the  sea  of  great  width  extended  south  (Fig.  484)  from  Alaska  to 
Wyoming,  Utah,  and  the  Black  Hills  of  South  Dakota.  The  preva- 


FIG.  484.  —  Map  showing  the  probable  outline  of 
North  America  during  a  portion  of  the  Upper  Jurassic. 
(Modified  after  Schuchert.) 


MESOZOIC  ERA:    THE  AGE  OF   REPTILES 


513 


lence  of  sandstones,  with  only  occasional  limestone  beds,  shows  that 
the  sea  was  a  shallow  one.  Since  the  fossils  of  some  of  the  beds  are 
of  marine  species,  closely  resembling  those  of  Siberia  rather  than  those 
of  California  of  the  same  age,  we  must  suppose  that  a  mediterranean 
sea  was  connected  at  the  north  with  the  ocean,  and  that  a  long  land 
barrier  separated  it  from  the  Pacific.  After  a  comparatively  short 
existence  this  great  bay  was  drained  by  elevation  of  the  land  ;  and  its 
site  was  covered  in  the  southern  portion  by  a  widespread,  continental 
formation  (Morrison)  which  contains  skeletons  of  dinosaurs  and  other 
reptiles  and  a  few  mammalian  remains.  Because  of  the  absence  of 
marine  fossils,  it  is  not  yet  certain  whether  these  beds  are  late  Juras- 
sic or  early  Cretaceous,  or  whether  the  lower  portions  belong  to  the 
earlier  period  and  the  upper  to  the  later. 

Mountain  Forming  in  the  West.  —  In  the  west,  where  the  Sierra 
Nevada  Mountains  now  stand,  Jurassic  sediments  derived  from 
extensive  lands  on  the  east  had  been  accumulating  in  a  great  subsiding 
trough  (geosyncline),  until  they  had  attained  a  maximum  thickness  of 
five  or  six  thousand  feet.  The  sediments  deposited  in  this  trough, 
including  Triassic,  Jurassic,  and  Paleozoic,  attained  the  enormous 
thickness  of  nearly  25,000  feet.  Near  the  close  of  the  period, 
this  huge  accumulation  of  sediments  began  to  yield  to  great  lateral 
compression  and  was  folded  and  upheaved  into  the  first  Sierra 
Nevada  Mountains,  which  perhaps  rivaled  in  height  any  in  existence 
to-day ;  they  may,  however,  have  been  eroded  almost  as  rapidly  as 
they  rose  and  therefore  never  have  reached  a  great  elevation.  Dur- 
ing the  folding  great  quantities  of  igneous  rocks,  especially  granites, 
were  forced  into  the  folded  sediments,  forming  upon  cooling  batho- 
liths  and  stocks.  The  muds  and  sands  in  the  neighborhood  of  the 
intrusions  were  also  changed  to  schists  and  other  metamorphic 
rocks ;  and  even  at  a  distance  shales  were  metamorphosed  to  slates. 
At  about  the  same  time,  the  Coast  Ranges,  the  Cascades,  and  farther 
north  the  Klamath  Mountains  began  their  growth.  It  should  not 
be  inferred  from  the  above  that  the  present  height  of  the  Sierra 
Nevadas  was  the  result  of  these  movements:  the  Sierra  Nevadas 
of  to-day,  as  will  be  explained  later,  are  the  result  of  a  great  fault 
on  the  east,  which  occurred  at  a  much  more  recent  date. 

Jurassic  of  Other  Continents.  —  The  greater  portion  of  Europe 
and  Asia  was  above  the  sea  during  the  earlier  part  of  the  period,  but  a 
progressive  submergence  soon  began,  culminating  in  the  Upper 
Jurassic,  at  which  time  the  two  continents  were  traversed  by  straits 


HISTORICAL  GEOLOGY 


and  seas  which  cut  them  into  a  number  of  large  and  small  islands. 
The  submergence  of  central  and  northern  Russia  is  of  especial  in- 
terest, since  in  this  basin  was  developed  a  peculiar  fauna  which 
spread  into  the  great  mediterranean  sea  of  the  western  interior  of 
North  America.  An  arm  of  the  sea  covering  the  site  of  the  Hima- 
layas separated  India  from  northern  Asia.  This  Upper  Jurassic  sub- 
mergence in  Europe  and  Asia  was  one  of  the  greatest  in  all  the  re- 
corded geological  history  of  these  continents. 


LOWER  CRETACEOUS  (COMANCHEAN) 

Atlantic  and   Gulf   Coasts.  —  No  marine  deposits  of  the  Lower 
Cretaceous  (Fig.  485)  have  been  found  on  the  Atlantic  coast,  but 

a  belt  of  continental 
sediments,  stretch- 
ing from  Marthas 
Vineyard  island  to 
Georgia  (the  Poto- 
mac group),  occurs, 
which  seldom  attains 
a  thickness  of  more 
than  600  feet.  The 
lesson  which  these 
deposits  teach  of  the 
physical  condition 
at,  and  immediately 
preceding,  the  time 
of  their  formation  is 
interesting.  They 
show  that  at  the 
close  of  the  Jurassic 
the  eastern  portion 
of  the  continent  had 
been  reduced  to  a 
comparatively  level 
plain  over  wide 
areas,  the  surface  of 
which  was  covered 
deeply  with  weath- 


FIG.  485.  —  Map  showing  the  probable  outline  of 
North  America  during  a  portion  of  the  Lower  Creta- 
ceous. Continental  deposits  are  shown  in  solid  black. 
(Modified  after  Schuchert.) 


ered  rock,  resulting 


MESOZOIC   ERA:    THE  AGE  OF   REPTILES  515 

from  the  decay  of  the  underlying  formations,  which  the  slow-moving 
streams  of  that  time  were  unable  to  transport.  At  the  beginning  of 
the  Lower  Cretaceous,  however,  the  Piedmont  and  Appalachian  re- 
gions were  raised,  perhaps  along  the  axis  of  the  Appalachian  tract, 
while  the  land  nearer  the  coast  was  but  little  disturbed,  either  remain- 
ing comparatively  level  or  being  depressed  into  long  troughs,  some- 
what similar  to  those  of  the  Triassic.  Under  these  new  conditions, 
the  streams  in  the  higher  regions  began  again  to  erode.  On  account 
of  the  abundance  of  loose,  weathered  material,  the  streams  in  their 
courses  to  the  sea  soon  had  all  the  sediment  they  could  carry  and  as 
soon  as  a  lower  gradient  was  reached  dropped  their  loads.  This 
resulted  in  the  formation  of  deltas,  flood  plains,  marshes,  and  shallow 
lakes.  The  deposits  formed  in  this  way  were  gravels,  composed  of 
the  quartz  of  quartz  veins  and  quartzites,  clay  from  the  decayed 
feldspar,  shales,  and  slates,  and  arkose  in  the  immediate  vicinity  of 
feldspar-bearing  rocks. 

The  most  striking  feature  of  the  Lower  Cretaceous  geography  is 
the  expansion  of  the  Gulf  of  Mexico  towards  the  west  and  northwest 
and  the  deep  subsidence  of  its  floor,  upon  which  were  deposited  a 
great  thickness  of  limestones.  Large  areas  in  Mexico,  Texas,  and  New 
Mexico  were  covered  at  this  time.  In  this  sea  the  Ouachita  Moun- 
tains stood  out  as  a  promontory,  as  is  shown  by  the  ancient  shore 
line  which  has  been  traced  around  their  foot.  The  sediments  have  a 
thickness  of  5000  feet  on  the  Rio  Grande  and  are  even  thicker  in 
Mexico.  At  the  base  of  this  marine  formation  is  one  which  is  in  part 
a  littoral  deposit  but  is  mostly  marine  (Trinity). 

Western  Interior.  —  In  the  western  interior  non-marine  formations, 
sometimes  including  coal  beds,  occur  (Morrison  which  may  be  Juras- 
sic, Kootenai,  Cleverly,  Lakota,  Fuson). 

Pacific  Coast.  —  On  the  Pacific  coast  the  conditions  were  very 
favorable  for  erosion,  because  of  the  newly  raised  Sierra  Nevadas 
which  were  being  rapidly  cut  away,  the  material  derived  from  them 
forming  a  thick  deposit  in  the  Sacramento  valley.  In  addition  to 
this  area,  other  narrow  strips  were  submerged  east  of  the  present 
coast  of  British  Columbia  and  Alaska,  during  portions  of  the  period. 

Lower  Cretaceous  of  Other  Continents.  —  In  Europe,  as  in  North 
America,  the  Lower  Cretaceous  formations  are  largely  of  continental 
origin  and  are  not  as  widespread  as  those  of  the  Upper  Cretaceous. 
In  general,  it  can  be  said  that  important  geographical  changes  oc- 
curred in  various  parts  of  the  earth  at  the  closeof  the  Lower  Cretaceous, 


Si6 


HISTORICAL  GEOLOGY 


as  are  recorded  in  the  unconformities  between  the  Lower  and  Upper 
Cretaceous  strata  and  in  the  difference  in  their  distributions. 


UPPER  CRETACEOUS  (CRETACEOUS) 

The  Upper  Cretaceous  was  a  period  of  great  subsidence  (Fig.  486), 
no  other  in  the  earth's  history  since  the  Paleozoic  being  comparable 
to  it.  Not  only  were  portions  of  the  Atlantic  and  Pacific  coasts  sub- 
merged, but  a  vast 
inland  sea  covered 
for  a  time  the  cen- 
tral portion  of  North 
America,  from  the 
Gulf  of  Mexico  to  the 
Arctic  Ocean  (Fig. 
486),  separating  it 
into  two  land  masses. 
Atlantic  and  Gulf 
Coasts.  —  The  sea 
spread  over  the 
coastal  plains  of  the 
Atlantic  and  Gulf 
states,  and  strata 
composed  of  sands, 
clays,  chalk,  and 
"green  sands"  (glau- 
conite)  were  accumu- 
lated. The  map  (Fig. 
486)  shows  the  sup- 
posed distribution 
better  than  a  written 
description.  At  this 
time  the  eastern  half 
of  the  continent  was 
probably  a  compara- 
tively flat  plain  (Kit- 


FIG.  486.— -Map  showing  the  probable  outline  of  North 
America  during  a  portion  of  the  Upper  Cretaceous.  The 
inland  or  epicontinental  seas  were  widespread.  Conti- 
nental deposits  are  shown  in  solid  black.  (Modified  after 
Schuchert.) 


tatmny  peneplain)  to  which  it  had  been  reduced  during  the  long  ages 
of  the  earlier  Mesozoic,  notwithstanding  occasional  warpings. 
Across  this  plain  the  Potomac,  Susquehanna,  and  Delaware  rivers 
meandered,  probably  in  very  much  the  same  courses  as  to-day. 


MESOZOIC   ERA:    THE  AGE  OF   REPTILES  517 

Pacific  Coast.  —  On  the  Pacific  coast  Upper  Cretaceous  sediments 
occur  in  California  and  northward  at  points  as  far  distant  as  Alaska, 
where  they  are  sometimes  conformable  and  sometimes  unconformable. 
Usually  they  are  not  thick,  but  in  one  locality  (California),  at  least, 
they  apparently  reach  the  great  thickness  of  25,000  feet. 

Western  Interior.  —  The  geography  of  the  western  interior  can 
be  roughly  divided  into  (i)  an  epoch  when  the  sea  was  excluded,  and 
beds,  mainly  of  non-marine  (Dakota)  sediments,  were  laid  down ; 
(2)  an  epoch  of  pronounced  extension  of  the  sea  during  which  a 
great  thickness  of  marine  sediments  (Colorado  and  Montana)  ac- 
cumulated ;  and  (3)  an  epoch  during  which  the  land  was  so  low  that 
slight  oscillations  produced  conditions  which  resulted  in  the  forma- 
tion of  bodies  of  salt,  brackish,  or  fresh  water  (Laramie). 

(1)  The  sediments  of  the  first  epoch  (Dakota)  probably  covered 
an  area  2000  miles  long  by  1000  miles  wide,  which  stretched  from 
Canada  on  the  north  into  Texas  on  the  south,  and  from  Minnesota 
and  Iowa  on  the  east  to  beyond  the  present  site  of  the  Rocky  Moun- 
tains on  the  west.     The  formation  is  largely  sandstone,  though  it 
contains  much   conglomerate  and   clay,  and  some  lignite.     At  this 
time  marshes  and  lagoons  existed  near  the  shores,  while  inland  slug- 
gish streams  were  depositing  fine  sediment  over  the  bottom.     The 
presence  of  brackish  water  fossils  in  beds  of  this  age  in  Kansas  in- 
dicates marine  conditions  at  certain  times,  or  at  least  a  low  shore 
on  which  fresh  and  salt  water  were  mingled.     The  porous  beds  thus 
formed  are  now  the  great  water-bearing  strata  of  the  Great  Plains, 
the  water  of  these  porous  sandstones  being  derived  from  the  rains 
which  fall  upon  and  the  streams  which  flow  over  their  upturned  edges 
in  the  mountainous  regions.     Although    of   such  wide  extent,  the 
Dakota  sandstones  have  a  fairly  uniform  thickness  of  only  200  to 
300  feet. 

(2)  This  epoch  (Dakota)  was  followed  by  one  of  extensive  submer- 
gence which  resulted  in  the  formation  of  a  great  sea   (Colorado), 
stretching  from  the  Gulf  of  Mexico  to  the  Arctic  Ocean  and  covering 
the  Great  Plains  of  Canada  and  the  United  States  and  the  site  of 
the  Rocky  Mountains,  with  the  possible  exception  of  some  large  and 
small  islands.     As  the  sea  encroached  upon  the  land,  muds  were 
first  deposited,  but  as  it  deepened  and  widened  the  waters  became 
clearer,  and  chalk  and  limestone  were  laid  down  locally,  while  in  the 
extensive  bordering  swamps  peat  accumulated  to  form  important 
beds  of  coal  which  are  best  developed  in  Wyoming  and  Utah.     This 


518  HISTORICAL  GEOLOGY 

larger  sea  gave  place  later  to  a  somewhat  more  constricted  one 
(Montana),  along  the  edges  of  which  the  conditions  were  favorable 
for  coal  formation.  This  is  shown  by  thick  coal  beds  of  this  time  in 
Montana,  Wyoming,  California,  Utah,  and  New  Mexico.  It  is  prob- 
able that  neither  of  these  seas  was  of  great  depth. 

(3)  The  closing  epoch  of  the  western  interior  was  the  Laramie. 
The  evidence  points  to  a  land  so  low  that  a  slight  oscillation  either 
raised  or  submerged  it.  When  the  latter  occurred,  the  sea  overspread 
the  land,  and  marine  sediments  were  deposited;  as  the  sea  was 
filled  in  by  sediments  or  was  partially  drained  by  elevation,  swamps 
and  marshes  were  formed  and  peat  accumulated  in  sufficient  quan- 
tities to  form  later  some  workable  beds  of  coal.  There  is  probably  as 
much  coal  in  the  Cretaceous  formations  of  the  west  as  in  the  Carbonif- 
erous of  the  eastern  United  States,  though  usually  of  a  poorer  quality. 

There  is  some  disagreement  as  to  where  the  line  between  the  latest 
Cretaceous  (Laramie)  and  the  earliest  Tertiary  (Eocene)  should  be 
drawn.  A  formation  (Lance)  in  Wyoming,  containing  dinosaurian  re- 
mains and  other  typical  Mesozoic  animals  but  with  plants  that  have 
a  Tertiary  aspect,  is  separated  from  the  Laramie  below  by  an  uncon- 
formity involving,  it  is  thought,  the  removal  of  over  20,000  feet  of 
strata.  This  formation  is  placed  by  some  in  the  Tertiary,  although 
others  believe  that  it  should  be  included  in  the  Mesozoic. 

The  maximum  thickness  of  the  Upper  Cretaceous  in  the  western 
interior  is  about  24,000  feet,  making  it  one  of  the  great  periods  of  the 
earth. 

Upper  Cretaceous  of  Other  Continents.  —  Not  only  was  the  Upper 
Cretaceous  a  period  of  great  submergence  in  North  America,  but 
in  other  continents  as  well.  Large  tracts  of  Europe  were  beneath 
the  sea  at  this  time. 

Limestones  were  deposited  in  southern  Europe,  and  chalk  (p.  523), 
to  a  thickness  of  several  hundred  feet,  accumulated  in  France  and 
England;  the  latter  has,  however,  little  development  elsewhere  on 
the  continent.  In  Asia  the  land  area  was  much  smaller  than  now, 
the  Himalaya  region,  as  well  as  large  tracts  in  India  and  elsewhere, 
being  covered  with  water.  Australia  and  South  America  show  a 
similar  extension  of  the  seas.  The  summits  of  much  of  the  eastern 
Andes  of  South  America,  to  a  height  of  14,000  feet  or  more,  are 
formed  of  Upper  Cretaceous  beds. 

The  Cretaceous  Peneplain.  —  Before  the  close  of  the  era,  not  only 
North  America  but  Europe  and  Asia  appear  to  have  been  reduced 


MESOZOIC  ERA:    THE   AGE  OF   REPTILES 


519 


by  erosion  to  low,  monotonous  plains  upon  which  few,  if  any,  eleva- 
tions of  great  height  remained  (p.  114).  This  being  the  case,  the  era 
as  a  whole  must  have  been  one  of  great  quiet,  during  which  crustal 
movements  were  uncommon.  One  should  remember,  however,  that 
at  the  close  of  the  Triassic  the  faulting  and  elevation  of  the  sandstones 
and  shales  of  that  period  occurred ;  that  at  the  close  of  the  Jurassic, 
the  Sierra  Nevadas  were  raised,  but  that  before  the  end  of  the  Upper 
Cretaceous  even  these  elevations  had  for  the  most  part  disappeared. 
The  Appalachians  were  largely  worn  down  to  base  level,  and  the 
Laurentian  region  of  Canada  was  a  comparatively  flat  plain.  Under 
these  conditions  the  streams  of  that  time  flowed  in  meandering 
courses  to  the  ocean,  and  the  climate  was  probably  uniform,  warm, 
and  humid. 

Mountain-making  Movements  at  the  Close  of  the  Mesozoic.  — 
During  the  closing  stages  of  the  Upper  Cretaceous,  great  crustal 
disturbances  began  which  resulted  in  the  formation  of  mountain 
ranges  from  Alaska  to  the  southern  tip  of  South  America.  These 
movements,  following  the  long  period  of  quiet  just  described,  were 
not  sudden,  but  were  anticipated  by  upwarping  in  Colorado,  Wyo- 
ming, and  other  places,  as  the  presence  of  more  abundant  coarse  sedi- 
ments indicates.  The  great  Rocky  Mountains  of  Canada  and  the 
United  States  had  their  birth  at  this  time,  but  not  their  full  growth 
until  later.  The  structure  of  these  mountains  is  in  marked  con- 
trast to  that  of  the  Appalachians,  whose  elevation  was  the  result 
of  great  lateral  movements  which  folded  and  crowded  together  the 
strata.  Although  horizontal  compression  was  important  in  the  for- 
mation of  the  Rocky  Mountains,  the  result  of  the  vertical  move- 
ments is  much  more  conspicuous  (p.  364).  The  growth  was  also 
assisted  by  faulting.  In  Utah  the  Wasatch  and  Uinta  mountains 
and  in  British  Columbia  the  Gold  Range  were  also  raised. 

That  the  deformation  did  not  take  place  previous  to  the  Laramie 
is  proved  by  the  fact  that  the  strata  of  this  stage  and  those  of  greater 
age  are  folded  with  equal  intensity,  while  the  overlying  Tertiary 
rocks  are  less  disturbed,  showing  that  the  deformation  took  place 
before  the  deposition  of  the  latter.  In  some  areas,  however,  the 
lowest  Eocene  (Fort  Union  and  Wasatch)  show  as  steep  dips  and 
were  apparently  as  much  disturbed  as  the  underlying  Cretaceous, 
indicating  later  deformation.  These  disturbances  were  accompanied 
by  volcanic  eruptions  and  intrusions  of  lava.  The  laccoliths  forming 
the  Henry  Mountains  of  Utah  were  elevated  by  lava  which  was 


520  HISTORICAL  GEOLOGY 

forced  into  the  strata  at  this  time.  At  this  time,  too,  a  large  part 
of  the  continent  of  North  America  was  affected  by  movements  of 
greater  or  less  strength,  so  that  at  the  close  of  the  era,  dry  land 
extended  from  the  Sierra  Nevadas  on  the  west  to  the  "  Fall  Line" 
on  the  east. 

With  the  formation  of  the  Rocky  Mountains,  the  fourth  great  range 
of  the  North  American  continent  came  into  existence ;  the  Taconics 
being  formed  at  the  close  of  the  Ordovician,  the  Appalachians  at 
the  close  of  the  Paleozoic,  and  the  Sierra  Nevadas  at  the  close  of  the 
Jurassic. 

Duration  of  the  Mesozoic.  —  Many  facts  point  to  a  great  duration 
for  this  era.  (i)  The  erosion  of  an  immense  thickness  of  rocks  from 
the  Appalachian  Mountains  and  the  reduction  of  the  continent  to  a 
peneplain  was  accomplished  during  the  era  and  must  have  taken  an 
almost  inconceivable  length  of  time.  (2)  The  first  Sierra  Nevada 
Mountains,  although  formed  in  the  latter  half  of  the  era,  were  not 
only  raised  —  possibly  to  a  great  height  —  but  were  also,  later, 
worn  down  to  a  peneplain.  (3)  During  the  Upper  Cretaceous 
alone,  24,000  feet  of  sediments  —  almost  five  miles  —  were  deposited, 
being  worn  from  the  land  and  carried  little  by  little  to  the  seas  by 
the  streams.  (4)  The  evolution  in  the  animal  and  plant  life  of  the 
era  is  very  striking,  from  the  standpoint  of  both  form  and  structure. 
It  does  not  seem  possible  that,  under  the  conditions  existing  at  that 
time  as  we  understand  them,  these  changes  could  have  been  brought 
about  rapidly. 

No  matter  upon  what  basis  the  estimate  is  made;  whether  the 
time  necessary  for  the  erosion  of  thousands  of  feet  of  strata,  or  that 
required  for  the  deposition  of  great  piles  of  sediment,  the  length 
of  the  era  must  have  been  enormous.  An  estimate  of  9,000,000 
years  has  been  suggested,  but  should  be  taken  merely  as  an  approxi- 
mation. It  may  be  too  large,  or  several  millions  of  years  too  short. 


REFERENCES  FOR  PALEOGEOGRAPHY 

CHAMBERLIN  and  SALISBURY,  —  Geology,  Vol.  3,  pp.  1-38;  59-79;  106-130;   137-172. 
SCHUCHERT,  CHAS.,  —  Paleogeography  of  North  America:    Bull.  Geol.  Soc.  America, 

Vol.  20,  1910,  pp.  427-606. 
SCHUCHERT,  CHAS.,  —  Climates  of  Geologic  Time:  Carnegie  Institution  of  Washington, 

Pub.  192,  1914,  pp.  280-284. 

SCOTT,  W.  B.,  —  An  Introduction  to  Geology,  pp.  657-667;  678-683  ;  700-713. 
STANTON,  T.  W.,  —  Outlines  of  Geologic  History  (Willis  and  Salisbury),  pp.  182-199. 


MESOZOIC  ERA:    THE  AGE  OF   REPTILES  521 

LIFE  OF  THE  MESOZOIC 

In  the  early  days  of  the  study  of  geology  it  was  believed  by  many 
that  life  ceased  to  exist  at  the  close  of  the  Paleozoic  and  was  re-created 
at  the  beginning  of  the  Mesozoic.  This  belief  was  based  upon  the 
great  dissimilarity  of  the  life  of  the  two  eras  and  upon  the  apparent 
absence  of  fossils  in  the  intermediate  strata.  As  a  more  careful 
study  of  these  rocks  was  made,  and  new  exposures  were  discovered, 
fossils  were  found  which,  though  rare,  proved  that  the  life  was  in 
many  respects  transitional.  The  change  in  vegetation  between  the 
two  eras  was  not  cataclysmic,  as  was  formerly  supposed,  though  "  it 
was  rapid  or  almost  sudden."  (D.  H.  Scott.)  The  animal  life  suf- 
fered even  more  than  the  plant,  very  few  of  the  Paleozoic  genera 
surviving  in  the  following  era.  The  transition,  in  other  words,  ap- 
parently took  place  with  great  rapidity  and  affected  all  classes  of  life. 
If  a  change  in  the  character  of  the  rocks  is  made  a  basis  for  separation, 
it  is  found  that  although  great  unconformities  occur,  yet  in  many 
places  it  does  not  seem  possible  to  tell  where  the  dividing  line  should 
be  drawn.  For  example,  in  America  (Kansas  and  Wyoming)  be- 
tween horizons  yielding  Permian  fossils  and  those  yielding  Mesozoic 
there  are  "  at  least  one  thousand  feet  of  continuous,  conformable, 
uninterrupted,  and  homogeneous  deposits  of  red  sandstone  which 
may  belong  to  one  period  or  to  both,"  and  in  Europe  the  Permian 
in  many  places  merges  into  the  Mesozoic  (Triassic)  so  insensibly  that 
it  is  impossible  to  state  where  one  ends  and  the  other  begins. 

Comparison  of  the  Life  of  the  Paleozoic  and  the  Mesozoic.  - 
The  dominant  plants  and  animals  of  the  Paleozoic  disappeared,  for 
the  most  part,  with  the  Permian.  The  lepidodendrons,  sigillarias 
with  the  exception  of  a  few  stragglers,  Calamites,  Cordaites,  spheno- 
phylls,  and  a  number  of  important  genera  of  ferns  had  vanished ; 
and  their  places  were  taken  by  a  flora  of  very  different  character,  so 
that  the  forests  of  this  era  were  very  unlike  those  of  the  preceding  in 
general  appearance. 

With  the  close  of  the  Paleozoic,  the  abundant  corals  of  that  era  had 
disappeared  and  were  replaced  by  a  new  type,  differing  (p.  524)  both 
in  structure  and  appearance. 

No  cystoids  or  blastoids  survived.  The  crinoids  are,  with  the 
exception  of  two  genera,  of  a  type  quite  different  from  those  of  the 
Paleozoic.  The  race  had  reached  its  zenith  and  its  decline  had  begun, 
though  now  and  then  a  species  made  its  appearance  which  by  its 


522 


HISTORICAL  GEOLOGY 


local  abundance  seemed,  for  a  time,  to  give  promise  of  regaining  the 
former  importance  of  the  race.  Sea  urchins,  which  for  the  millions 
of  years  of  the  Paleozoic  had  remained  in  a  subordinate  position,  were 
replaced  by  forms  of  modern  structure  and  occupied,  to  some  degree, 
the  place  vacated  by  the  crinoids,  cystoids,  and  blastoids. 

The  brachiopods  of  the  Paleozoic  were,  as  far  as  external  appear- 
ance is  concerned,  of  two  classes ;  those  with  long  hinge  lines,  giving 
them  a  square-shouldered  look,  and  those  with  short  hinge  lines  and 
sloping  shoulders.  Of  these,  the  square-shouldered  and  most  charac- 
teristic type  soon  disappeared,  and  only  a  comparatively  few  genera 
of  the  sloping-shouldered  type  survived.  The  brachiopods,  as  was 
true  of  so  many  other  classes,  after  attaining  considerable  importance 
gave  way  to  other  classes  of  animals.  In  this  case,  as  they  decreased 
in  abundance,  the  pelecypods  and  gastropods  increased.  The  com- 
paratively simple-sutured  cephalopods  of  the  Paleozoic,  such  as  the 
Orthoceras  and  the  angled  goniatite  (p.  459),  were  quite  suddenly  re- 
placed by  an  abundance  of  cephalopods  with  complicated  sutures. 
The  Orthoceras,  which  lived  throughout  the  whole  of  the  Paleozoic, 
had  a  few  survivors  in  the  Triassic,  but  these  soon  became  extinct. 

The  fishes  of  the  Triassic  resemble  those  of  the  Permian  in  most 
particulars,  but  many  of  the  Permian  genera  are  wanting.  The  Age 
of  Amphibians  passed  with  the  Permian ;  and  although  the  Stego- 
cephalia  (p.  485)  lived  on  into  the  Triassic,  they  disappeared  before 
its  close ;  and  the  insignificant  frogs  and  salamanders  of  the  present 
are -the  sole  representatives  of  that  once  varied  and  conspicuous 
race. 

The  cause  of  the  revolution  in  life  at  the  close  of  the  Paleozoic,  as 
has  been  seen,  must  be  found  in  the  very  different  physical  conditions 
which  were  present  at  this  time,  since  not  one  or  two  but  many  orders 
of  animals  and  plants  either  became  extinct  or  were  profoundly 
affected.  The  formation  of  great  mountain  ranges,  the  withdrawal 
of  epicontinental  seas  in  America,  Europe,  and  elsewhere,  must 
have  produced  a  climate  markedly  different  from  that  of  the  Carbon- 
iferous, since  the  ocean  currents,  with  their  great  stores  of  heat,  would 
be  forced  to  take  courses  different  from  those  which  they  formerly 
held.  Moreover,  the  circulation  of  the  air  would  be  affected  by  the 
high  mountain  ranges.  Besides  these  more  evident  causes,  it  is 
possible  that  a  radical  change  in  climate  resulted  from  the  withdrawal 
of  carbon  dioxide  during  the  Carboniferous,  and  that  this,  combined 
with  the  above-mentioned  and  other  physical  changes,  caused  the 


/ 


MESOZGIC  ERA:    THE  AGE  OF   REPTILES  523 

extinction  of  those  species  which,  because  of  their  lack  of  variability, 
could  not  adapt  themselves  to  the  new  conditions. 

Plan  of  Study.  —  For  the  sake  of  continuity  in  the  study  of  the 
various  groups  of  animals  and  plants  described,  the  life  of  the  four 
periods  of  the  Mesozoic  will  not  be  studied  separately,  but  the  periods 
in  which  the  genera  and  species  occur  will  often  be  referred  to. 


INVERTEBRATES 

Chalk.  —  Chalk  is  composed  largely  of  the  remains  of  Foraminifera. 
Although  these  unicellular  organisms  have  been  found  in  Paleozoic 
strata,  being  abundant  in  the  Lower  Carboniferous  (Mississippian), 
it  was  not  until  the  Jurassic  that  they  attained  a  great  development. 
Although  conditions  were  very  favorable  for  their  increase  in  the 
Jurassic  of  Europe  (but  not  of  America)  and  still  more  so  in  the  Cre- 
taceous, they  were  even  more  important  as  rock  builders  in  the 
Tertiary. 

The  best  known  chalk  deposits  are  those  of  which  the  cliffs  of  Dover, 
England,  and  Dieppe,  France,  form  a  part;  and  because  of  their 
conspicuous  character,  the  name  Cretaceous  —  Age  of  Chalk  —  was 
given  to  the  period  in  which  they  occur.  In  the  United  States  also, 
Cretaceous  chalk  is  extensive.  Chalk  and  chalky  limestone  many 
hundred  feet  in  thickness  are  found  in  the  Lower  Cretaceous  series  of 
Texas;  and  another  deposit  in  the  Upper  Cretaceous  extends  from 
Texas  northward  through  the  Great  Plains  region,  in  Kansas, 
Colorado,  and  Nebraska.  However,  this  name  is  not  altogether 
appropriate,  since  by  no  mean.s  all  the  rocks  of  that  period  are  composed 
of  chalk.  It  seems  probable  that  the  chalk  of  the  Cretaceous  was  not 
deposited  in  seas  of  great  depth  as  is  true  of  the  Globigerina  (chalk) 
ooze  of  to-day,  which  in  portions  of  the  ocean  is  being  laid  down  at  a 
depth  of  12,000  feet  or  more,  but  that  the  water  was  only  moderately 
deep.  The  occurrence  in  the  chalk  of  certain  mollusks  which  do  not 
seem  to  be  of  deep-sea  species  indicates  this.  Therefore,  there  is 
little  reason  to  believe  that  the  great  chalk  beds  of  the  Cretaceous  were 
deposited  at  depths  of  thousands  of  feet,  and  that  the  ocean  bottom 
was  later  raised  to  form  dry  land.  An  explanation  for  the  purity  of 
the  chalk,  if  deposited  in  comparatively  shallow  water,  is  to  be  found 
in  the  conditions  existing  at  the  time.  .  As  a  result  of  the  low  relief 
of  the  land  with  its  thick  covering  of  vegetation,  there  was  little 
erosion,  and  the  scanty  sediments  were  laid  down  but  a  short  distance 


524 


HISTORICAL  GEOLOGY 


from  the  shore.  Consequently,  the  Foraminifera  which  throve  in 
great  abundance  near  the  shores  as  well  as  in  the  waters  of  the  deep 
seas,  as  they  do  to-day,  were  not  upon  their  death  covered  with 
clastic  sediments,  but  in  the  course  of  time  built  up  thick  deposits 
of  lime,  composed  largely  of  their  own  remains.  The  genera  and 
species  of  Foraminifera  have  generally,  as  might  be  expected  from 
their  low  organization,  a  long  range  in  time  :  some  of  the  species  which 
occur  in  the  Paleozoic  are  still  living. 

Flint  nodules,  varying  in  shape  and  ranging  in  size  from  that  of  a 
walnut  to  two  feet  in  length,  are  of  common  occurrence  in  certain 
portions  of  many  chalk  beds.  They  are  composed  largely  of  siliceous 
protozoa  (Radiolaria),  sponge  spicules,  and  silica  that  has  no  or- 
ganic form.  These  flint  nodules  were  probably  formed  by  concre- 
tionary action,  the  silica  scattered  rather  uniformly  throughout  the 
deposits  being  brought  together  to  form  masses  of  varying  size. 

Sponges.  —  Sponges  appeared  in  the  Triassic  of  Europe  in  small 
numbers  and  became  so  numerous  in  certain  localities  during  the 
Jurassic  as  to  form  thick  strata  with  their  remains.  They  were  still 
more  abundant  in  the  Cretaceous  of  Europe,  though  not  common  in 
America. 

Corals.  — The  Paleozoic  corals  (Tetracoralla)  did  not  immediately 
give  place  to  the  modern  type  (Hexacoralla,  Fig.  487  A,  B),  but  a  few 
lingered  for  a  short  time  in  the  Triassic.  Be- 
fore the  close  of  that  period,  the  new  type 
(Hexacoralla)  became  so  thoroughly  established 
as  to  build  coral  reefs  where  conditions  were 
favorable.  It  was  not,  however,  until  the 

Jurassic  that  ex- 
tensive reefs 
were  formed  by 
their  remains. 
The  general  ap- 
pearance of  the 

Cretaceous    cor- 

FIG.    487.  —  Mesozoic    corals :    A,    Thecosmilia    trichotoma    ajs  Js  not  unn'ke 
(Triassic  and  Jurassic) ;  B,  Thamnastreza  prolifera  (throughout      , 
the  Mesozoic).  that  of  the  corals 

of  to-day. 

Crinoids.  —  This  class  (Fig.  488  A,  B)  was  rare  both  near  the  be- 
ginning and  with  some  exceptions  (Uintacrinus  of  the  Niobrara 
Chalk  of  Kansas),  near  the  close  of  the  Mesozoic,  but  became  abun- 


B 


MES020IC  ERA:    THE  AGE  OF  REPTILES 


525 


dant,  though  not  diversified,  in  the  Jurassic,  at  which  time  the 
crinoids  attained  their  greatest  size  and  beauty.  The  stem  of  one 
has  been  traced  seventy  feet  without  reaching  either  end.  The 
"  head  "  in  some  individuals  (Fig.  488  A)  is  as  large  as  a  feather  duster 
and  similar  to  it  in  appearance.  The  genus  to  which  these  large 
specimens  belong  (Pentacrinus)  is  still  found  in  the  West  Indian 

seas.      In  America  the  class  appears 
to  have  been  rare  throughout  the  era. 
Although  the  structure  of  the  Meso- 
zoic     and     Tertiary     crinoids     differs 
markedly  from  that  of  the  Paleozoic, 
perhaps  the  most  conspicuous  external 
difference    lies   in  the   great   develop- 
ment and  subdivision  of  the  arms  and 
the   relatively   small   body   (calyx)   of 
the  later  type.     All 
Paleozoic       crinoids 
were  attached  to  the 
sea  bottom  by  stems, 
and    this    was    also 
true    of    the    great 
majority  of  the  Ju- 
rassic genera,  but  a 
few      free-swimming 
forms  began  then  and 


B 


FIG.  488.  —  Mesozoic  crinoids  :   A,  Pentacrinus  fossilis; 
B,  Apiocrinus  parkinsoni  (without  arms). 


have  continued  to 
the  present.  In  these 
unattached  forms, 

the  animal  begins  its  existence  fixed  to  the  bottom  by  a  stem,  as  did 
its  ancestors,  but  later  becomes  free. 

Sea  Urchins  (Echinoids). — A  new  type  of  sea  urchin  (Fig.  489 
A—C),  which  had  a  few  forerunners  in  the  later  Paleozoic,  soon  en- 
tirely replaced  the  older  type.  One  marked  difference  between  the 
two  groups  lies  in  the  number  of  rows  of  plates  forming  the  "  shell," 
which  was  variable  in  the  old,  but  in  the  new  was  constant.  Early 
in  the  era,  a  fivefold  symmetry  (Fig.  489  A)  was  the  rule,  but  later 
a  twofold  or  bilateral  symmetry  characterized  the  greater  number 
of  species.  Sea  urchins  with  club-shaped  spines  (Fig.  489  A)  were 
abundant  in  the  Jurassic  and  Cretaceous.  Inconspicuous,  rare,  and 
little  changed  throughout  the  ages  of  the  Paleozoic,  sea  urchins  had  a 


526 


HISTORICAL  GEOLOGY 


rapid  development  early  in  the  Mesozoic,  became  abundant  in  the 
Jurassic  and  Cretaceous,  assuming  the  place  formerly  held  by  the 

crinoids,  and  finally  cul- 
minated in  the  Tertiary. 
Starfish.  —  Starfish  are 
not  a  conspicuous  race 
in  the  Mesozoic. 

Brachiopods.  —  Aside 
from  the  abundance  of  a 
few  genera  at  different 
times  in  the  Triassic  and 
Jurassic,  there  is  little  of 
interest  in  this  class  in 
the  Mesozoic.  Most  of 
the  species  (Fig.  490  A- 
D)  belong  to  the  genera 
that  are  living  in  the 
seas  of  to-day  and  are 
almost  exclusively  of 
A  few 

of  the  Paleozoic  long- 
hinged  type  (Spiriferina, 
etc.)  survived  for  a  short  time,  but  were  inconspicuous.  After  their 
long  period  of  ascendancy  in  the  Paleozoic,  brachiopods  became 
unimportant  and  have  remained  in  a  subordinate  position  ever  since. 
Pelecypods. — Almost  in  proportion  as  the  brachiopods  declined 
the  pelecypods  (Fig.  491  A-J}  increased,  both  in  numbers  and 
variety.  This  rapid  development  is  shown  in  the  Triassic,  where  only 
about  one  fourth  were  of 
Paleozoic  genera.  In  the  Ju- 
rassic, the  oyster  tribe  is  con- 
spicuous and  is  represented 

not   only    by   the  true  oyster  ,          J^ 

11             ,              ,                 ,  1*10.490.  —  Mesozoic  brachiopods :  A,Teru- 

but    by    Others    that,    although  bratula  humboldtensis  (Triassic) ;    B,  Rhyncho- 

belonging    to    the    same    order,  nella  czquiplicata    (Triassic) ;    C,  Rhynchonella 

have  a   different  external  ap-  Z™thoPho™  (Jurassic) ;  A  Lingula  brevirostra 

fr^         ,             1?.  (Jurassic). 
pearance   (Gryphaea,   Fig.  491 

C,  Exogyra,  etc.)  and  are  much  more  characteristic.  In  the  Jurassic 
and  still  more  in  the  Cretaceous,  a  number  of  pelecypod  genera 
appear  which  depart  radically  from  the  typical  forms  in  which  the 


FIG.    489.  —  Mesozoic   echinoderms  :    A,    Cidaris 

coronata  (Jurassic) ;  B,  Cassidulus  subconicus  (Upper     three     families. 
Cretaceous) ;    C,  Diplopodia  texanum  (Lower  Creta- 
ceous). 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


527 


two  valves  are  alike.  In  some  of  these  (Diceras,  Fig.  491  F)  each 
valve  is  horn-shaped ;  in  others  (Requienia,  Fig.  491  /)  one  valve  is 
long  and  spirally  twisted  and  the  other  flat,  with  a  low  spiral ;  in 
others  (Radiolites,  Fig.  491  G)  one  valve  has  the  appearance  of  a  cup 
coral  and  the  other  is  flat  with  prolongations  extending  into  the 


D 


J 


FIG.  491.  —  Mesozoic  pelecypods :  A,  Halobia  (Daonella)  lommeli  (Triassic) ;  S, 
Trigonia  clavellata  (Jurassic) ;  C,  Gryph&a  arcuata  (Mesozoic) ;  D,  Inoceramus  vanux- 
emi  (Upper  Cretaceous) ;  E,  Aucella  pioche  (Lower  Cretaceous) ;  F,-  Diceras  arietinum 
(Jurassic) ;  G,  Radiolites  cornu-pastoris  (Upper  Cretaceous) ;  H,  Eumicrotis  curta 
(Jurassic) ;  7,  Camptonectes  bellistriatus  (Jurassic) ;  /,  Requienia  patagiata  (Lower 
Cretaceous).  C,  F,  G,  and  /  present  forms  very  unlike  the  typical  pelecypod. 

CLELAND    GEOL.  —  34 


528 


HISTORICAL  GEOLOGY 


horn-shaped  valve.  These  irregular,  unsymmetrical  bivalves  are 
usually  firmly  attached  by  one  valve,  their  irregular  development 
being  due,  to  some  degree,  probably  largely,  to  this  fact.  It  is 
interesting  to  note  that  these  extraordinary  forms  appear  contem- 
poraneously with  the  extravagantly  modified  cephalopods.  A  char- 
acteristic Cretaceous  genus  is  Inoceramus  (Fig.  491  £>),  also  found 
in  Jurassic  deposits. 

Gastropods.  —  The  Mesozoic  gastropods  (Fig.  492  A,  B)  were,  as 
a  whole,  less  simple  than  those  of  the  Paleozoic,  although  many  of 
the  older  type,  in  which  the  mouth  of  the  shell  is  a  complete  ring, 

lived    on.      In   one    branch,    a   tube 

^ira^^  was    developed    through    which    the 

K  ^^^~    waste  waters  of  the  body  were  car- 

•  W    ried  and  emptied  some  distance  from 

J&  the   opening    into   which    the    fresh 

Jm  mn      waters  entered,  a  structure  the  ad- 

Jl^n  vantage  of  which  is  obvious.     Forms 

fc^Sg^  ^  of  this  type  were  lacking  in  the  Paleo- 

|jHv  zoic,  but  became  common  before  the 

^wA  W  B  close  of  the  Mesozoic.     Towards  the 

close  of  the  era  many  of  the  genera 
which  reached  their  highest  develop- 
ment in  the  Tertiary  and  recent  times 
appeared. 

Cephalopods.  —  The  Paleozoic  types  of  cephalopods  (Fig.  407 
A-D)  are  represented  in  the  Triassic  strata  by  orthoceratites  and 
goniatites  and  occur  with  the  fringe-sutured  ceratites  (Fig.  493  /) 
and  the  complex-sutured  ammonites  (Fig.  493  A),  but  soon  disappear. 
Ammonites.  —  Ammonites  "  developed  with  wonderful  rapidity 
from  the  first  rare  members  [in  the  Upper  Silurian  or  Devonian] 
into  numerous  families,  hundreds  of  genera,  and  thousands  of  species, 
reaching  their  acme  in  the  Jurassic."  "  In  the  Cretaceous  they 
gradually  declined,  dropping  off  one  at  a  time,  until  all  are  gone 
before  the  end."  (J.  Perrin  Smith.)  In  numbers,  diversity  of  form,  and 
ornamentation,  ammonites  are  remarkable.  Especially  towards  the 
end  of  the  race  (in  the  Upper  Cretaceous)  unusual  forms  appeared. 
At  this  time  —  and  occasionally  in  the  Triassic  and  Jurassic —  many 
began  to  uncoil;  some  were  coiled  during  the  early  part  of  their 
life,  but  as  they  approached  old  age  became  less  coiled  (Scaphites, 
Fig.  493  L) ;  others  formed  open  coils  (Crioceras,  Fig.  494  A) ;  some 


FIG.  492.  —  Mesozoic  gastropods 

A,  Anchura  americana  (Cretaceous) 

B,  Pyropsis  bairdi  (Cretaceous). 


FIG.  493.  —  Mesozoic  cephalopods  :  A,  Tropites  subbullatus  (Triassic),  side  and  front 
views;  B,  Development  of  sutures  in  Tropites;  Cy  Lytoceras  fimbriatum  (Jurassic); 
D,  Perisphinctes  achilles  (Jurassic) ;  E,  F,  Meekoceras  gracilitatis  (Lower  Triassic), 
front  and  side  views ;  G,  Sagenites  herbichi  (Upper  Triassic) ;  H,  Turrilites  catenatus 
(Cretaceous) ;  /,  Ceratites  nodosus  (Triassic) ;  /,  Baculites,  showing  the  complexity  of 
the  sutures  (one  segment  is  shaded  to  emphasize  this) ;  K,  Cardioceras  cordatum  (Juras- 
sic), side  and  front  views;  L,  Scaphites  nodosus  (Cretaceous). 


53° 


HISTORICAL  GEOLOGY 


were  turreted  (Turrilites,  Fig.  493  H) ;  one  common  form  (Baculites, 
Fig.  493  /)  became  straight  like  the  Orthoceras  ;  others  assumed  forms 
which  seem  to  have  been  entirely  a  matter  of  accident,  as  is  shown 

especially  well  in 
a  specimen  from 
Japan  (Nipponites 
mirabilis,  Fig.  494 
C).  Many  sug- 
gestions have  been 
made  to  account 
for  the  "  death 
contortions "  of 
the  ammonites, 
but  none  is  satis- 
factory. The  one 
most  in  favor  is 
that  they  mark 
the  senility  of  the 
race. 

The  descent  of 
ammonites  from 
goniatites  (p.  459) 
is  shown  in  two 
ways  :  (i)  by  com- 
paring specimens 
from  successively 
older  formations 
and  noting  the  pro- 
gressive changes, 

FIG.  494.  —  Cretaceous  ammonites  :  A,Cnoceras;  B,  Nosto-  .  . 

ceras  stantoni  (Cretaceous).  In  this  specimen  the  death  of  and  (2)  by  Study- 
the  animal  was  probably  caused  by  its  own  growth,  as  the  ing  the  oldest  and 
edge  of  the  living  chamber  is  almost  in  contact  with  the  younffCSt  portions 
lowest  whorl  of  the  spiral;  C,  Restoration  of  Nipponites,  a  ,. 

remarkable  genus  from  Japan.  of  the  shell  ot   an 

individual       (Fig. 

493  B).  In  these  shells  every  stage  in  the  growth  of  the  individual  is 
preserved,  so  that  if  the  shell  of  a  full-grown  ammonite  is  separated 
along  its  septa  from  the  apex  to  the  living  chamber  and  its  sutures 
studied,  it  is  found  that  they  increase  in  complexity  —  the  first  suture 
or  two  made  by  the  animal  when  young  being  simple,  like  those  of  the 
Silurian  nautilus ;  then  follow  a  few  like  those  of  the  Devonian  gonia- 


MESOZOIC  ERA:    THE  AGE  OF   REPTILES 


531 


tites  (Fig.  434  A,  B) ;  the  complicated  ammonite  sutures  beginning 

when  the  whorl  is  only  two  or  three  millimeters  in  diameter.     In 

other  words,  each  individual  ammonite  recapitulates  the  history  of 

its  race.     It  is  consequently  possible  by  studying 

a    well-preserved    individual    to    tell    what    its 

genealogical  tree  was.     In  no  other  animal  can 

the  evolution  of  the  race  be  so  well  studied.     It 

should    be    borne    in    mind,    however,    that   the 

record  of  some  of  the  stages  of  development  is 

often  omitted  by  "  acceleration." 

Since  many  of  the  species  had  a  very  short 
life,  they  are  especially  important  in  showing 
that  widely  separated  strata  are  of  the  same  age. 
It  should  be  remembered,  in  this  connection,  that 
ammonites  were  free-swimming  or  crawling  ani- 
mals, and  also  that  upon  their  death  the  gases  of 
purification  caused  them  to  float.  They  were 
consequently  moved,  either  by  their  own  volition 
or  by  the  ocean  currents  after  their  death,  over 
wide  areas,  and  hence  are  excellent  "  index 
fossils."  The  ammonites  contributed  largely  to 
the  Jurassic  limestones,  but  of  all  this  great 
horde  of  shelled  cephalopods,  the  simple-sutured 
nautilus  alone  survived  the  Mesozoic. 

Naked  Cephalopods  (Belemnites). —  In  Jurassic 
deposits  straight,  cigar-shaped  fossils  (Fig.  495) 
are  sometimes  found  in  great  abundance.  These 
belemnites  are  usually  3  to  5  inches  in  length, 
although  some  specimens  several  feet  long  have  FIG.  495.  —  Res- 
been  found.  Ink  bags  are  associated  in  some  toration  of  a  Belem- 
,  .  ,  .  .  111  mte.  The  solid, 

specimens,  showing  that  their  possessors  darkened    cigar-shaped  "guard" 

the  water  to  escape   their   enemies,    as    do   the 
squids  of  the   present.     These   are  the   internal 


(lower  end)  is  a  com- 
mon fossil  in  the  Juras- 

r  ...  ,     ,     .         sic     and    Cretaceous, 

shells  or  naked  cephalopods  which  resembled  the    Next    above    the 

squids  of  to-day  in  general  appearance.  This 
class  is  first  known  from  the  Triassic,  but  once 
started,  the  race  rapidly  increased,  culminating  in 
the  Jurassic  and  declining  rapidly  in  the  Cretaceous.  Only  one  surviv- 
ing genus  (Spirula)  is  living  to-day.  The  solid  internal  shells  of  the 
belemnites  constitute  a  considerable  part  of  some  Jurassic  limestones. 


"guard"  is  the  phrag- 
mocone,  and  above 
this  the  proostracum. 


532 


HISTORICAL  GEOLOGY 


Crustaceans.  —  Crustaceans  (Fig.  496  A,  B)  of  a  very  modern 
appearance  took  the  place  of  the  trilobites  and  eurypterids.  In 
America  few  fossils  of  this  group  have  been  found,  but  in  the  Jurassic 
lithographic  limestone  of  Bavaria  many  beautifully  preserved  speci- 
mens have  been  col- 
lected. It  is  possible 
that  this  class  was  as 
abundant  in  America 
as  in  Europe,  but  if 
so,  the  conditions  for 
the  preservation  of 
the  remains  were  not  favorable. 
Ancestral  long-tailed  crustaceans 
of  the  lobster  and  shrimp  type 
(Macrura)  began  in  small  num- 
bers in  the  Triassic,  and  a  few 
survivors  of  these  ancient  forms 
are  living  in  the  deep  seas  of  the 
present.  Crabs  are,  in  general, 
crustaceans  of  the  lobster  type 
in  which  the  tail  is  abbreviated 
and  turned  under  the  body  and 
the  shell  widened  and  otherwise 
modified  (Brachyura).  Crabs 
did  not  appear  until  the  Jurassic 
and  were  derived  from  the  long- 
tailed  series,  as  numerous  speci- 
mens intermediate  between  the 
two  show. 

Insects.  —  Insects  are  better 
known  from  the  Jurassic  than 
from  any  other  portion  of  the 
era,  probably,  as  in  the  case  of 


FIG.  496.  —  Mesozoic  crustaceans  :  A, 
Penceus  meyeri  (Jurassic) ;  S,  Eryon  pro- 
pinquus  (Jurassic). 


the  crustaceans,  because  the  conditions  favorable  for  the  preservation 
of  their  remains  were  better  than  at  other  times.  True  cockroaches 
and  beetles  are  known  from  the  Triassic ;  and  practically  all  of  the 
groups  of  to-day  were  present  in  the  Jurassic,  with  the  exception 
of  those  depending  upon  flowering  plants  for  their  food.  Crickets, 
locusts,  and  cockroaches  (Orthoptera),  May  flies,  dragon  flies,  and 
caddis  flies  (Neuroptera)  occur.  Wood  beetles  are  found  associated 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES  533 

with  driftwood  in  the  Jurassic;  and  flies  (Diptera),  plant  lice,  and 
aquatic  bugs  (Hemiptera)  are  known.  The  absence  of  insects  depend- 
ing upon  the  pollen  and  nectar  of  flowers  is  probably  indirect  evidence 
that  flowering  plants  were  not  yet  in  existence  in  the  Jurassic. 

REFERENCES  FOR  INVERTEBRATES 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3,  pp.  48-58 ;  80-94  5  134-136 ;  186-190. 
GRABAU  AND  SHIMER,  —  North  American  Index  Fossils. 

SCOTT,  W.  B.,  —  An  Introduction  to  Geology,  pp.  670-673;  684-690;   716-718. 
ZITTEL-£ASTMAN, —  Textbook  of  Paleontology,  2d  ed.  (for  description  and  bibliography). 

FISHES  AND  AMPHIBIANS 

At  the  beginning  of  the  Mesozoic,  less  modification  in  structure 
is  noticeable  in  this  class  than  in  others  to  be  considered,  but  the 
changes  were  by  no  means  inconsiderable.  • 

The  shark  tribe  (Fig.  497  A)  has  had  a  long  and  varied  history. 
It  began  in  the  Silurian  and  abounds  still  in  the  warm  seas  of  the 
present.  These  fish  were  abundant,  both  in  species  and  individuals, 
in  the  Lower  Carboniferous  (Mississippian),  but  during  the  Permian 
declined  rapidly,  almost  to  extinction.  In  the  early  Mesozoic, 
however,  they  once  more  began  to  increase  and  were  common  before 
its  close.  The  cobblestone-pavement  toothed  shark  lived  on  and  is 
represented  to-day  by  one  genus,  the  Port  Jackson  shark  (Cestracion). 
A  possible  explanation  of  this  curious  fluctuation  is  as  follows  : 
in  the  Carboniferous,  being  the  most  powerful  animals  and  having  no 
enemies,  they  multiplied  until  their  increase  was  checked  by  their 
very  numbers.  Then  the  overspecialized  forms  and  those  that 
failed  to  respond  to  changed  conditions  dropped  out,  leaving  the 
best  to  survive.  These,  then,  gradually  increased  to  the  Middle 
Tertiary  when,  through  a  change  in  climate,  they  became  again 
comparatively  rare.  (Lucas.) 

The  skates  and  rays  (Fig.  498)  are  sharks  in  which  the  body  has 
been  admirably  adapted  to  bottom  living  and  probably  should  be 
regarded  as  "  the  culminating  forms  of  the  specializing,  bottom- 
living  sharks  of  the  Mesozoic."  (Dean.)  The  skates  of  the  Meso- 
zoic and  Tertiary,  without  doubt,  mimicked  the  color  of  the  ocean 
bottom  and  glided  along  inconspicuously,  just  as  their  living  descend- 
ants do.  The  teeth  of  all  skates  are  simple,  crushing,  pavement 
teeth,  suited  for  crushing  the  shellfish  and  crustaceans  upon  which 
they  live.  They  are  first  known  in  the  Jurassic  and  are  abundant  in 
the  seas  of  to-day. 


534 


HISTORICAL  GEOLOGY 


D 

FIG.  497.  —  A,  jaw  and  teeth  of  the  shark,  Synechodus  (Cretaceous) ;  B,  ganoid 
fish,  Dapedius  (Triassic) ;  C,  ganoid  fish,  Aspidorhynchus  (Jurassic) ;  D,  teleost  fish, 
Portheus  (Cretaceous). 


MESOZOIC  ERA:    THE  AGE  OF   REPTILES 


535 


The  lungfish  almost  disappeared  from  the  seas  of  the  Mesozoic,  but 
a  few  (the  best  known  of  which  is  Ceratodus)  have  succeeded  in  living 
on  in  small  numbers  to  the  present. 

Ganoids  (Fig.  497  B,  C)  were  the  common  fish  of  the  Triassic  and 
Jurassic.  Although  they  had  no  bony  skeleton,  they  are  well  pre- 
served because  of  their  thick, 
enameled  scales  which  were  excep- 
tionally well  suited  for  fossiliza- 
tion.  The  ganoids  are,  as  a  rule, 
rather  small  fish  and  never  attained 
the  size  of  sharks.  As  the  modern 
fishes  with  bony  skeletons  (teleosts) 
increased  during  the  Cretaceous 
and  Tertiary,  the  ganoids  gradu- 
ally disappeared  until,  at  present, 
only  a  few  species  are  in  existence. 
Two  of  the  commonest  living 
ganoids  are  the  gar  pike  and  the 
sturgeon,  both  of  which  are  ex- 
tremely plentiful  in  some  localities. 

The  bony  fishes  (teleosts),  de- 
scendants of  the  ganoids,  have 
been  found  in  small  numbers  in 
the  Lower  Jurassic,  but  probably 
began  in  the  Triassic.  They  held 
a  subordinate  place,  however,  until 
the  Cretaceous,  when  they  ap- 
peared in  great  numbers.  Among 
the  teleosts  of  the  Cretaceous  were 
herring,  cod,  salmon,  mullet,  perch, 
and  catfish.  One  characteristic 
Cretaceous  type,  Portheus  (Fig. 
497  D),  should  be  mentioned.  It 
was  a  teleost  that  occasionally  at- 
tained a  length  of  fifteen  feet  and 
was  provided  with  large,  flattened, 
irregular  teeth.  The  suddenness  of  the  appearance  of  teleosts  was 
due  to  the  fact  that,  once  they  were  established  and  able  to  compete 
with  the  fish  of  that  time,  there  was  no  hindrance  to  their  migration, 
and,  in  a  comparatively  few  years,  geologically,  they  had  spread  into 


FIG.  498.  —  An  ancestral  skate, 
Rhinobatus  (Jurassic). 


536  HISTORICAL  GEOLOGY 

the  seas  of  the  whole  world.  Most  of  the  ganoids  became  extinct, 
either  because  of  their  inability  to  compete  with  the  teleosts  in  the 
search  for  food,  or  because  of  climatic  and  other  conditions. 

REFERENCES   FOR   FISHES 

DEAN,  B.,  —  Fishes,  Living  and  Fossil. 

EASTMAN,  C.  R.,  —  Ann.  Rept.  New  Jersey  Geol.  Surv.  for  1904. 
WOODWARD,  A.  S.,  —  Vertebrate  P  alee  ontology. 
ZITTEL-EASTMAN,  —  Textbook  of  Paleontology. 

Amphibians.  —  The  amphibians  reached  their  greatest  develop- 
ment in  the  Permian,  but  were  present  in  considerable  numbers  in 
the  Triassic,  after  which  their  remains  are  seldom  found.  Individ- 
uals of  this  class  attained  their  greatest  size  in  the  Triassic,  Masto- 
donsaurus  (so-called  because  of  its  bulk)  having  a  skull  four  feet  long 
and  probably  attaining  a  length  of  15  or  20  feet.  Although  large 
for  an  amphibian,  the  size  is  not  great  as  compared  with  some  modern 
crocodiles.  In  general  appearance,  Mastodonsaurus  resembled  the 
modern  salamander,  but  it  differed  in  several  essential  points  of  struc- 
ture. Its  teeth  were  of  the  complicated  labyrinthine  type  (p.  485), 
and  the  skull  was  roofed  over  with  bony  plates  (Stegocephalia). 
It  is  possible  that  bony  plates  protected  the  chest,  but  if  so,  positive 
proof  is  lacking.  The  Stegocephalia  became  extinct  before  the  close 
of  the  Triassic,  and  thus  far  with  the  exception  of  two  specimens  of 
frogs  from  Wyoming  no  amphibian  remains  have  been  found  in 
Jurassic  rocks.  The  cause  of  the  extinction  of  this  great  amphibian 
order  is  probably  to  be  found  in  the  highly  developed  reptiles,  large 
and  small,  with  which  they  had  to  compete.  A  few  specimens  of 
salamanders  of  modern  type,  differing  little  in  general  appearance  from 
the  salamander  of  to-day,  though  of  a  different  genus,  have  been  found 
in  the  Cretaceous.  Because  of  the  lack  of  fossil  evidence,  the  an- 
cestry of  the  modern  amphibians  is  not  known. 

REPTILES 

Reptiles  with  Mammalian  Characters.  —  The  reptiles  with  mam- 
malian characters,  as  far  as  fossil  evidence  shows,  began,  and  were 
represented  by  many  genera  in  the  Permian  (p.  490),  but,  since 
they  apparently  attained  their  greatest  development  in  the  Triassic, 
their  discussion  has  been  postponed  to  this  chapter.  This  group 
of  reptiles  is  included  under  the  term  Theromorpha  (Greek,  ther, 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


537 


beast,  and  morphe,  form),  because  of  the  strong  resemblance,  both 
in  teeth  and  skeleton,  to  mammals.  They  are  remarkable  in  pos- 
sessing not  only  mammalian  characters,  but  amphibian  as  well 


FIG.  499.  —  Skeleton  of  a  mammal-like  (theromorph)  reptile,  Endothiodon. 
(Courtesy,  American  Museum  of  Natural  History,  City  of  New  York.) 

and  occupy  a  position  intermediate  between  mammals  and  amphib- 
ians. It  seems  probable  that  the  Theromorpha  include  the  pro- 
genitors of  the  mammals.  (Broom,  1911.)  One  amphibian  character 
is  seen  in  the  backbone,  the  bodies  of  the  vertebrae  of  which  are 
hollow  at  both  ends  (amphicoelous)  and,  in  some  cases,  are  only  partly 
connected  with  bone.  The  teeth 
of  certain  genera  (Cynognathus) 
are  of  three  kinds  as  in  mammals : 
incisors,  canines,  and  molars.  In 
some  cases  the  limbs  are  decidedly 
mammal-like  in  structure.  An- 
other group  of  theromorphs  have 
toothless  jaws  (Figs.  499,  500), 
covered  with  horn  like  a  turtle's 
(Oudenodon),  and  some  possess,  in 
addition,  two  long  canine  teeth 

(Dicynodon).     It  is  possible  that  the   former  (Oudenodon)  is  the 
female  of  the  latter. 

The  theromorphs  were  all  land  animals,  with  limbs  for  the  sup- 
port of  the  body,  but  they  varied  greatly  in  appearance  and  habit. 


FIG.  500.  —  Skull  of  a  theromorph 
with  beak-like  jaw  (Oudenodon).  The 
animal  was  herbivorous.  The  skull  is 
one  and  a  half  feet  long. 


538 


HISTORICAL  GEOLOGY 


Some  (Pareiasaurus,  Fig.  501  A,  B)  were  as  large  as  rather  small  cattle, 
about  nine  feet  in  length  and  standing  about  three  and  a  half  feet 
high,  but  with  short  legs  and  small,  peg-like  teeth,  showing  that  they 
were  herbivorous.  They  are  believed  to  have  been  tortoise-like  in 
habits,  and  probably  protected  themselves  by  digging  in  the  ground. 
Associated  with  these  in  the  same  beds  are  carnivorous  thero- 
morphs  (Fig.  502),  some  with  skulls  two  feet  in  length,  with  long, 


FIG.  501.  —  A,  skeleton,  and  B,  restoration  of  the  herbivorous  mammal-like  (thero- 
morph)  Pareiasaurus.  The  length  is  about  eight  feet.  The  surface  ornamentation 
of  the  restoration  is  entirely  fanciful.  (After  Amalitzky.) 

tiger-like  teeth.  Attention  has  already  been  called  to  the  fact  that 
as  soon  as  herbivorous  animals  appear  in  any  age,  carnivores,  often 
closely  related  to  the  herbivores,  also  occur  and  prey  upon  their 
less  agile  neighbors.  So  among  the  theromorphs  we  find  some  of 
massive  build  being  destroyed  and  devoured  by  their  swifter,  carniv- 
orous relatives. 

The  theromorphs  diverged  with  such  rapidity  in  the  Permian 
that,  by  its  close,  various  groups  appeared,  differing  slightly  from 
one  another,  as  has  been  seen.  They  survived  the  severe  changes 


MESOZOIC  ERA:    THE  AGE  OF   REPTILES 


539 


FlG-  S02-  —  Skull  of  a  large,  carnivorous,  mammal- 


which  brought  the  Paleo- 

zoic to  a  close  and  spread 

over  three  continents,  but 

became  extinct  before  the 

beginning  of  the  Jurassic. 

It  has  been  suggested  that 

their    rapid    development 

and    great  variation   may 

have  been  due  to  a  more 

oxygenated  atmosphere  re- 

suiting     from     the    with- 

drawal  of  the  carbon  di- 

oxide which  was  abstracted  from  the  atmosphere  to  form  coal  in  the 

Carboniferous.     It  is  possible  that  their  extinction  was  due  to  com- 

petition with  the  better  organized  reptiles  of  the  Triassic. 

REFERENCES   FOR   REPTILES   WITH   MAMMALIAN   CHARACTERS 

BROOM,  R.,  —  South  African  Fossil  Reptiles:  Am.  Museum.  Jour.,  Vol.  13,  1913,  pp. 

335-346,  and  Vol.  14,  1914,  pp.  139-143. 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  105-117. 
LANKESTER,  E.  R.,  —  Extinct  Animals,  pp.  209-222. 

DINOSAURS 

The  preeminent  land  animals  of  the  Mesozoic  were  the  dinosaurs 
(Greek,  deinos,  terrible,  and  saurus,  reptile),  which  occupied  the 
place  in  nature  now  held  by  the  land  mammals.  Some  were  larger 
than  the  largest  animals  of  the  present  day,  with  the  exception  of  a 
few  of  the  whales,  while  others  were  as  small  as  a  common  fowl  ;  some 
walked  in  a  more  or  less  erect  position,  while  others  moved  about  on  all 
fours  ;  some  had  limbs  as  light  as  birds,  while  the  limb  bones  of  others 
were  the  largest  and  heaviest  known  ;  some  were  covered  with  a  bony 
armor,  and  others  were  without  such  protection;  some  were  very 
agile,  others  were  slow-moving  ;  some  were  carnivorous,  others  her- 
bivorous ;  all  were  alike  in  having  very  small  brains.  All  the  conti- 
nents of  the  world,  including  Australia,  were  occupied  by  them. 

The  dinosaurs  may  be  separated  into  four  groups:  (i)  carnivores 
(Theropoda),  (2)  unarmored  quadrupeds  (Sauropoda),  (3)  unar- 
mored  bipeds  (unarmored  Predentata),  and  (4)  armored  dinosaurs 
(armored  Predentata).  Of  these,  the  first  only  was  carnivorous, 
the  others  being  herbivorous. 


540 


HISTORICAL  GEOLOGY 


Carnivorous  Dinosaurs.  —  The  most  striking  features  of  the  car- 
nivorous dinosaurs  were  the  bipedal  habit  and  the  disparity  in  size 
between  the  fore  and 
hind  limbs,  a  char- 
acter which  increased 
as  the  race  became 
older,  until  in  later 
forms  (Tyranno- 
saurus, Fig.  503)  the 
arms  are  so  absurdly 
small  that  it  is  diffi- 
cult to  conjecture 
their  use.  As  the 
fore  limbs  decreased 
in  size  and  gradually 
relinquished  their 
function,  although 
never  entirely  aban- 
doning it,  the  hind 
legs,  in  addition  to 
their  duty  of  support- 
ing the  weight  of  the  body,  had  to  assume  a  grasping  function  as 
well,  and  the  claws  became,  in  consequence,  great  talons,  differing 
thus  markedly  from  those  of  an  earlier  type  (Anchisaurus).  This 
grasping  function,  however,  was  perhaps  trans- 
ferred to  the  teeth  quite  as  much  as  to  the  hind 
limbs.  The  earlier  forms  probably  walked  on 
all  fours,  but  as  the  fore  limbs  became  smaller, 
they  stalked  about  on  their  hind  legs,  or  pos- 
sibly leaped  about  in  kangaroo  fashion  with  the 
forward  part  of  the  body  lifted  from  the  ground 
and  balanced  by  the  powerfully  developed  tail. 
A  second  group  (Compsognathus,  Fig.  504) 

differed  from  the  above 
in     being     lighter     in 
FIG.  504.  —  A  bird-like  carnivorous  dinosaur,  Comp-      build,    with     the     fore 
sognathus,  about  two  feet  long.     (After  Abel.)  limb       developed       for 

grasping  its  prey. 

1  he  skull  is  very  light  and  bird-like  in  some  genera  (Anchisaurus) ; 
and,  although   quite  large   in  others  (Tyrannosaurus),  it  is  always 


FIG.  503.  —  Skull  of  Tyrannosaurus,  a  gigantic  car- 
nivorous dinosaur.  The  skull  is  four  and  a  half  feet 
long  and  the  animal  was  sixteen  feet  high  when  standing. 
(After  Prof.  H.  F.  Osborn.) 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES  541 

relatively  delicate.  The  skeleton  is  very  light,  as  would  be  expected 
of  animals  of  their  habits,  and  the  limb  bones  are  hollow.  An 
improvement  in  the  teeth  is  noticeable  from  period  to  period ;  those 
of  the  earliest  (Anchisaurus),  although  plainly  for  eating  flesh,  are 
not  the  perfect  instruments  possessed  by  those  of  later  date  (Allo- 
saurus,  Fig.  505),  which  are  long  and  somewhat  flattened,  with  ser- 
rated edges.  It  is  not  known  that  the  carnivorous  dinosaurs  were 
especially  ornamented ;  one  genus  (Ceratosaurus),  however,  pos- 
sessed a  horn  on  the  nose,  and  a  row  of  small  bones  embedded  in  the 


FIG.  505.  —  Restoration  of  Allosaurus,  a  carnivorous  dinosaur.  The  small  size 
of  the  fore  limbs  as  compared  with  the  hind  is  striking.  (Restoration  by  C.  R.  Knight, 
under  the  direction  of  Professor  Osborn.  Copyright,  American  Museum  of  Natural 
History.) 

skin  down  the  middle  of  the  back,  but  aside  from  this,  ornamentation 
was  rare.  They  varied  greatly  in  size,  from  animals  as  small  as  a  cat 
to  the  largest  carnivorous  land  animals  that  ever  lived.  Tyran- 
nosaurus  was  40  feet  long,  with  teeth  projecting  from  two  to  six  inches 
from  the  jaw.  It  is  possible  that  this  last  was  developed  to  prey 
upon  the  great  armored  dinosaurs  (p.  545)  which  attained  their 
greatest  size  and  most  perfect  protection  in  the  Upper  Cretaceous, 
shortly  before  the  extinction  of  the  race. 

The  carnivorous  dinosaurs  are  the  earliest  known,  beginning  in 
the  Triassic  and  living  throughout  the  whole  of  the  Mesozoic. 

Unarmored  Quadrupedal  Dinosaurs  (Sauropoda). — These  were 
the  largest  animals  of  the  time.  A  study  of  the  skeleton  and  restora- 


54* 


HISTORICAL  GEOLOGY 


tion  of  Brontosaurus  (Fig.  506)  or  an  allied  form  gives  a  truer 
conception  of  the  animal  than  any  written  description.  The  long 
neck  with  its  absurdly  small  head,  the  large  body,  stout  limbs,  and 
long  tail  make  an  animal  differing  from  any  now  living.  Certain 
characters  of  the  skeleton  are  unusual.  The  leg  bones,  ribs,  and  tail 
bones  are  solid  and  heavy ;  the  head  and  the  vertebrae  of  the  neck  and 
back,  on  the  contrary,  being  constructed  so  as  to  combine  minimum 


FIG.  506.  —  Skeleton  and  restoration  of  Brontosaurus.  These  herbivorous  dinosaurs 
grew  to  be  sixty  feet  long.  (Model  by  C.  R.  Knight  under  the  direction  of  Professor 
Osborn.  Copyright,  American  Museum  of  Natural  History.) 

weight  with  the  large  surface  necessary  for  the  attachment  of  the 
huge  muscles.  The  significance  of  the  remarkably  heavy  bones  of 
the  lower  portion  of  the  skeleton,  combined  with  the  unusual  lightness 
in  the  upper  portion,  is  that  the  animals  lived  in  the  water  a  large  part 
of  the  time.  Under  such  conditions,  the  greater  the  weight  of  the 
bones,  the  greater  would  be  the  ease  of  walking  with  the  body  partly 
submerged  in  water.  The  lightness  of  the  head  and  the  vertebrae  of 
the  neck  would  be  of  advantage  in  making  rapid  movement  of  these 
members  possible. 

The  teeth  are  long  and  either  cylindrical  or  somewhat  spoon-shaped 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES  543 

and  are  set  rather  far  apart,  a  shape  and  arrangement  fitting  them 
for  biting,  but  not  for  mastication.  The  brain  is  smaller  than  the 
spinal  cord. 

The  reptiles  of  this  family  grew  to  be  as  much  as  80  feet  long  and 
stood  1 6  or  more  feet  high,  and  some  are  believed  to  have  weighed 
35  to  40  tons.  They  lived  on  flat  plains,  such  as  those  at  the  mouth  of 
the  Amazon  to-day,  occupied  by  interlacing  streams  and  small  lakes 
in  abandoned  river  channels ;  in  a  warm  climate  with  luxuriant  vege- 
tation. That  the  water  was  fresh  is  shown  by  the  fossil  remains  with 
which  they  are  associated,  such  as  fresh-water  plants,  and  shells,  fish, 
crocodiles,  turtles,  and  other  dinosaurs.  They  went  on  land  occa- 
sionally but  not  habitually,  since  the  great  weight  of  the  solid  bones 
would  impede  their  movements,  thus  rendering  them  less  able  to 
escape  their  enemies.  In  the  water  they  could  swim  with  ease, 
propelled  by  their  long  tails.  Their  food  was  either  floating  plants, 
or  such  as  were  loosely  attached  to  the  bottom  or  banks ;  but  they 
probably  sometimes  cropped  foliage  growing  20  feet  above  the  water, 
which  their  long  necks  enabled  them  to  reach.  The  character  of 
the  teeth  precludes  the  possibility  of  hard,  tough  vegetation,  since 
these  are  weak  and  not  adapted  to  grinding.  The  lack  of  grinding 
teeth  made  it  necessary  for  them  to  bolt  their  food,  and  it  is  interesting 
to  note  the  occurrence  of  polished  flint  pebbles  associated  with  the 
remains,  which  may  have  been  "  stomach  stones  "  or  "  gastroliths," 
used  in  grinding  the  food  after  it  had  been  swallowed. 

These  huge,  four-footed  creatures  were  probably  descended  from 
the  carnivorous  dinosaurs,  either  before  or  after  the  latter  acquired 
the  bipedal  habit.  When  the  carnivorous  race  became  widespread 
and  competition  more  severe,  certain  of  them  probably  had  a  mixed 
diet  at  first,  which  in  time  became  entirely  herbivorous.  After 
this  change  was  established,  the  increase  in  size  was  largely  a  matter 
of  abundance  of  food  and  lack  of  enemies.  Although  the  body  in- 
creased in  bulk  and  changed  in  structure,  the  teeth  failed  to  be  modi- 
fied to  a  great  degree,  but  retained  many  of  their  ancestral  characters 
to  the  end  of  the  race.  The  unarmored  quadrupeds  first  appeared 
either  at  the  close  of  the  Triassic  or  at  the  beginning  of  the  Jurassic, 
and  survived  into  the  Lower  Cretaceous.  Their  extinction  may 
have  been  caused  by  a  change  in  climate;  by  starvation  as  the 
result  of  the  disappearance  of  the  water  plants  upon  which  they 
fed ;  by  the  arrival  or  development  of  powerful  enemies ;  or  in 
other  ways. 

CLELAND   GEOL. — 3$ 


544 


HISTORICAL  GEOLOGY 


Unarmored  Bipedal  Herbivorous  Dinosaurs  (Unarmored  Pre- 
dentata).  — The  dinosaurs  of  this  group  were  similar  in  general  ap- 
pearance to  the  carnivores,  but  differed  in  their  less  graceful  build. 
Some  of  them  attained  a  large  size,  being  as  much  as  30  feet  in  length 
and  standing  15  feet  high  (Iguanodon  and  Trachodon).  The  hind 
legs  of  some  were  twice  as  long  as  the  fore.  The  heads  varied  con- 
siderably in  different 
genera,  being  long  and 
rather  slender  in  most, 
but  flat  and  ducklike  in 
one  specialized  form,  the 
duck-billed  dinosaur 
(Trachodon,  Figs.  507, 
508) .  They  were  all  alike 
in  having  the  front  of  the 
jaw  toothless  and  covered 
with  horn.  The  rear 
portion  of  the  jaws,  how- 
ever, was  in  one  genus 
(Trachodon)  provided 
with  a  battery  of  chop- 
ping and  shearing  teeth, 
composed  of  45  to  60 
vertical  and  10  to  14 
horizontal  rows  (Fig. 
509),  though  the  rows 
were  not  all  in  use  at  the 
same  time,  the  total  num- 
ber of  teeth  in  some  indi- 
viduals being  more  than 
2000.  Others  (Iguano- 
don and  Camptosaurus) 

had  only  one  row  of  shearing  teeth  in  use  at  one  time.  The  teeth 
were  replaced  as  rapidly  as  they  were  worn  out.  Teeth  of  this 
sort  indicate  that  their  possessors  chopped  or  sheared  their  food 
and  were  able  to  live  on  tough,  hard  vegetation,  such  as  the 
cycads  and,  perhaps,  even  the  siliceous  horsetails  of  the  period. 

They  had  three  toes  on  the  hind  feet,  terminating  in  hoofs  (Tra- 
chodon), or  claws  (Camptosaurus).  The  fore  limbs  had  three  well- 
developed  fingers,  with  one,  or  sometimes  two  other  rudimentary 


FIG.  507.  —  Skeleton  of  the  herbivorous,  duck- 
billed dinosaur,  Trachodon.  (After  American 
Museum  of  Natural  History.) 


MESOZOIC  ERA:    THE  AGE  OF   REPTILES 


545 


ones. 


In  a  mummified  specimen  (Trachodon)  found  in  Wyoming, 
the  epidermis,  which  is  covered  with  flat,  bony  scales,  is  seen  to  be 
extremely  thin  and  the  markings  exceedingly  fine  and  delicate  for  an 


FIG.  508.  —  Restoration  of  the  herbivorous,  duck-billed  dinosaur,  Trachodon. 
(Restoration  under  the  direction  of  Professor  Osborn.  Copyright,  American  Museum 
of  Natural  History.) 

animal  of  such  dimensions  (Fig.  510).  The  same  specimen  shows 
that  the  fore  feet  were  webbed,  the  skin  reaching  beyond  the  fingers 
and  forming  a  sort  of  paddle.  Since  these  animals  had  strong, 
powerful  hind  legs  and  were  without  armor,  it  is  evident  that  their 
existence  depended  upon  their  ability  to  escape  their  carnivorous 
enemies  by  speed. 
Some  (Camptosaurus 
and  Iguanodon)  ap- 
parently lived  on 
the  dry  land,  while 
others  (Trachodon)  FIG.  509.  —  Portion  of  the  lower  jaw  of  Trachodon. 
were  amphibious.  ™e  numerous  teeth  form  a  kind  of  Pavement.  (After 

11        Gilmore.) 
The  latter  were  able, 

when  on  land,  to  run  rapidly,  and  when  in  the  water  to  swim,  per- 
haps, with  the  speed  of  the  crocodile,  as  is  indicated  by  the  great, 
flattened,  crocodile-like  tail. 

Armored  Dinosaurs  (Armored  Predentata). — The  reptiles  of 
this  group  were  of  two  very  different  types.  A  representative  of 


546 


HISTORICAL  GEOLOGY 


one  family  is  Stegosaurus  (Greek,  stegos,  roofed,  and  saurus,  reptile), 
an  animal  of  greater  bulk  than  an  elephant.  The  restoration  (Figs. 
511,  512)  shows  two  rows  of  broad  plates  on  either  side  of  the  back- 
bone, varying  from  a  few  inches  to  two  feet  in  height  and  less  than  an 
inch  in  thickness,  except  where  they  were  embedded  in  the  skin,  and 
with  spines  near  the  end  of  the  tail  six  inches  to  over  three  feet  in 
length.  The  stout  fore  limbs  are  much  smaller  than  the  hind,  but  a 

study  of  the  joints  shows  that  the 
creatures  were  quadrupeds.  The 
front  of  the  jaw  was  toothless  and 
covered  with  horn  as  in  the  preced- 
ing group.  The  teeth  in  the  back 
of  the  mouth  were  weak  shearing 
teeth,  not  strong  enough  to  masti- 
cate the  coarser  vegetation  of  the 
time ;  and  they  must,  therefore, 
have  fed,  for  the  most  part,  on 
succulent  plants.  It  is  possible 
that  they  lived  on  the  land  bor- 
dering marshes.  One  of  the  most 
remarkable  features  of  this  unique 
reptile  is  to  be  seen  in  its  nervous 
system.  The  brain  is  estimated 
to  have  weighed  only  about  two 
and  a  half  ounces  (about  one- 
fiftieth  that  of  an  elephant  of 
smaller  size),  while  the  enlarge- 
ment of  the  spinal  cord  above  the 
hips  is  twenty  times  larger  than 
the  brain.  This  "  hind  brain  " 

was  probably  the  nervous  center  for  the  great  muscles  of  the  tail. 
It  is  likely  that  Stegosaurus  did  not  face  its  enemy,  but  protected 
itself  by  swinging  its  long,  powerful  tail,  which,  however,  was  not 
very  flexible.  The  long  hind  limbs  suggest  the  possibility  of  con- 
siderable speed,  and  the  fore  limbs  are  so  constructed  as  to  make  it 
possible  for  the  animal  to  pivot  the  body  rapidly  so  as  to  keep  the 
tail  to  the  enemy.  These  reptiles  were  descended  from  unarmored, 
herbivorous  dinosaurs  with  a  bipedal  habit. 

Two  causes  of  the  extinction  of  this  family  are  suggested  :  the  change 
in  the  vegetation  to  modern  plants  (p.  568),  and  the  senility  of  the  race, 


FIG.  510.  —  The  skin  of  Trachodon. 
The  illustration  is  from  a  photograph 
of  the  impression  made  by  the  skin  on 
the  sediments  in  which  it  was  buried. 
(After  Professor  Osborn.) 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


547 


indicated  by  the  spinose  character.  It  is  certainly  true  that  an 
animal  of  such  bulk,  so  ornamented,  would  not  be  likely  to  vary  to 
such  an  extent  as  to  meet  radically  new  conditions.  Stegosaurus 
and  its  armored  ancestors  have  been  found  only  in  the  Jurassic. 

Another  family  of  armored  dinosaurs,  differing  widely  from  Stego- 
saurus, is  represented  by  Triceratops  (Greek,  tri-,  three,  and  ceras,  horn) 


FIG.  511.  —  Skeleton  of  Stegosaurus,  an  armored  herbivorous  dinosaur. 
(After  R.  S.  Lull.) 

-  5  J3)»  one  of  the  largest  dinosaurs  of  the  time  (Cretaceous),  with  a 
length  of  about  20  feet.  The  noticeable  feature  of  Triceratops  is 
the  skull  with  its  two  enormous  horns,  three  feet  long  and  six  inches 
in  diameter  at  the  base,  one  above  each  eye,  and  a  shorter  one  on  the 
nose.  (In  a  closely  related  genus  the  horn  on  the  nose  was  long  while 
those  above  the  eyes  were  short.)  The  skull  projected  over  the  neck 
like  a  great  bony  frill  and  was  fringed  with  short,  bony  points.  The 


HISTORICAL  GEOLOGY 


FIG.  512.  —  Restoration  of  Stegosaurus.     (After  F.  A.  Lucas.) 

front  of  the  jaw  was  sharp  and  parrot-like,  and  covered  with  horn, 
while  the  rear  of  the  jaw  was  provided  with  shearing  teeth.  One 
genus  of  horned  dinosaur  (Torosaurus)  had  the  largest  head  of  any 
land  animal,  the  skull  being  nearly  nine  feet  long.  The  body, 
as  well  as  the  head,  was  protected,  as  is  indicated  by  various 
spines  and  plates  found  associated  with  the  skeleton,  which  were 
evidently  embedded  in  the  skin  during  life,  doubtless  for  pro- 


FIG.  513. —  Triceratops,  an  armored  dinosaur.  It  was  about  twenty  feet  long. 
(Restoration  by  C.  R.  Knight  under  the  direction  of  Professor  Osborn.  Copyright, 
American  Museum  of  Natural  History.) 


MESOZOIC   ERA:    THE  AGE  OF    REPTILES  549 

tection.  The  toes,  five  in  front  and  three  behind,  were  provided 
with  hoofs. 

Triceratops,  unlike  Stegosaurus,  faced  its  enemies,  as  do  cattle 
to-day.  Punctures  of  the  skull  and  frill  over  the  neck,  and  broken 
horn  cores  are  frequently  found,  showing  that  Triceratops  often  had 
combats  with  other  animals.  These  creatures  had  the  largest  heads 
and  smallest  brains  for  their  bulk  of  any  of  the  reptiles,  and  were 
unquestionably  extremely  stupid,  depending  upon  their  size  and 
armor  for  protection.  During  the  life  of  the  race,  the  animals  in- 
creased in  size  and  developed  longer  horns  and  a  more  complete  frill 
over  the  neck.  Triceratops  had  a  relatively  brief  career,  beginning 
in  the  Cretaceous  and  disappearing  with  its  close. 

Summary  of  Dinosaurs.  —  Dinosaurs  are  first  known  from  the 
Triassic,  at  which  time  they  were  numerous  and  diversified,  as  is 
shown  by  the  great  number  and  variety  of  footprints  in  the  Triassic 
sandstone,  although  few  skeletons  have  been  found.  This  class 
became  more  abundant,  larger,  and  more  varied  in  the  Jurassic, 
culminating  either  in  that  period  or  in  the  Cretaceous.  During  the 
Mesozoic,  they  became  more  and  more  specialized,  the  specialization 
culminating  in  Stegosaurus  and  Triceratops  among  the  herbivores  and 
in  Tyrannosaurus  among  the  carnivores.  After  becoming  adapted 
to  widely  different  conditions  of  life,  assuming  many  strange  forms 
and  spreading  over  all  the  continents  of  the  world,  they  disappeared 
with  the  Mesozoic  and  left  no  descendants. 

Migration  and  Extinction  of  Dinosaurs.  —  Our  knowledge  of  the 
reptilian  life  of  the  Mesozoic  lands  is  almost  entirely  confined  to  that 
which  lived  in  the  delta  and  coastal  swamps  of  the  era :  the  bronto- 
saurs,  the  trachodons,  the  tyrannosaurs  are  all  dinosaurs  that  lived 
either  in  swamps  or  on  their  margins.  Of  the  upland  reptiles  little  is 
known.  At  the  close  of  the  Jurassic  the  gigantic  dinosaurs  were  al- 
most completely  wiped  out,  doubtless  because  of  the  draining  of  the 
swamps  in  which  they  lived  and  their  inability  to  adapt  themselves 
to  other  conditions.  Early  in  the  Cretaceous,  however,  the  swamps 
were  again  populated  by  other  huge  dinosaurs  as  well  as  many  of 
smaller  size.  This  fauna  was  a  new  one  and  was  not  descended  from 
that  of  the  previous  period.  Either  it  (i)  migrated  from  some  region 
as  yet  unknown  or,  more  probably,  (2)  was  developed  from  surviving 
small,  active  denizens  of  the  uplands  which  are  as  yet  practically 
unknown.  Towards  the  close  of  the  Mesozoic  (in  the  late  Cretaceous) 
the  modern  type  of  vegetation  was  probably  associated  with  the 


550 


HISTORICAL  GEOLOGY 


dominance  of  mammals  on  the  uplands  and,  although  the  dinosaurs 
held  on  in  the  swamp  regions  and  had  adapted  themselves  more  or 
less  to  the  new  vegetation,  they  had  probably  become  extinct  in  the 
uplands  long  before  the  close  of  the  period.  When  the  elevation  that 
divides  the  Mesozoic  from  the  Tertiary  occurred,  it  caused  the  disap- 
pearance of  this  swamp  fauna,  but  in  the  following  period  (Tertiary) 
we  find  a  swamp  fauna  being  developed  again,  not  from  upland 
dinosaurs  but  from  the  mammals  which  had  taken  their  place  (p.  590). 

Size  as  a  Factor  in  Extinction.  —  "It  is  a  well-known  mechanical  principle  that  the 
strength  of  a  beam  varies  in  proportion  to  its  cross  section ;  its  weight  in  proportion 
to  its  mass.  Hence,  a  beam  twice  as  large  lineally  as  another  of  the  same  shape  will 
be  four  times  as  strong  and  eight  times  as  heavy.  Its  strength,  in  proportion  to  its 
weight,  varies  inversely  as  its  lineal  dimensions.  Or,  to  have  a  beam  support  a  load 
proportioned  to  its  length,  its  diameter  must  be  increased  by  \/2  for  every  doubling  of 
lineal  dimensions. 

"Apply  this  principle  to  the  skeletons  and  muscles  of  animals,  and  it  will  appear 
that  the  bones  must  become  more  massive  and  the  muscles  (whose  strength  of  pull 
varies  with  their  cross  section)  heavier  with  increase  of  size  in  the  above  proportion. 
But  the  proportionately  heavier  muscles  must  mean  a  proportionately  greater  amount 
of  food  required  to  supply  power.  If  one  animal  is  twice  as  large  lineally  as  another, 
the  length  of  its  limbs  will  be  twice  as  great,  but  its  weight  will  be  eight  times  as  great. 
In  order  to  support  that  weight,  the  bones  and  muscles  must  be  eight  times  as  strong. 
Since  their  strength  depends  upon  their  cross  section,  their  diameter  must  be  V8  times 
as  large.  To  move  the  greater  bulk  of  the  larger  animal  will  require,  on  account  of 
its  more  massive  build,  somewhat  more  than  eight  times  as  much  expenditure  of 
energy.  This  energy  is  supplied  from  the  food  which  it  finds  in  its  path.  Now  the 
larger  animal,  supposing  its  movements  to  be  in  proportion  to  its  size,  will  traverse 
a  path  which  will  be  twice  as  long  and  its  reach  will  be  twice  as  wide.  In  other  words, 
the  larger  animal,  with  the  expenditure  of  eight  times  as  much  energy,  will  cover  a 
food  area  four  times  as  large  as  that  covered  by  the  smaller  one.  If  conditions  be 
equal,  it  will  find  and  secure  four  times  as  much  food  in  the  same  length  of  time,  but 
as  we  have  seen,  it  will  consume  more  than  eight  times  as  much  energy  in  doing  so. 
From  this  it  will  follow  that  the  larger  animal  must  use  more  than  twice  as  much  time 
in  securing  the  necessary  food  to  maintain  its  activities  as  the  animal  half  as  large  in 
lineal  dimensions. 

"Quite  obviously,  this  will  fix  a  definite  limit  of  size  which  will  be  reached  when  the 
animal  expends  practically  all  of  its  time  in  securing  and  eating  food.  After  that 
point  is  reached,  further  increase  in  size  can  be  obtained  only  when :  (i)  food  becomes 
more  abundant;  (2)  the  race  becomes  adapted  to  a  more  abundant  but  hitherto  un- 
suitable kind  of  food ;  (3)  new  adaptations  are  evolved  for  more  rapid  securing  and 
digesting  of  food ;  (4)  the  animal  is  relieved  of  the  support  of  part,  or  of  most  of  its 
weight,  by  adopting  an  aquatic  life.  It  is  then  subject  to  a  new  series  of  conditions 
f  involving  a  limit  of  size,  indeed,  but  a  much  higher  one  than  for  terrestrial  animals. 
It  is  a  matter  of  common  observation  that  while  very  large  animals  spend  nearly  all 
their  time  in  eating,  small  animals  spend  a  small  proportion  of  theirs,  and  most  of  it  in 
other  activities. 


MESOZOIC  ERA:    THE   AGE  OF  REPTILES 


551 


"Now,  as  long  as  food  is  abundant,  the  larger  individuals  of  a  race  have  the  better 
chances,  both  to  repulse  or  escape  enemies,  and  to  drive  off  rivals  of  their  own  kind. 
Therefore,  the  tendency  is  for  a  race  to  increase  steadily  in  size  so  long  as  food  is  abun- 
dant, up  to  a  maximum  above  indicated.  But  if  a  scarcity  of  food  ensues,  the  larger 
animals  may  all  be  suddenly  swept  out  of  existence,  and  if  the  smaller  ones  have  been 
eliminated  through  the  gradual  evolution  of  the  race  during  the  period  of  plenty,  the 
whole  race  may  become  extinct,  or  be  driven  to  other  regions,  where  if  it  is  unable  to 
adapt  itself  successfully  to  its  new  environment,  it  will  finally  disappear. 

"For  these  reasons,  it  seems  probable  that  we  should  regard  all  large  animals  as, 
so  to  speak,  on  the  verge  of  extinction.  They  may  not  cross  the  verge  for  a  long  time, 
but  they  are  always  easily  pushed  across  by  some  unfavorable  climatic  or  environ- 
mental changes."  (Manuscript  of  W.  D.  Matthew.) 


REFERENCES  FOR  DINOSAURS 
GENERAL 

DEPERET,  CHAS.,  —  Transformations  of  the  Animal  World. 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  124-186. 

JAEKEL,  O.,  —  Die  Wirbeltiere. 

LANKESTER,  E.  R.,  —  Extinct  Animals,  pp.  199-202. 

LUCAS,  F.  A.,  —  Animals  of  the  Past,  pp.  90-1 10. 

LUCAS,  F.  A.,  —  Animals  before  Man  in  North  America,  pp.  143-172. 

LULL,  R.  S.,  —  Dinosaurian  Distribution:  Am.  Jour.  Sci.,  Vol.  29,  1910,  pp.  1-39. 

LULL,  R.  S.,  —  Nature1  s  Hieroglyphics:  Pop.  Sci.  Monthly,  Vol.  66,  1904,  pp.  139-149. 

MARSH,  O.  C.,  —  Dinosaurs  of  North  America:   Sixteenth  Ann.  Rept.,  U.  S.  Geol. 

Surv.,  1896,  pp.  143-244. 

VON  REICHENBACH,  E.  F.  S.,  —  Lehrbuch  der  Paldozoologie,  Vol.  2. 
WOODWARD,  A.  S.,  —  Fertebr ate  Paleontology. 
ZITTEL-EASTMAN,  —  Textbook  of  Paleontology. 

CARNIVOROUS  DINOSAURS  (THEROPODA) 

MATTHEW,  W.  D.,  —  Allosaurus,  a  Carnivorous  Dinosaur,  and  its  Prey:  Am.  Museum 

Jour.,  Vol.  8,  1908,  pp.  3-5. 
The  Tyrannosaurus :  Am.  Museum  Jour.,  Vol.  10,  1910,  pp.  1-8. 

UN  ARMORED  QUADRUPED  DINOSAURS  (SAUROPODA) 

African  Dinosaurs,  a  Review:  Geog.  Jour.,  Vol.  42,  1913,  p.  3^9- 

MATTHEW,  W.  D.,  —  The  Mounted  Skeleton  of  Brontosaurus :    Am.  Museum  Jour., 

Vol.  5,  1905,  pp.  63-70. 
WIELAND,  G.  R.,  —  Dinosaurian  Gastroliths:  Science,  Vol.  23,  1906,  pp.  819-821. 

BIPEDAL  HERBIVOROUS  DINOSAURS  (UNARMORED  PREDENTATA) 

BROWN,  B., —  The  Trachodon  Group:  Am.  Museum  Jour.,  Vol.  8,  1908,  pp.  51-56. 
HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  158-171. 
OSBORN,  H.  F.,  —  A  Dinosaur  Mummy:  Am.  Museum  Jour.,  Vol.  11,  1911,  pp.  7-11. 


552  HISTORICAL  GEOLOGY 

ARMORED  DINOSAURS  (ARMORED  PREDENTATA) 

HATCHER,  MARSH,  LULL, —  The  Ceratopsia:  Mon.  U.  S.  Geol.  Surv.,  Vol.  49,  1907. 
LULL,  R.  S.,  —  The  Evolution  of  Ceratopsia:  Seventh  Intern.  Zobl.  Congr.  Proc.,  1910. 
LULL,  R.  S.,  —  The  Armor  of  Stegosaurus:  Am.  Jour.  Sci.,  Vol.  29,  1910,  pp.  201-210. 

Crocodiles.  —  The  Triassic  ancestral  crocodiles  have  so  many 
characters  in  common  with  the  primitive  reptiles  (of  the  Pelycosaur 
type,  p.  490)  and  dinosaurs  that  the  order  to  which  they  belong  is 
determined  with  difficulty.  Among  the  changes  that  the  crocodiles 
underwent  during  the  Mesozoic,  the  following  may  be  mentioned, 
(i)  The  vertebrae  were  biconcave  (amphicoelous)  in  the  Triassic, 
Jurassic,  and  most  of  the  Cretaceous,  as  in  fish,  and  not  concave  in 
front  and  convex  behind  (procoelous)  as  in  modern  genera.  (2)  The 
older  crocodiles  had  the  opening  of  the  nasal  passages  into  the  mouth 
placed  far  forward,  whereas  living  crocodiles  have  them  placed  in 
the  extreme  back  of  the  mouth.  This  change  in  position  is  of  advan- 
tage in  that  it  makes  it  possible  for  the  animal  to  breathe  while  it  is 
drowning  its  prey.  The  early  Mesozoic  crocodiles  were  probably 
obliged  to  go  to  the  land  to  devour  their  food.  The  marine  crocodiles 
were  doubtless  descended  from  a  group  that  lived  in  rivers,  and  these, 
in  turn,  from  terrestrial  or  amphibious  ancestors,  although  the 
earliest  known  crocodiles  are  marine. 

Shortly  before  the  close  of  the  Jurassic,  a  side  branch  (Thalat- 
tosuchia)  appeared,  which  were  thoroughly  adapted  to  a  marine 
existence.  They  were  covered  with  a  bare  skin,  without  scales,  and 
the  tail  ended  in  a  long  fin.  The  fore  limbs  were  paddle-like,  while 
the  hind  limbs  were  less  modified,  probably  because  of  the  necessity 
of  visiting  the  shore  for  egg-laying.  After  a  brief  existence,  this 
family  disappeared. 

The  crocodiles  underwent  a  marked  change  early  in  the  Cre- 
taceous, at  which  time  the  more  modern  crocodiles  and  gavials  were 
developed. 

Marine  Reptiles. — One  of  the  most  significant  features  of  reptilian 
evolution  is  the  way  in  which  the  reptiles,  after  they  had  become 
adapted  to  land  life,  were  enabled  by  their  superior  organization  and 
greater  activity  as  ^V-breathing  animals,  to  re-invade  the  sea  re- 
peatedly and  successfully.  Hardly  had  the  reptiles  become  well- 
established  upon  the  land,  before  some  took  to  the  water  and  became 
perfectly  adapted  to  a  marine  existence.  Members  not  only  of  one 
but  of  several  classes  of  reptiles  were  so  modified.  It  is  not  remark- 


MESOZOIC   ERA:    THE  AGE  OF  REPTILES  553 

able  that  some  of  the  land  reptiles  should  have  changed  their  habits, 
when  it  is  remembered  that  in  the  shallow  waters  bordering  the  land 
there  was  an  abundant  supply  of  fish  for  food,  and  also  that  there  was 
probably  some  overcrowding  on  the  land  which  would  force  the 
weaker  species  to  take  the  food  that  was  not  to  the  liking  of  their 
stronger  neighbors. 

Ichthyosaurus   (Greek,  ichthus,  fish,   and  saurus,  reptile). — The 
most  conspicuous  features  of  reptiles  of  this  order  (Fig.  514)  are  the 


FIG.  514.  —  Ichthyosaurus,  showing  both  the  skeleton  and  the  "shadow"  made 
by  the  carbon  of  the  fleshy  parts  of  the  body.  (Courtesy,  American  Museum  of 
Natural  History.) 

heavy  body,  with  its  pointed  head  and  numerous  teeth,  and  the 
powerful  tail,  with  its  vertical  fin,  adapted  for  rapid  propulsion. 
Some  individuals  grew  to  be  40  feet  long,  although  the  usual  size 
was  very  much  less.  The  jaws  of  some  individuals  were  five  feet 
long  and  were  furnished  with  200  conical  teeth.  The  eyes  were 
large,  not  only  in  proportion  to  the  size  of  the  skull,  but  in  the 
largest  species  actually  attained  in  some  perhaps  the  size  of  the 
human  skull,  and  were  provided  with  a  ring  of  radiating,  bony 
plates  (sclerotic  plates),  like  those  of  the  early  amphibians,  which 
were  apparently  for  the  purpose  of  focusing  the  eye,  as  well  as  for 
protection. 

The  limbs  consisted  of  paddles,  made  up  of  three  or  more  rows  of 
polygonal  bones,  the  whole  being  covered  with  a  leathery  membrane. 
The  skin  was  smooth  and  without  scales.  The  vertebrae  were  bicon- 
cave, as  in  fishes.  That  Ichthyosaurus  was  carnivorous  is  shown  by 


554  HISTORICAL  GEOLOGY 

the  contents  of  the  abdomen,  which  often  contains  fish  scales  and  the 
remains  of  shelled  cephalopods  (belemnites,  p.  531). 

They  were  remarkably  well  adapted  to  aquatic  life,  as  is  shown  by 
the  paddle-like  limbs ;  by  the  outline  of  the  body,  which  was  so  modi- 
fied as  to  permit  movement  through  the  water  with  as  little  resistance 
as  possible;  by  the  sharp  teeth  for  the  catching  and  retention  of 
slippery  prey.  The  occurrence  of  undigested,  immature  young 
within  the  ribs  of  a  number  of  specimens  indicates  that  their  offspring 
were  produced  alive. 

Although  only  the  later  stages  of  the  evolution  of  the  ichthyosaurs 
are  known,  yet  it  is  evident  that  they  were  descended  from  land 
reptiles.  This  is  shown  by  the  structure  of  the  limbs  of  the  earlier 
forms,  which  were  more  like  the  legs  of  land  animals  than  were  those 
of  the  later  species. 

The  following  progressive  changes,  fitting  the  animal  for  marine 
existence,  have  been  traced:  (i)  The  limb  became  more  paddle-like 
and  less  leg-like,  both  in  the  structure  of  the  skeleton  and  in  the 
external  shape.  (2)  The  head  became  longer  and  better  adapted 
for  catching  fish  and  other  slippery  animals.  (3)  The  eyes  became 
larger  and  more  efficient  for  seeing  in  the  water.  (4)  The  neck  be- 
came shorter.  (5)  The  body  gradually  became  more  fishlike  in 
shape  and  could  move  through  the  water  more  rapidly  and  with  less 
resistance. 

Ichthyosaurs  began  in  the  Triassic,  culminated  in  the  Jurassic, 
and  lived,  for  a  short  time,  in  the  Upper  Cretaceous.  During  the 
Jurassic  they  appear  to  have  been  very  abundant  and  to  have  oc- 
cupied every  sea. 

Plesiosaurus  (Greek,  plesios,  near,  and  saurus,  reptile). — These 
marine  reptiles  are  characterized  (Fig.  515)  by  a  short,  stout  body, 
a  short  tail,  and  usually  by  a  long  neck  and  small  head.  The  tail 
was  probably  of  greater  use  in  steering  than  as  an  organ  of  propulsion, 
the  powerful,  paddle-like  limbs  being  for  that  purpose.  These  pad- 
dles had  five  digits,  but  each  digit  was  made  up  of  a  large  number 
of  small  bones,  in  some  cases  as  many  as  20.  Plesiosaurs  varied 
greatly  in  size,  some  being  30  to  40  feet  long,  but  they  usually  did  not 
attain  a  greater  length  than  6  to  15  feet.  One  American  species 
(Elasmosaurus),  for  example,  was  40  feet  long,  with  a  small  head  and  a 
neck  22  feet  in  length.  "The  other  extreme  was  Pliosaurus,  equally 
huge  in  bulk,  but  with  a  skull  nearly  5  feet  long  and  a  neck  of  only  a 
foot  and  a  half."  Most  of  the  smaller  Plesiosaurs  had  small  heads. 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


555 


The  skin  was  smooth,  without  scales.  The  sharp,  flaring  teeth  show 
that  the  creatures  lived  on  animal  food,  possibly  on  small  fish  or  some 
of  the  cephalopods  which  were  so  numerous  in  the  seas  of  the  time. 
Judging  from  the  shape  of  the  body,  they  probably  swam  slowly, 
depending  upon  stealth  rather  than  speed  in  capturing  their  prey. 


FIG.  515.  —  Restoration  of  Plesiosaurus.     (Courtesy,  American  Museum  of  Natural 

History.) 

It  has  been  shown  by  a  study  of  the  neck  vertebrae  that  the  neck  was 
too  stiff  for  very  quick  movements,  but  would,  nevertheless,  be  of 
great  assistance  both  in  capturing  prey  and  in  enabling  an  animal 
quickly  to  reach  the  surface  for  air.  Within  the  body  cavity  of  some 
skeletons,  a  large  number  of  polished  pebbles  have  been  found  — 
in  one  case  a  peck  of  them  —  from  the  size  of  a  hen's  egg  to  that  of 
a  baseball.  These  "  gizzard  stones  "  were  doubtless  of  use  in  grind- 
ing the  food,  which  was  swallowed  whole.  If  the  plesiosaurs  fed,  to  any 
extent,  on  the  shelled  cephalopods,  some  such  apparatus  must  have 
been  extremely  useful.  Plesiosaurs  ranged  from  the  Triassic  to  the 
end  of  the  Mesozoic  and  reached  their  greatest  size  in  the  Cretaceous, 
and,  perhaps,  their  greatest  abundance  in  the  Jurassic.  They  were 
not  closely  related  to  the  ichthyosaurs  and  were  probably  descended 
from  a  different  race  of  land  reptiles. 

Mosasaurus  (Sea  Lizards).  — As  the  ichthyosaurs  disappeared  in 
the  Upper  Cretaceous,  their  place  was  taken  by  the  mosasaurs  (Figs. 
516,  517),  long,  slender  reptiles,  with  a  scaly  skin  like  that  of  modern 
snakes,  which  attained  a  length  of  35  feet  or  more,  although  usually 
smaller.  The  heads  were  pointed  and  provided  with  sharp,  stout, 
pointed  teeth.  The  jaws  were  so  constructed  as  to  make  it  possible 
for  the  animal  to  swallow  an  object  of  almost  the  diameter  of  itself. 


556 


HISTORICAL  GEOLOGY 


This  was  accomplished  by  a  hinge  in  each  half  of  the  lower  jaw  (Fig. 
516)  which  permitted  it  to  bow  outward  when  open.  The  articulation 
of  the  jaw  with  the  skull  also  assisted  in  this  process.  The  limbs  were 


FIG.  516.  —  Skeleton  of  a  Cretaceous  mosasaur  about  sixteen  feet  long. 
(After  Williston.) 

not  as  greatly  modified  as  in  the  ichthyosaurs,  but  were  completely 
paddle-like  and  resembled  those  of  the  whale.  The  great  speed  with 
which  it  could  be  propelled  by  its  tail  made  the  catching  of  its  fish 
food  an  easy  matter.  Mosasaurs  were  descended  from  land  animals 
and  may  have  sprung  from  the  same  stock  as  modern  reptiles. 
They  were  not  well  established  until  the  Upper  Cretaceous,  in  which 
period  they  rapidly  diverged  and  swarmed  the  Atlantic  and  Gulf 


FIG.  517.  —  Restoration  of  a  Cretaceous  mosasaur.  (Painted  by  C.  R.  Knight 
under  the  direction  of  Prof.  H.  F.  Osborn.  (Copyright,  American  Museum  of  Natural 
History.) 

coasts  and  the  interior  seas.  They  had  a  wide  distribution,  being 
found  in  North  and  South  America,  Europe,  and  as  far  south  as 
New  Zealand.  They  disappeared  with  the  Mesozoic,  after  having 
had  a  comparatively  short  life. 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES  557 


REFERENCES   FOR   MARINE  REPTILES 

ICHTHYOSAURS 


HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  59-75. 
LANKESTER,  E.  R.,  —  Extinct  Animals,  pp.  225-229. 

OSBORN,  H.  F.,  —  Ichthyosaurs:   Century  Mag.,  Vol.  69,  1905,  pp.  414-422. 
WILLISTON,  S.  W.,  —  Water  Reptiles  of  the  Past  and  Present,  pp.  107-125. 


PLESIOSAURS 


HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  78-84. 
MATTHEW,  W.  D.,  —  The  New  Plesiosaur:  Am.  Museum  Jour.,  Vol.  10,  1910,  pp. 

246-250. 
WILLISTON,  S.  W.,  —  Water  Reptiles  of  the  Past  and  Present,  pp.  73-101. 

MOSASAURS 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  187-198. 

LUCAS,  F.  A.,  —  Animals  of  the  Past,  pp.  48-56. 

OSBORN,   H.   F.,  —  A  Complete  Mosasaur  Skeleton:    Am.  Mus.  Nat.  Hist.  Mem., 

Vol.  i,  1899,  pp.  167-188. 
WILLISTON,  S.  W.,  —  Water  Reptiles  of  the  Past  and  Present,  pp.  148-167. 

GENERAL 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3. 

WILLISTON,  S.  W.,  —  Water  Reptiles  of  the  Past  and  Present,  pp.  59-72. 

WOODWARD,  A.  S.,  —  Vertebrate  Paleontology. 

ZITTEL-EASTMAN,  —  Textbook  of  Paleontology. 

Turtles.  —  It  is  an  interesting  fact  that,  although  turtles  are  so 
widely  different  from  other  forms  at  the  present,  yet,  even  when  first 
known,  —  in  the  Upper  Triassic,  —  they  are  as  typically  turtle-like 
as  now.  Jurassic  turtles  were  abundant,  had  a  world-wide  distribu- 
tion, and  were  closely  related  to  existing  genera.  The  first  strictly 
marine  turtles  (in  which  the  feet  are  modified  to  form  "  flippers  ") 
have  been  found  in  the  Cretaceous,  one  of  which,  Archelon,  was  of 
great  size,  the  head  measuring  three  feet  in  length,  the  total  length  of 
the  animal  being  12  to  14  feet.  In  this  case,  the  shell  proper  had  dis- 
appeared, and  the  broadened  ribs  were  possibly  covered  with  a  soft 
skin,  as  in  some  living  marine  turtles  (Dermochelys).  Land  turtles 
did  not  appear  until  the  Tertiary. 

A  number  of  suggestions  as  to  the  origin  of  turtles  have  been  offered, 
but  since  the  earliest  known  species  are  far  from  being  generalized, 
the  whole  matter  is,  as  yet,  in  doubt. 


558 


HISTORICAL  GEOLOGY 


REFERENCES   FOR  TURTLES 

HAY,  O.  P.,  —  The  Fossil  Turtles  of  North  America:   Carnegie  Institution  of  Wash- 
ington, Pub.  No.  75,  1908. 

WIELAND,  G.  R.,  —  Archelon,  etc.:  Am.  Jour.  Sci.,  Vol.  2,  1896,  pp.  401-412. 
WILLISTON,  S.  W.,  —  Water  Reptiles  of  the  Past  and  Present,  pp.  216-241. 

Flying  Reptiles  (Pterosaurs).  —  Either  because  of  the  overcrowd- 
ing of  the  land,  or  for  some  other  reason,  a  race  of  flying  reptiles  was 

developed  during  the  Jurassic  and 
Lower  Cretaceous,  and  occupied  the 
realm  of  the  air,  in  which  there  was 
no  competition. 

The  pterosaurs  (Figs.  518,  519, 
520)  are  as  extraordinary,  in  many 
ways,  as  any  animal  that  ever  lived. 
They  had  a  short  body,  hollow 
bones,  a  rather  large  but  light  head, 
and  jaws  which  at  the  beginning  of 
the  race  were  provided  with  slender 
teeth,  but  which  in  some  highly 
specialized  later  genera  were  tooth- 
less and  sheathed  with  horn,  as  in 
modern  birds.  The  most  remark- 
able and  characteristic  features,  how- 
ever, were  the  large,  membranous 
wings,  supported  by  one  greatly 
elongated  finger,  the  fourth.  The 
breastbone,  to  which  the  muscles  of 
flight  were  attached,  was  large  and 
keeled,  and  the  shoulder  girdle  was 
FIG.  518.  —  A  Jurassic  pterosaur  strong.  Some  had  long  tails  with  a 
(Rhamphorynchus).  (After  Von  Rei-  kind  of  mdder  at  the  extremity,  and 
chenbach.)  Length  about  twenty 
inches.  others  were  tailless.  1  he  pterosaurs 

varied  greatly  in  size ;  some  were  as 

small  as  sparrows,  some  were  the  size  of  partridges,  while  others  were 
the  largest  flying  creatures  that  ever  lived,  the  wings  measuring 
over  20  feet  from  tip  to  tip  (Fig.  520). 

One  of  the  best  known  and  least  specialized  genera  of  the  Jurassic 
pterosaurs  (Dimorphodon  ;  Greek,  dimorphos,  two-formed,  and  odont-, 
tooth)  (Fig.  519)  had,  as  the  name  implies,  two  kinds  of  teeth,  those 
in  front  of  the  jaw  being  sharp  and  strong  and  fitted  for  tearing,  while 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


559 


those  in  the  back  of  the  jaw  were  small  and  sharp,  with  a  sawlike 
edge.  This  pterosaur  could  probably  walk  on  all  fours  or  on  its  hind 
legs  alone.  When  standing  on  its  hind  legs,  it  was  less  than  two  feet 


FIG.  519. — A  Jurassic  -pterosaur  (Dimorphodon).  The  extreme  length  from  the  tip 
of  the  nose  to  the  end  of  the  tail  was  a  little  more  than  three  feet.  (Modified  after 
Seeley.) 

high,  and  its  wings  had  a  spread  of  a  little  more  than  four  feet.  The 
wings  (Fig.  519)  were  formed  by  a  naked  membrane,  without  feathers 
or  hair,  stretching  from  the  body  to  the  greatly  elongated  fourth  fin- 
ger. Although  the  least  specialized  of  the  pterosaurs,  they  possessed 
few  characters  connecting  them  with  other  reptiles. 

Perhaps  the  most  highly  specialized  animal  that  ever  existed  was 
a  pterosaur  (Pteranodon;  Greek,  pteron,  wing,  and  a-odont-,  without 
a  tooth)  that  lived 
in  the  Upper  Creta- 
ceous. In  this  animal 
(Fig.  520)  it  would 
seem  that  every- 
thing possible  was 
sacrificed  for  flight. 
The  upper  portion  of 
the  body,  the  wing, 
shoulder,  and  breast 
were  all  extraordi-  pIG  520  _  Skeleton  of  Pteranodon,  the  most  highly 
narily  strong,  while  specialized  of  the  pterosaurs  (Cretaceous).  Everything 

i-Vip  Inwpr  nnrtmn   nf    was  sacrificed  for  flight  and  feeding.    The  wings  measured 
lower  portion  o     almost  menty  feet  from  dp  to  dp     (After  £aton  } 

the    body    and    hind 

limbs  were  very  weak.  The  head  was  highly  developed,. being  long 
and  slender,  with  a  dagger-like  beak  and  toothless  jaws.  The  head 
was  about  four  feet  long,  the  body  only  slightly  longer.  It  is  thought 

CLELAND    GEOL.  —  36 


560  HISTORICAL  GEOLOGY 

that,  notwithstanding  its  large  size,  it  was  so  lightly  built  that  in  life 
it  did  not  weigh  more  than  25  pounds.  In  fact,  the  bones  of  the 
largest  specimen,  even  as  petrified,  do  not  weigh  more  than  5  or  6 
pounds.  When  not  sailing  in  the  air,  pteranodons  probably  spent 
their  time  suspended  from  cliffs  or  trees  by  their  slender,  clawed  fingers. 
Pteranodons  lived  upon  fish,  as  is  shown  by  the  fishbones  and  scales 
found  within  their  skeletons.  Because  of  the  small  pelvis,  we  must 
suppose  that  if  they  laid  eggs,  the  eggs  were  very  small. 

Because  of  the  high  degree  of  specialization  of  the  earliest  ptero- 
saurs, nothing  definite  can  be  said  as  to  their  ancestry,  but  it  is  possi- 
ble that  pterosaurs,  carnivorous  dinosaurs,  and  birds  all  sprang  from 
a  common  ancestor  (such  as  Euparkeia).  (Broom.)  Although  flying 
animals  they  were  not  the  ancestors  of  birds.  The  first  evidence  of 
their  appearance  has  been  found  in  the  later  Triassic,  but  they  did  not 
reach  North  America  until  after  the  middle  of  the  Jurassic,  at  which 
time  they  swarmed  over  the  epicontinental  seas.  None  lived  into  the 
Upper  Cretaceous. 

REFERENCES  FOR  PTEROSAURS 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3,  especially  pp.  101  and  179. 
EATON,  G.  F.,  —  Pteranodon:  Am.  Jour.  Sci.,  Vol.  17,  1904,  pp.  318-320. 
HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  199-210. 
LUCAS,  F.  A.,  —  Animals  before  Man,  pp.  209-219. 
LUCAS,  F.  A., —  The  Greatest  Flying  Creature,  etc.:    Smithsonian  Inst.  Kept.,  1901, 

pp.  654-659. 

SEELEY,  H.  G.,  —  Dragons  of  the  Air. 

WILLISTON,  S.  W.,  —  Winged  Reptiles:  Pop.  Sci.  Monthly,  Vol.  60,  1902,  pp.  314-322. 
WILLISTON,  S.  W.,  —  The  Wing  Finger  of  Pterodactyls:  Jour.  Geol.,  Vol.  19,  1911, 

pp.  696-705. 

WOODWARD,  A.  S.,  —  Vertebrate  Paleontology. 
ZITTEL-EASTMAN,  —  Textbook  of  Paleontology. 

TOOTHED  BIRDS 

Archaeopteryx.  —  If  the  skeletons  of  the  earliest  known  bird  had  not 
had  feathers  associated  with  them,  it  is  probable  that  they  would  have 
been  described  as  belonging  to  the  Reptilia,  with  some  birdlike  char- 
acters. This  oldest  bird  (Archaeopteryx;  Greek,  archaios,  old,  and 
pterux,  a  wing)  (Fig.  521)  was  about  the  size  of  a  small  crow,  with  a 
small,  stout,  birdlike  head  and  a  birdlike  brain,  but  its  jaws,  instead 
of  being  of  horn  as  in  modern  birds,  were  provided  with  sharp,  conical 
teeth.  The  wing  was  peculiar  in  having  three  reptile-like  claws, 
by  means  of  which  the  bird  could  crawl  about  the  trees,  instead  of 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


flying.     The  hind  limb  was  much  like  that  of  modern  birds  and  had 
four  digits.     The  vertebrae  were  biconcave,  as  in  fish  and  some  rep- 
tiles.    The  tail  was  one  of  the  most  peculiar  features  in  that  it  was 
vertebrated,  with  a  pair  of  feathers  springing  from  each  joint.     In 
modern      birds,     the 
feathers  are  arranged     |F 
like   the   sticks  of  a 
fan.       Archaeopteryx 
was  not  well  adapted 
for  flying,  as  is  shown 
by    the     poorly    de- 
veloped    breastbone. 
With    the    exception 
of    occasional     short 
flights,     it     probably 
soared    somewhat    as 
flying  squirrels  do  to- 
day.    Birds  probably 
did    not    have    dino- 
saurian  ancestors,  but 
were  presumably  de- 
rived from  a  group  of 
primitive,     dinosaur- 


FIG.  521.  —  Restoration  of  Archczopteryx  (Jurassic). 
The  long  vertebrated  tail,  clawed  wings,  and  teeth  are 
well  shown.  (Modified  after  Hutchinson.) 


like  reptiles  that  were 

capable  of  running  on 

their   hind   legs.      Archaeopteryx   is    not   known   to   have    lived   in 

America ;    and  only  a  few  specimens  have  been  found  in  Europe,  all 

of  which  are  from  the  Jurassic. 

Hesperornis.  —  This  bird  (Fig.  522)  was  adapted  for  life  in  water 
instead  of  in  air.  It  was  the  largest  bird  of  its  time,  attaining  a 
length  of  nearly  six  feet.  The  jaws  were  supplied  with  small  teeth 
which,  instead  of  being  set  in  sockets  as  in  Ichthyornis  (p.  562)  were 
in  grooves.  As  in  snakes,  the  jaws  were  so  constructed  as  to  per- 
mit the  bird  to  swallow  large  prey.  The  tail  was  vertebrated,  but  was 
intermediate  between  Archaeopteryx  and  modern  birds.  Hesperornis 
was  perfectly  adapted  for  aquatic  life.  Wings  were  wanting,  and  only 
a  rudimentary  bone  was  left  to  show  that  a  wing  existed  in  its  remote 
ancestors.  The  feet  were  modified  in  a  manner  not  found  in  any  other 
bird,  living  or  fossil,  being  so  joined  to  the  leg  as  to  turn  edgewise 
as  the  foot  was  brought  forward.  The  resistance  of  the  water  was  in 


562  HISTORICAL  GEOLOGY 

this  way  lessened.  This  adaptation  to  aquatic  conditions  may  have 
been  so  perfect  that  not  only  flying,  but  walking  as  well,  was  aban- 
doned. The  bird  was  covered  with  soft  feathers,  as  fossil  impressions 
show.  Hesperornis  lived  only  in  the  Upper  Cretaceous. 


FIG.  522.  —  Hesperornis,   a  diving,   toothed   bird  of  the  Cretaceous.      (Restoration 
under  the  direction  of  F.  A.  Lucas.) 

Ichthyornis  (Greek,  ichthus,  fish,  and  ornis,  a  bird). — This  bird 
(Fig.  523)  was  about  as  large  as  a  pigeon  and  must  have  looked  very 
much  like  a  modern  bird.  It  was,  however,  radically  different  in  some 
particulars.  Its  slender  jaws  were  toothed,  the  teeth  being  small 
and  set  in  sockets,  twenty  on  each  side  below  and  fewer  above.  The 
vertebrae  were  biconcave,  like  those  of  fishes  and  many  extinct  reptiles 
but  no  modern  bird.  The  tail  was  about  midway  between  the  verte- 
brated  tail  of  Archaeopteryx  and  those  of  the  birds  of  the  Tertiary  and 
to-day.  The  strongly  keeled  breastbone  for  the  attachment  of  the 
muscles  proves  that  it  was  a  powerful  flyer.  Although  Ichthyornis 
shows  a  distinct  advance  over  Archaeopteryx  in  its  less  vertebrated  tail, 
its  power  of  flight,  and  the  loss  of  the  claws  on  the  fore  limbs,  an  equal 
or  greater  change  is  to  be  seen  between  the  Cretaceous  birds  and  those 
of  the  Tertiary. 

It  is  interesting  to  speculate  upon  the  cause  of  the  abandonment 
of  teeth  for  a  horny  jaw  both  by  birds  and  pterosaurs.  A  toothed 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


563 


jaw  would  insure  the  retention  of  every  fish  captured,  but  would 

prove    a    hindrance    to    its     being    swallowed     quickly.     Possibly 

toothed  birds  and  pterosaurs  were  obliged  to  go  to  land  before  being 

able  to  devour  their  food, 

but  those  with  horny  beaks 

could  bolt  their  food  on  the 

wing. 

Fossil  birds  are  compara- 
tively rare,  even  from  the 
rocks  of  periods  when  birds 
were  abundant,  because  of 
the  lightness  of  the  skele- 
tons, which  caused  the  car- 
casses to  float  on  the  seas 
for  a  long  time  before  sink- 
ing to  the  bottom,  with  the 
result  that  the  skeletons 
were  usually  devoured  by 
fish  or  beasts  of  prey  before 
they  had  a  chance  to  be 
buried  in  the  sediments. 
Bird  fossils  are  rare  in  the 
Mesozoic  also  since  they  are 
not  now  and  were  not  then  to  JIG.  523.  —  Ichthyornis,  a  small,  toothed  bird 
,  with  strong  powers  of  flight  (Cretaceous).  (After 

any  extent  swamp  dwellers,   Marsh  ) 

and  what  is  known  of  Meso- 
zoic land  life  is  chiefly  limited  to  the  fauna  of  the  swamps.     Because 
of  this,  it  is  probable  that  only  a  small  part  of  the  bird  life  of  the  Cre- 
taceous is  known. 

REFERENCES   FOR  BIRDS 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  211-220. 

LUCAS,  F.  A.,  —  Animals  of  the  Past,  pp.  70-89. 

MARSH,   O.    C.,  —  Birds   with    Teeth:    Third   Ann.   Kept.,  U.  S.  Geol.  Surv.,   188, 

pp.  45-88. 

NEWTON,  —  A  Dictionary  of  Birds. 
WOODWARD,  A.  S.,  —  Vertebrate  Paleontology. 
ZITTEL-£ASTMAN,  —  Textbook  of  Paleontology. 

MAMMALS 

A  few  very  small   lower  jaws   have   been  discovered  in  Triassic 
deposits  of  America  and  Europe  (Dromatherium,  Fig.  524),  which 


564 


HISTORICAL  GEOLOGY 


have  been  considered  by  some  students  as  reptilian  and  by  others 
as  mammalian.  If  they  are  reptilian,  they  are  theromorphs;  if 
mammalian,  either  monotremes  (egg  layers  that  suckle  their  young, 

like  Platypus),  or  marsupials  (mam- 
mals that  produce  their  young  in  an 
immature    condition,    like   the   opos- 
sum).    It  is  suggestive,  however,  to 
FIG.  524.  — Jaw  of  either  a  primi-  note  that    creatures   that   are   either 
tiro  mammal  or  a  theromorph  reptile  reptiles  with  strong  mammalian  char- 
(Inassic),    twice    the    natural    size.  i  -  •'"•  i 

(After  Emmons.)  acters  or  mammals  with  well-marked 

reptilian    characters    existed     before 

true  mammals  made  their  appearance.  With  one  exception,  the  few 
mammalian  remains  found  in  the  Mesozoic  are  of  small  size,  indicat- 
ing animals  not  larger  than  rats.  Those  of  the  Jurassic  and  Creta- 
ceous appear  to  be  insectivores  and  to  be  either  monotremes  (egg- 
laying  mammals)  or  marsupials,  but  none  clearly  belong  to  the 
highest  type  of  mammals  (placentals,  common  mammals  of  to-day) 
nor  are  they  closely  related  to  other  forms. 


FIG.  525.  —  Table  showing  the  probable  relationships  of  verteorates,  with  tneir 
geological  distribution. 


MESOZOIC   ERA:    THE  AGE  OF   REPTILES  565 

The  probable  relationships  of  the  mammals  and  other  vertebrates 
are  shown  in  the  table  (Fig.  525). 


REFERENCES  FOR  MAMMALS 

SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  642-644. 
WOODWARD,  A.  S.,  —  Vertebrate  Paleontology,  pp.  246-260. 


PLANTS 

The  vegetation  of  the  Mesozoic  is  of  great  interest,  since  it  was 
during  this  period  of  world  history  that  the  now  dominant  types  of 
plants  were  introduced.  Mesozoic  plant  life,  as  indeed  does  the  plant 
life  of  all  geological  ages,  affords  a  reliable  clue  to  the  climatic  and 
physical  conditions  which  prevailed  during  the  several  periods,  and 
incidentally  offers,  to  some  degree,  an  explanation  of  the  striking 
changes  which  took  place  in  the  animal  life. 

In  discussing  the  vegetation  of  the  Mesozoic,  a  division  into  Lower 
and  Upper  should  perhaps  be  made,  because  of  the  introduction  of 
modern  plants  (angiosperms)  in  the  Lower  Cretaceous  and  the  sub- 
ordination of  the  typical  early  Mesozoic  plants  in  the  Upper  Creta- 
ceous. 

Owing  to  considerations,  physical  and  otherwise,  concerning  which 
there  is  not  complete  agreement,  the  lower  part  of  the  Triassic  affords 
but  scant  remains,  and  it  is  not  until  we  come  to  the  upper  part 
(Rhaetic)  that  the  plant  remains  can  be  really  dignified  as  a  flora.  In 
North  America  there  are  less  than  150  species,  and  the  entire  Triassic 
flora  of  the  world  probably  does  not  exceed  300  or  400  forms. 

Horsetails.  —  The  horsetails,  which  entirely  replaced  the  cala- 
mites  of  the  Carboniferous,  do  not  appear  to  have  differed  markedly 
from  those  now  living,  except  that  they  were  often  of  larger  size, 
some  having  been  reported  that  are  from  five  to  eight  inches  in  diam- 
eter. It  is  presumed  that  they  formed  dense  growths,  like  canebrakes, 
in  or  along  swamps,  marshes,  or  lakes,  as  do  certain  of  their  living 
representatives  to-day,  the  largest  of  which  —  a  South  American 
species  —  is  an  inch  in  diameter  and  20  or  30  feet  in  height. 

Cycads.  —  Among  the  most  characteristic  and  abundant  plants  of 
the  Triassic  and  Jurassic  was  the  great  group  of  cycads  (using  the  term 
in  the  broad  sense  to  include  the  Bennettitales  and  Cycadales).  They 
were  similar  in  general  appearance  to  those  of  the  present,  but  differed 
in  some  important  characters.  Fossil  cycad  trunks  (Fig.  526)  are 


566 


HISTORICAL  GEOLOGY 


generally  short  and  stout,  never  apparently  reaching  a  greater  height 
than  three  or  four  feet,  and  usually  much  less.  As  in  modern  cycads, 
a  crown  of  long,  stiff  leaves  sprang  from  the  top  of  the  trunk,  which  was 
scarred  throughout  by  the  leaf-bases  of  previous  leaves.  Some 
fossil  cycads  from  South  Dakota  are  so  completely  preserved  that  such 
delicate  structures  as  immature  leaves,  flowers,  pollen,  and  some  seeds 
with  their  contained  embryos,  are  retained  in  remarkable  perfection. 


FIG.  526.  —  A  group  of  cycad  trunks  (Bennettites).     (After  Wieland.) 

Because  of  this  perfection  of  preservation,  almost  as  much  is  known 
of  the  structure  of  this  extinct  group  as  of  its  living  relations.  The 
position  of  the  seed-bearing  cone  and  the  large  leaves  bearing  the  pol- 
len sacks  of  a  fossil  cycad  is  well  shown  in  the  diagram  (Fig.  527). 
Cycads,  which  appear  to  have  grown  on  the  dryer  lowlands  about 
the  swamps,  had  their  origin  in  the  Permian,  reached  their  greatest 
abundance  in  the  Jurassic,  and  are  rare  after  the  close  of  the 
Mesozoic. 

Ferns  were  common  throughout  the  era,  wherever  the  conditions 
were  favorable  for  their  growth. 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES 


567 


Br 


Gymnosperms.  —  The 

conifers  (evergreen  trees 
of  to-day)  lived  on  the 
higher  lands  during  the 
Mesozoic  and  were  repre- 
sented by  pines,  cy- 
presses, yews,  and  arau- 
carias  (monkey-puzzle), 
the  last  being  especially 
abundant  in  the  Jurassic. 
The  sequoia  (redwoods 
and  "  big  trees  "  of  Cali- 
fornia) had  a  notable  de- 
velopment in  the  Upper 
Cretaceous.  Because  of 
the  resinous  character  of 
the  wood  of  the  conifer- 
ous trees,  it  was  often  FIG.  527.  —  Diagram  of  a  complete  cone  of  Ben- 
preserved,  sometimes  in  a  .nettitf  ^enlarged  (After  Wieland.)  The 

large  leaves,  M,  bear  pollen  sacks,  and  the  central 
remarkable  degree  of  per-  cone  S  is  seed-bearing. 

fection.     On  the  whole, 

the  Mesozoic  conifers  were  not  very  different  in  general  appearance 
from  those  of  to-day.  The  early  Triassic  forms,  however,  were  some- 
what dwarfed,  while  those  of  the  later  Triassic  were  gigantic  trees, 
often  over  100  feet  in  length  and  from  four  to  eight  feet  in  diameter. 
The  maidenhair  tree,  ginkgo,  was  abundant  and  had  a  world-wide 

distribution  in  the  Lower 
Mesozoic.  This  once  nu- 
merous family  is  now  rep- 
resented by  but  one  spe- 
cies, which  probably  would 
have  been  long  since  ex- 
tinct had  it  not  been 
preserved  by  cultivation 
about  the  Buddhist 
temples  in  Japan  and 
China.  The  modern  ginkgo 
comes  of  a  long-lived 
family.  Evidence  is  at 
FIG.  528.  —  Leaves  of  the  modern  ginkgo  tree.  hand  indicating  that,  if 


568  HISTORICAL  GEOLOGY 

not  the  existing  genus,  at  least  a  closely  related  one  lived  in  the 
Paleozoic.  The  impressions  of  the  leaf,  seeds,  and  male  cones  of  Ju- 
rassic trees  are  very  similar  to  those  of  the  trees  now  living  (Fig.  528). 

Angiosperms.  —  The  flowering  plants  are,  at  present,  the  common- 
est of  all  plants,  four  sevenths  of  the  existing  species  belonging  to 
this  class.  They  have,  however,  a  much  shorter  known  history  than 
the  conifers  and  various  other  groups,  since  no  positive  evidence 
is  at  hand  of  their  existence  prior  to  the  Lower  Cretaceous.  At  the 
beginning  of  this  period,  the  horsetails,  cycads,  conifers,  and  ferns 
were  the  common  and  conspicuous  forms ;  but,  before  its  close,  flower- 
ing plants  of  both  divisions  (monocotyledons,  represented  to-day  by 
palms,  lilies,  and  grasses,  and  dicotyledons,  of  which  the  elm,  rose, 
and  clover  are  examples)  had  become  prominent.  The  sassafras,  fig, 
willow,  magnolia,  tulip  tree,  laurel,  and  others  have  been  recognized. 
In  the  Upper  Cretaceous  the  flowering  plants  became  even  more 
conspicuous,  and  are  represented,  among  many  others,  by  palms, 
beeches,  birches,  chestnuts,  and  poplars. 

The  introduction  of  flowering  plants  was,  perhaps,  the  most  impor- 
tant and  far-reaching  event  in  the  whole  history  of  vegetation,  not 
only  because  they  almost  immediately  became  dominant,  but  also 
because  of  their  influence  upon  the  animal  life  of  the  succeeding  peri- 
ods. Hardly  had  flowers  appeared,  before  a  great  horde  of  insects 
which  fed  upon  their  honey  or  pollen  seem  to  have  sprung  into  exist- 
ence. The  nutritious  grasses  and  the  various  nuts,  seeds,  and  fruits 
afforded  a  better  food  for  non-carnivores  than  ever  before  in  the  his- 
tory of  the  world.  It  was  to  be  expected,  therefore,  that  some  new 
type  of  animal  life  would  be  developed  to  take  advantage  of  this 
superior  food  supply.  As  we  shall  see  in  the  discussion  of  the  Ter- 
tiary, the  mammals,  which  kept  a  subordinate  position  throughout 
the  Mesozoic,  rapidly  took  on  bulk  arid  variety  and  acquired  posses- 
sion of  the  earth  as  soon  as  they  became  adapted  to  this  new  food, 
quickly  supplanting  the  great  reptiles  of  the  Mesozoic. 

The  flowering  plants  (angiosperms)  had  their  origin,  as  far  as  is 
known,  on  both  sides  of  the  North  Atlantic  during  the  Lower  Cre- 
taceous. Some  of  the  earliest  of  these  are  somewhat  generalized, 
but  do  not  give  a  positive  clue  to  the  group  from  which  they  were 
descended.  Just  when  and  where  they  began  we  do  not  know,  but 
once  started  they  spread  rapidly  and  widely,  and  before  the  close  of 
the  Lower  Cretaceous  had  reached  California,  Alaska,  Greenland,  and 
Bohemia. 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES  569 

REFERENCES  FOR  PLANTS 

CHAMBERLIN  and  SALISBURY,  —  Geology,  Vol.  3. 

KNOWLTON,  F.  H.,  —  The  Relations  of  Paleobotany  to  Geology:  Am.  Naturalist,  Vol.  46, 

1912,  pp.  207-215. 
SEWARD,  A.  C.,  —  Fossil  Plants. 
SCOTT,  D.  H.,  —  Studies  in  Fossil  Botany. 
SCOTT,  D.  H.,  —  Evolution  of  Plants. 

SCOTT,  D.  H.,  —  Paleobotany:   Encyclopedia  Brittanica,  I2th  edition. 
SCOTT,  W.  B.,  —  An  Introduction  to  Geology,  pp.  668,  684,  714-715. 
STOPES,  M.  C.,  —  Ancient  Plants. 
WIELAND,  G.  R.,  —  American  Fossil  Cycads. 


CLIMATE 

Triassic.  — The  climate  of  the  Triassic  of  North  America,  Central 
Europe,  and  North  Africa  seems,  as  a  whole,  to  have  been  arid,  al- 
though some  areas  of  considerable  extent  had  sufficient  rainfall  to 
produce  a  luxuriant  vegetation.  The  proofs  of  aridity  are  to  be  found 
in  the  widespread  occurrence  of  gypsum  and  salt,  and  in  the  preva- 
lence of  "  red  beds  "  (rock  of  a  red  color).  It  is  well  known  that  the 
deposition  of  salt  and  gypsum  is  the  result  of  evaporation  in  excess  of 
supply,  such  as  can  happen  only  in  arid  regions.  The  explanation 
of  the  red  color  of  sedimentary  rocks  is  not  so  clear.  If  organic  matter, 
either  animal  or  plant,  is  plentiful  in  sediments,  the  contained  iron 
will  be  in  the  form  of  the  gray  iron  carbonate  instead  of  the  red  iron 
oxide.  At  the  present  day,  for  example,  although  the  rocks  of  the 
southern  Appalachians  are  weathered  to  red  clay  many  feet  deep, 
the  sediments  derived  from  them  are  gray  when  deposited,  because  of 
the  reduction  of  the  iron  oxide  by  the  plant  debris  which  they  inclose. 
A  less  abundant  flora,  due  to  decreased  rainfall,  might  readily  result 
in  the  deposit  of  red  sediments  without  reduction.  On  the  other  hand, 
attention  has  been  called  to  the  fact  that,  probably  in  many  cases, 
red  sediments  were  laid  down  in  regions  where  the  rainfall  was  un- 
doubtedly not  small.  (White.) 

The  Triassic  red  sandstones  and  shales  of  the  Connecticut  valley, 
with  their  innumerable  reptilian  footprints,  indicate  aridity  in  another 
way.  It  was  formerly  thought  that  these  deposits  were  laid  down  in  a 
great  estuary  of  the  sea,  under  conditions  similar  to  those  of  the  Bay 
of  Fundy  to-day,  in  which  the  difference  between  high  and  low  tide 
was  great.  As  a  result,  during  several  hours  of  the  day,  extensive 
mud  flats  were  uncovered,  upon  which  the  saurians  of  that  time  walked 


570 


HISTORICAL  GEOLOGY 


or  ran  in  search  of  food  or  water  and  left  their  tracks.  It  has  been 
shown,  however,  that  a  number  of  hours  at  least  must  elapse  under 
known  conditions,  before  mud  can  dry  sufficiently  to  form  sun  cracks 
and  to  retain  the  footprints  of  animals.  If  these  conclusions  are 
correct,  we  must  believe  that  the  deposits  of  the  Connecticut  valley 
and  New  Jersey  were  laid  down  in  river  valleys  analogous  to  the  Great 
Valley  of  California  and  other  structural  basins.  In  the  shallow 
lakes  which  occurred,  such  for  example  as  Tulare  Lake  in  California, 
the  depth  of  the  water  was  greatly  reduced  by  evaporation  during 
longer  or  shorter  periods,  and  the  bottom  of  the  shallower  portions 
of  the  seas  was  exposed  for  several  days  or  weeks  at  a  time. 

There  is  abundant  proof,  however,  that  in  certain  regions  the  rain- 
fall during  the  Triassic  was  plentiful,  due  probably,  as  to-day,  to  the 
presence  of  mountain  ranges  which  caused  abundant  precipitation 
on  one  side  and  deserts  on  the  other.  In  Virginia,  for  example,  beds 
of  coal  aggregating  30  to  40  feet  in  thickness  indicate  long-continued 
swamp  conditions.  Horsetails  four  to  five  inches  in  diameter,  ferns  of 
large  size,  some  of  them  tree  ferns,  prove  that  the  climate  was  favorable 
for  luxuriant  growth.  The  petrified  trees  of  Arizona,  some  of  which 
were  eight  feet  in  diameter  and  more  than  1 20  feet  high,  do  not  indicate 
aridity,  nor,  for  that  matter,  do  they  prove  a  moister  climate  than  that 
of  Arizona  to-day,  in  which  the  great  pines  south  of  Flagstaff  flourish. 
The  complete  or  nearly  complete  absence  of  rings  in  the  tree  trunks 
indicates  that  there  were  no,  or  but  slight,  seasonal  changes,  due  to 
alternations  of  heat  and  cold  or  wet  and  dry  periods. 

Jurassic. — The  presence  of  luxuriant  ferns,  many  of  them  tree  ferns, 
horsetails  of  large  size,  and  conifers,  the  descendants  of  which  live 
in  warm  regions,  all  point  to  a  moist,  warm,  subtropical  climate  during 
the  greater  part  of  the  Jurassic ;  although  arid  regions  unquestion- 
ably existed.  The  animals  also  indicate  a  warmer  climate  in  the 
northern  regions  than  at  present.  Saurians  and  ammonites  lived 
within  the  Arctic  circle,  and  corals  3000  miles  farther  north  than  now. 
The  presence  in  the  late  Jurassic  of  rings  in  the  tree  trunks  of  northern 
species  shows  that  slight  seasonal  changes  occurred. 

Cretaceous.  —  The  climate  of  the  Lower  and  Upper  Cretaceous 
seems  to  have  been  milder  than  at  present,  even  that  of  Greenland 
being  temperate  or  warm  temperate.  The  distribution  of  marine 
fossils  indicates  the  existence  of  climatic  zones  according  to  latitude, 
but  the  vegetation  does  not  show  this  so  clearly ;  for  example,  oaks, 
maples,  and  magnolias  grew  in  Greenland  and  nearly  as  far  north  as 


MESOZOIC  ERA:    THE  AGE  OF  REPTILES  571 

Alaska,  in  the  Lower  Cretaceous.  If  a  cold,  but  not  frigid,  polar  sea 
existed  from  which  currents  extended  southward,  the  apparent  con- 
tradiction in  the  evidence  of  the  plants  and  animals  would  be  ex- 
plained. 

REFERENCES  FOR  CLIMATE 

KNOWLTON,  F.  H.,  —  Succession  and  Range  of  Mesozoic  and  Tertiary  Floras:  Outlines 

of  Geologic  History  (Willis  and  Salisbury),  pp.  201-207. 
LULL,  R.  S.,  —  The  Life  of  the  Connecticut  Trias:   Am.  Jour.  Science,  Vol.  33,  1912, 

PP-  397-422. 
SCHUCHERT,  CHAS.,  —  Climates  of  Geologic  Time:  Carnegie  Institution  of  Washington, 

Pub.  192,  1914,  pp.  263-298. 
WHITE  and  KNOWLTON,  —  Evidences  of  Paleobotany  as  to  Geological  Climate:  Science, 

Vol.  31,  1910,  p.  760. 

COAL 

Triassic.  —  Coal  beds  occur  in  the  four  systems  of  the  Mesozoic. 
In  the  United  States,  coal  of  the  Triassic  Age  was  worked  as  early  as 
1700  in  the  Virginia-North  Carolina  coal  fields,  but  these  deposits 
are  of  more  interest  historically  than  economically.  Coal  of  this 
age  occurs  also  in  Germany,  Sweden,  South  Africa,  and  Australia, 
and,  as  in  North  America,  is  composed  of  horsetails,  ferns,  and 
cycads.  Coal  in  commercial  quantities  occurs  in  Hungary,  in  several 
of  the  countries  of  Asia,  in  Australia,  and  New  Zealand,  in  Jurassic 
formations. 

Cretaceous.  —  The  Lower  Cretaceous  rocks  bear  coal  locally  in 
British  Columbia  and  Alaska.  The  great  coal-producing  system  of 
western  North  America  is  the  Upper  Cretaceous,  the  total  quantity 
and  extent  of  the  coal  formations  being  comparable  to  those  of  the 
Carboniferous.  The  quality  is,  however,  usually  inferior  to  that  of 
the  Carboniferous  coal,  being  largely  lignite,  although  some  bitumi- 
nous coal  of  excellent  quality  is  produced,  and  in  a  few  localities 
anthracite  coal,  made  from  bituminous  and  lignite  coal  by  the  intru- 
sion of  igneous  rocks,  is  worked.  It  is  interesting  in  this  connection 
to  note  the  presence  of  charred  wood  and  charcoal  in  some  of  the  Cre- 
taceous beds,  showing  the  existence  of  fire  during  the  period.  Al- 
though workable  coal  is  found  in  all  the  stages  of  the  Upper  Creta- 
ceous of  western  North  America,  that  of  the  Montana  and  Colorado 
stages  is  most  important.  The  so-called  Laramie  coal  has  been  found 
to  belong  largely  to  the  Montana  stage  of  the  Upper  Cretaceous  and 
to  the  lowest  stage  (Fort  Union)  of  the  Tertiary. 


CHAPTER  XXI 
CENOZOIC  ERA:  AGE  OF  MAMMALS.     TERTIARY  PERIOD 

Comparison  of  the  Life  at  the  Close  of  the  Mesozoic  and  the  Begin- 
ning of  the  Cenozoic.  — The  Age  of  Reptiles  apparently  came  abruptly 
to  a  close,  and  the  Age  of  Mammals  began.  In  the  last  stage  of  the 
Upper  Cretaceous  (Lance)  the  dinosaurs  were  in  the  "  climax  of  their 
specialization  and  grandeur."  The  bulky  Triceratops  (p.  548)  with 
his  great  horned  head,  the  amphibious  duck-bill  dinosaur  (Trachodon, 
p.  544)  as  well  as  other  armored  dinosaurs,  roamed  about  in  the  Rocky 
Mountain  region.  At  the  same  time  lived  the  swift  and  powerful 
Tyrannosaurus  (p.  540)  which  doubtless  preyed  upon  some  of  these 
herbivorous  relatives.  Associated  with  these  great  reptiles  were 
small  mammals  (p.  564)  of  lowly  organization  and  of  small  size.  "  One 
of  the  most  dramatic  moments  in  the  life  history  of  the  world  is  the 
extinction  of  the  reptilian  dynasty  which  occurred  with  apparent 
suddenness  at  the  close  of  the  Cretaceous,  the  very  last  chapter  in 
the  Age  of  Reptiles."  (Osborn.)  This  does  not  mean  that  the  reptiles 
were  wiped  out  of  existence  by  some  great  cataclysm,  but  that,  as 
measured  by  geologic  time,  the  wane  was  rapid.  What  cause  or 
causes  produced  this  great  result  cannot  be  stated  definitely. 

(i)  Change  in  vegetation  has  often  been  called  in  to  account  for  the 
extinction  of  various  groups  of  animals,  but  we  find  much  the  same 
vegetation  after  the  extinction  of  the  dinosaurs  as  when  they  were 
abundant  and  at  the  summit  of  their  specialization.  Such  trees  as 
the  fig,  banana,  sequoia,  ginkgo,  oak,  and  sycamore  passed  from  one 
period  to  the  other  without  alteration.  This  being  the  case,  a  change 
in  food,  unless  under  exceptional  conditions,  could  not  have  been  a 
cause  of  dinosaurian  extinction.  Moreover,  since  the  vegetation  re- 
mained so  nearly  the  same  at  the  critical  time,  it  is  not  probable  that 
the  climate  had  been  greatly  modified.  (2)  It  has  also  been  suggested 
that  the  cause  of  their  extinction  was  their  inability  to  compete  with 
the  more  agile  and  intelligent  mammals,  and  the  fact  that  their  young, 
not  having  the  maternal  care  of  these  higher  vertebrates,  were  easily 

572 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


573 


captured  and  destroyed  by  carnivorous  mammals.  Whatever  the 
cause  or  causes,  the  great  reptiles  —  marine,  flying,  and  terrestrial 
—  disappeared;  and  mammals  soon  occupied  all  the  places  in 
nature  formerly  held  by  them,  the  only  reptiles  surviving  being 
those  whose  habits  or  inconspicuous  form  saved  them  from  their 
competitors. 

The  Mesozoic  types  of  birds  with  toothed  jaws  and  vertebrated 
tails  (p.  560)  were  replaced  by  the  toothless  birds  with  which  we  are 
familiar. 

The  difference  between  the  invertebrate  life  at  the  close  of  the 
Mesozoic  and  at  the  beginning  of  the  Tertiary  is  not  great,  although 
the  species  are  different.  The  most  noticeable  feature,  perhaps,  is 
the  absence  of  an  abundant  and  varied  cephalopod  fauna  which  was 
so  conspicuous  in  the  Cretaceous  seas. 

Subdivisions  of  the  Cenozoic  Era.  —  The  Cenozoic  (Greek,  kainos, 
recent,  and  zoa,  life)  is  the  last  era  in  the  world's  history.  It  is  also 
called  the  Age  of  Mammals  because  of  their  predominance  and  im- 
portance from  the  beginning  of  the  era,  to,  and  including  the  present. 


Cenozoic 


Quaternary 


Tertiary 


f  Recent 

(  Pleistocene  (or  Glacial) 
Pliocene  (Greek,  pleion,  more,  and  kainos,  recent).      More 

than  half  of  the  mollusca  are  living  species. 
Miocene  (Greek,  melon,  less,  and  kainos,  recent).     Less  than 

half  of  the  mollusca  are  recent  species. 
Oligocene  (Greek,  oligos,  little,  and  kainos,  recent).      Less 

than  one  fourth  of  the  mollusca  are  recent. 
Eocene  (Greek,  eos,  dawn,  and  kainos,  recent).      With  few 

or  no  modern  species  of  mollusca. 


This  era  is  separated  into  two  periods,  Tertiary  and  Quaternary, 
the  first  lasting  until  the  appearance  of  the  great  ice  sheets  and  the 
second  from  that  time  to  the  present.  They  were  of  very  unequal 
duration,  the  former  being  several  millions  of  years  long,  the  lattec 
probably  less  than  one  million.  The  life  of  the  Tertiary  became  more 
and  more  modern  as  the  end  was  approached,  and  the  period  is  sub- 
divided into  four  epochs,  as  is  shown  by  the  above  table.  In  deter- 
mining the  age  of  the  rocks  of  the  Tertiary,  however,  the  percentage 
of  modern  species  is  not  computed,  but  the  separation  is  based  on  cer- 
tain species  which  had  a  short  life  and  are  characteristic  of  a  single 
epoch. 


574 


HISTORICAL  GEOLOGY 


PHYSICAL  GEOGRAPHY  OF  THE  TERTIARY.     EOCENE 


The  deformations  that  raised  the  Rocky  Mountains  and  drained 
the  western  interior  of  North  America  apparently  affected  the  conti- 
nent as  a  whole,  and  for  a  time  the  Atlantic  and  Pacific  coasts  were 
farther  out  than  in  the  period  under  discussion  (Fig.  529) ;  thus  por- 
tions of  the  Cretaceous  sea  bottom  were  exposed  to  erosion.  This  is 
shown  by  the  old  land  surfaces  (unconformities)  —  not,  however, 
universal  —  between  the  Eocene  and  the  underlying  formations,  on 
both  the  Atlantic  and  Pacific  borders  of  the  continent.  Since,  when 
traced  eastward,  the  Cretaceous  peneplain  disappears  beneath  Eocene 
deposits,  we  know  that  the  beginning  of  the  latter  epoch  was  marked 
by  submergence.  An  important  point  to  be  kept  in  mind  in  our  dis- 
cussion of  the  physical  geography  of  the  Tertiary  is  that  North 
America  has  been  a  relatively  stable  continent  since  the  close  of 
the  Cretaceous. 

Atlantic  and  Gulf  Coasts.  —  On  the  Atlantic  coast,  deposits  occur 
on  Marthas  Vineyard  island,  but  not  on  the  mainland  of  New  England 
or  Canada  (Newfoundland  was  probably  a  part  of  the  continent  at 
this  time),  and  extend  from  New  Jersey  into  Texas,  by  way  of  Ala- 
bama and  Mississippi,  then  up  to  the  mouth  of  the  Ohio  River  and 
thence  southwest.  The  Atlantic  deposits  of  this  period  are  usually 
loose  and  incoherent  sands,  clays,  and  green-sand  marls,  derived 
largely  from  the  Cretaceous  formations  but  also  to  some  extent  from 
older  formations.  In  the  Gulf  regions  the  rocks  are  more  consolidated, 
sandstones,  limestones,  and  shales  being  common.  Extensive  lignite 
deposits  occur  in  Texas  and  Louisiana,  which  may  become  valuable 
at  some  future  day  when  bituminous  coal  is  more  costly  than  now. 
These  lignite  beds  were  formed  from  the  peat  bogs  that  existed  on 
poorly  drained  portions  of  the  low-lying  coast,  just  as  peat  is  being 
formed  in  similar  regions  to-day. 

Pacific  Coast.  —  In  the  western  portions  of  the  continent  the  rocks 
of  the  period  are,  for  the  most  part,  sandstones  and  shales,  with  oc- 
casional conglomerates  and  tuffs,  which  rest  unconformably  on  the 
older  rocks  in  many  places,  but  in  others  are  conformable,  the  divi- 
sion being  determined  by  the  change  in  the  fauna.  The  diatoma- 
ceous  shales  which  occur  at  the  top  of  the  series  (in  the  vicinity  of 
Coalinga,  California)  should  be  mentioned,  since  they  are  believed 
to  be  the  source  of  important  deposits  of  petroleum. 

During  the  early  part  of  the  Eocene,  marine  conditions  prevailed 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


S7S 


over  a  considerable  territory,  but  these  later  gave  way  to  brackish 
or  fresh-water  swamp  conditions.  The  physical  history  during  the 
latter  part  of  the  period  is  one  of  persistent  but  frequently  interrupted 
submergence,  in  which  the  alternation  of  many  coal  beds  (some  work- 
able) with  deposits  of  fine  shale  and  coarse  sandstones  indicates  that, 
during  this  great  subsidence,  the  depth  of  the  water  frequently 
changed.  At  times  the  sinking  proceeded  more  rapidly,  and  the 
deepened  water  was  then  filled  with  sediment,  "  until  the  tide-swept 
flats  became  marshes  and,  for  a  time,  vegetation  flourished  vigorously 
in  the  moist  lowlands"  (Willis),  this  rotation  being  repeated  inter- 
mittently. This  condition  is  believed  to  have  prevailed  in  Alaska, 
western  Oregon,  and  the  Great  Valley  of  California.  Most  of  the  coal 
of  the  west  coast  belongs  to  this  epoch,  making  this  the  "  Eocene 
Carboniferous"  of  the  west.  In  the  later  Eocene,  elevation  and  ero- 
sion, accompanied  by  volcanic  outbursts  and  extensive  lava  flows, 
occurred  in  Oregon  and  Washington.  The  presence  of  Atlantic 
species  in  the  marine  deposits  shows  that  an  oceanic  connection, 
probably  in  the  Central  American  region,  was  in  existence  for  a  time. 
Western  Interior.  —  The  Eocene  deposits  of  the  western  interior 
(Fig.  529),  with  the  exception  of  a  few  small  areas  in  Colorado,  are  con- 
fined to  the  region  between  the  Sierra  Nevadas  and  the  Rocky  Moun- 
tains. It  is  thought  that  the  region  under  discussion  was  not  greatly 
elevated  above  sea  level,  although  the  summits  of  the  mountains 
probably  stood  sufficiently  high  above  the  general  level  of  the  plains 
to  permit  the  vigorous  erosion  which  was  in  progress  during  the  epoch, 
and  which  furnished  the  waste  to  form  a  great  thickness  of  sediments. 
The  mountains  and  hills,  formed  by  folding,  by  faulting,  by  warping, 
and  by  volcanic  debris,  inclosed  basins  and  valleys  in  which  the 
streams  deposited  the  sediments  obtained  from  the  steep  slopes  of 
the  higher  lands.  These  sediments  were  deposited  partly  in  lakes 
and  partly  in  alluvial  fans  in  front  of  the  valleys  which  the  streams 
had  cut  in  the  mountain  slopes.  The  most  important  deposits, 
however,  were  laid  down  in  flood  plains,  in  deltas,  and  in  swamps. 
From  time  to  time  the  area  of  deposition  shifted,  because  of  the  fill- 
ing up  of  old  basins  or  the  warping  of  the  land.  Lakes  were  also  in 
existence,  the  most  famous  being  one  in  Wyoming  in  which  the  Green 
River  formation  occurs,  consisting  of  impure  limestone  and  thin, 
fissile  calcareous  shales,  often  as  thinly  laminated  as  paper.  Be- 
tween the  leaves  of  these  shales  remains  of  plants,  insects,  and  fishes 
are  beautifully  preserved,  but  no  remains  of  mammals  are  found, 

CLELAND   GEOL. — 37 


576 


HISTORICAL  GEOLOGY 


except  in  the  form  of  footprints.  Since  these  sediments  were  depos- 
ited in  more  or  less  isolated  basins,  the  work  of  correlating  them  with 
each  other,  and  especially  with  the  marine  deposits  of  the  Atlantic 
and  Pacific,  has  been  difficult. 

The  Eocene  was  an  epoch  of  great  coal  formation,  especially  during 
the  earlier  portion   (Fort  Union).     The  great  lignite  deposits  that 

cover  one  half  of 
North  Dakota  were 
formed  at  this  time, 
as  were  also  exten- 
sive areas  in  Wy- 
oming and  Montana. 
The  Eocene  was 
brought  to  a  close  by 
crustal  movements 
of  some  importance 
which,  on  the  Pacific 
coast,  resulted  in  the 
draining  of  certain 
areas  and  the  lower- 
ing of  others  to  below 
sea  level.  In  the 
same  region  some 
mountain  ranges 
(Klamath)  were  again 
bowed  up  to  some 
extent,  and  others 
(Coast  Ranges)  began 
their  development. 
In  the  Great  Plains 
region  the  changes 
were  such  as  to  bring 
about  aggradation 
where  degradation 

had  formerly  prevailed.  The  interior  mountain  region  of  the  west 
was  elevated  and  drained,  and  in  subsequent  epochs  was  not  a  region 
of  extensive  deposition.  The  eastern  coast  remained  much  as  before. 
A  narrow  strip  of  land  was  added  to  both  the  Atlantic  and  Gulf  coasts. 
Eocene  of  Other  Continents.  —  The  evolution  of  the  continents  of 
Europe  and  Asia  was  not  so  far  advanced  at  the  beginning  of  the 


FIG.  529.  —  Map  showing  the  probable  outline  of 
North  America  during  a  portion  of  the  Eocene.  The 
continental  deposits  are  shown  in  solid  black.  (Modified 
after  Schuchert.) 


CENOZOIC  ERA:    AGE  OF   MAMMALS 


577 


Eocene  as  that  of  North  America.  Seas  covered  large  areas  that  are 
now  land,  and  there  were  probably  extensive  land  masses  which  are 
now  covered  by  the  ocean.  Europe  was  smaller  than  at  present  and 
at  times  was  entirely  separated  from  Asia  by  a  narrow  sea  on  the 
east  side  of  the  Ural  Mountains.  The  most  marked  feature  of  the 
Eocene  European  continent  was  the  greatly  expanded  Mediterranean 
Sea  which,  with  its  extensive  arms,  covered  the  sites  of  the  conspicu- 
ous mountains  of  the  present :  the  Pyrenees,  Apennines,  Alps,  and 
Urals.  Above  the  surface  of  this  sea  numerous  islands  probably  stood 
on  the  sites  of  some  of  the  ranges.  The  greater  part  of  Spain  seems 
to  have  been  separated  for  a  time  from  the  mainland  by  a  sea  which 
also  covered  a  portion  of  southern  France. 

Not  only  Europe,  but  Asia  and  Africa  as  well,  were  far  from  having 
attained  their  present  outlines.  The  greater  part  of  Africa  north  of 
the  equator  was  under  water,  and  an  extension  of  the  Mediterranean 
Sea  reached  to  the  Indian  Ocean.  Portions  of  Australia,  New  Zea- 
land, Patagonia,  and  the  West  Indies  were  also  submerged. 

In  portions  of  Europe  and  Africa  a  great  thickness  of  limestone, 
made  up  of  large  Foraminifera  (nummulites,  p.  626),  was  deposited ; 
besides  which,  an  immensely  thick  mass  of  sandstone  and  shale  which 
now  outcrops  on  the  Alps  was  also  laid  down.  The  nummulitic  lime- 
stone was  largely  used  in  the  construction  of  the  pyramids  of  Egypt. 
The  Eocene  strata  have  since  been  raised  to  great  heights,  as  is  indi- 
cated by  their  presence  on  the  Tibet  Plateau  at  an  altitude  of  20,000 
feet,  in  the  Himalayas  16,000  feet  above  the  sea,  as  well  as  high  up  on 
the  Alps,  Pyrenees,  Caucasus,  and  other  mountain  ranges. 

It  will  be  seen  from  the  above  imperfect  history  that  the  outlines 
and  mountainous  regions  of  Europe  and  Asia  were  very  different 
during  the  Eocene  from  what  they  are  to-day. 

OLIGOCENE 

The  Oligocene,  which  followed  the  Eocene,  is  sometimes  included  in 
the  latter,  but  is  usually  separated  from  it  because  of  the  distinctness 
of  the  two  series  in  Europe,  and  also  because  they  can  be  readily 
separated  in  North  America  whenever  fossils  occur. 

Atlantic  and  Gulf  Coasts.  —  The  Oligocene  does  not  have  a  wide 
distribution  on  the  Atlantic  coast,  but  is  well  represented  in  the  Gulf 
region,  where  2000  feet  of  strata,  rich  in  marine  invertebrates,  occur. 
The  Oligocene  in  these  regions  rests  upon  the  Eocene  without  a  break, 
the  two  series  being  distinguished  by  a  change  in  fauna,  A  great  de- 


5/8  HISTORICAL  GEOLOGY 

velopment  of  marls  and  limestone  of  this  age  in  Central  America  and 
the  West  Indies  shows  that  submergence  was  widespread  in  these 
regions.  An  island  was  raised  in  northern  Florida  early  in  the  epoch 
which,  by  further  arching  of  the  sea  bottom,  became  joined  to  the 
mainland  in  the  Miocene. 

Western  Interior.  —  On  the  Great  Plains  region  continental  de- 
posits of  this  epoch  occur  at  various  points  from  British  Columbia 
to  Mexico,  and  outcrop  from  two  to  three  hundred  miles  east  of  the 
Rocky  Mountains.  They  seldom  rest  upon  the  Eocene,  but  on  the 
worn  surfaces  of  the  Upper  Cretaceous,  showing  that  while  deposi- 
tion was  taking  place  in  the  mountain  basins  of  the  Eocene,  the  re- 
gion of  the  Great  Plains  was  an  open,  rolling  country,  traversed  by 
streams  which  were  degrading  its  surface.  "  A  picture  of  the  plains 
region  in  Oligocene  times  is  that  of  broad,  gentle,  eastward  slopes  from 
the  Rocky  Mountains,  plane  or  gently  undulating  and  not  moun- 
tainous, bearing  broad  streams  with  varying  channels,  sometimes 
spreading  into  shallow  lakes,  but  never  into  vast  fresh-water  sheets. 
Savannahs  were  interspersed  with  grass-covered  pampas  traversed 
by  broad,  meandering  rivers.  This  land  was  dry  in  dry  seasons,  but 
was  flooded  in  very  high-water  periods.  The  materials  were  partly 
erosion  products  of  the  Rocky  Mountains  and  Black  Hills,  such  as 
true  sandstones  and  conglomerates,  but  they  include  also  fine  layers 
of  volcanic  dust,  wind-borne  from  distant  craters  in  the  mountains, 
far  out  on  the  plains  of  Nebraska  and  Kansas."  (Osborn.) 

Pacific  Coast.  —  On  the  Pacific  coast  the  Oligocene  was  an  epoch  of 
elevation  and  erosion,  during  which  the  land  was  not  high  except  in 
a  few  places,  as  is  indicated  by  the  fine  character  of  most  of  the  sedi- 
ments. The  areas  of  deposition  on  what  is  now  land  were  compara- 
tively small. 

Oligocene  of  Other  Continents.  —  In  general,  the  distribution  of 
land  and  water  was  different  in  the  Oligocene  from  what  it  was  in  the 
preceding  epoch.  One  important  transgression  of  the  sea  covered 
Germany  and  Belgium  and  at  the  time  of  greatest  extension  joined 
the  North  Sea  with  the  Mediterranean  and  Aral  seas.  In  France 
and  Russia  large  areas  were  beneath  the  water.  In  the  Paris  basin 
the  presence  of  salt  and  gypsum  furnishes  a  clue  to  the  climate  during 
a  portion  of  the  epoch.  In  various  parts  of  Europe  (Germany,  Switz- 
erland, southern  France,  and  Bavaria)  extensive  swamps  were  present 
in  which  were  accumulated  the  lignite  deposits  that  are  now  workable 
to  some  extent. 


CENOZOIC   ERA:    AGE  OF   MAMMALS 


579 


MIOCENE 

The  outline  of  North  America  was  practically  the  same  in  the 
Miocene  (Fig.   530)   as   in   the   Eocene,  with   the  exception   of  the 
Mississippi  embayment  which  was  reduced  in  size,  and  the  Florida 
peninsula  which  was 
formed   later  in  the 
epoch. 

Atlantic  and  Gulf 
Coasts.  —  On       the 
Atlantic     and     Gulf 
coasts  the  strata  rest 
—  often  unconform- 
ably — on  the  Eocene 
or  Oligocene,  and,  in 
general,   occur   in    a 
narrow,    interrupted 
belt   parallel   to  the 
older      formations 
from  Marthas  Vine- 
yard southward.  The 
Miocene     strata     in 
some  localities  over- 
lap   the    Eocene    to 
landward,    com- 
pletely concealing  it. 
The  sediments  on  the 
Atlantic    coast    con- 
sist chiefly  of  sands, 
clays,       and       marls,          ^IG-    53°-  —  Map    showing    the    probable    outline    of 
with  occasion^   hrrk    N°rth  America  during  a  Portion  of  the  Miocene.     The 
Wltn  occasional  beds    continental  deposits  are  shown  in  solid  black.     (Modified 
of      diatomaceous    after  Schuchert.) 
earth  from  30  to  40 

feet  thick.  In  Florida,  Georgia,  and  in  the  Gulf  region  limestones 
are  the  rule.  The  deposits  of  this  epoch  on  the  Atlantic  and  Gulf 
coasts  are  comparatively  thin,  being  only  700  feet  thick  in  New 
Jersey,  400  in  Maryland,  and  even  less  in  North  Carolina. 

Economic  Products  of  the  Miocene.  —  The  economic  products  of 
the  strata  of  this  time  are  the  phosphates  of  Florida,  the  oil  of  Louisi- 
ana, and  the  diatomaceous  earth  of  the  Atlantic  coast. 


580  HISTORICAL  GEOLOGY 

Diatomaceous  earth  resembles  chalk  in  color,  but  is  lighter  in  weight, 
and,  since  it  is  composed  of  silica,  does  not  effervesce  with  acids.  On 
account  of  the  hardness  of  its  constituent  parts  and  its  extreme  fine- 
ness, it  is  used  as  a  base  in  the  manufacture  of  preparations  for  clean- 
ing and  polishing  silver,  nickel,  etc.1  Since  it  is  porous,  it  has  been 
used  as  an  absorbent  for  nitroglycerin  in  the  manufacture  of  dyna- 
mite. It  is  also  used  as  a  non-conductor  of  heat. 

The  valuable  phosphate  deposits  of  Florida  are  believed  by  some 
investigators  to  have  originated  by  the  leaching  of  guano,  or  bone 
beds,  and  the  deposition  of  the  phosphate  in  the  underlying  limestone, 
either  by  precipitation  in  the  pores  of  the  rock  or  by  replacing  the 
limestone  molecule  by  molecule.  The  phosphate  may,  however,  have 
been  disseminated  through  the  beds  in  small  quantities  and  later 
concentrated  as  the  more  soluble  limestone  was  dissolved  and  carried 
away. 

Western  Interior.  —  In  the  Great  Plains  region  east  of  the  Rocky 
Mountains,  the  conditions  traced  in  the  Oligocene  continued,  and  were 
probably  not  unlike  those  now  prevalent  where  the  flood  plains  of 
the  upper  Paraguay,  Amazon,  and  Orinoco  rivers  of  South  America 
are  confluent.  In  this  portion  of  South  America  is  a  region  larger  than 
that  occupied  by  the  Miocene  deposits  of  North  America,  with  all  the 
conditions  necessary  for  the  deposition  and  present  distribution  of 
sandstones,  clay,  and  conglomerates,  together  with  the  preservation 
of  animal  remains.  North  American  Miocene  formations  are  found 
from  Montana  into  Texas,  although  largely  covered  to  the  south  and 
east  by  later  deposits.  Sediments  of  this  age  occur  also  in  Montana, 
Nevada,  Colorado,  Oregon,  British  Columbia,  and  Alaska. 

A  lake  existed  in  Colorado  at  this  time  (Florissant)  which  is  inter- 
esting because  of  the  excellent  preservation  of  many  insects  and 
plants  in  its  deposits.  It  lay  in  a  narrow  valley  in  the  vicinity  of 
active  volcanoes,  whose  numerous  eruptions  spread  ashes  over  its 
surface,  burying  the  insects  and  plants  which  had  been  carried  into  it. 

Pacific  Coast.  — The  restricted  seas  of  the  Oligocene  on  the  Pacific 
coast  were  much  expanded  during  the  Miocene,  although  at  no  time, 
as  will  be  seen  by  consulting  the  map  (Fig.  530),  was  a  large  portion 
of  what  is  now  land  in  that  region  submerged.  The  southern  portion 
of  the  Great  Valley  of  California  (San  Joaquin)  was  beneath  the  sea 
early  in  the  epoch  (Vaqueros),  and  in  this  bay  a  great  thickness  of 
marine  sediments,  consisting  of  sands  and  clays  with  some  conglom- 
1  Volcanic  ash  is  also  used  for  this  purpose. 


CENOZOIC  ERA:    AGE  OF  MAMMALS  581 

crates,  was  laid  down.  The  variation  in  the  lithological  character 
of  the  deposits  within  short  distances  is  believed  to  have  been  caused 
by  the  rather  local  elevation  of  land  due  to  faulting  and  subsequent 
stream  rejuvenation.  The  California  earthquake  rift  (Fig.  275,  p.  277) 
is  first  known  to  have  been  a  plane  of  movement  at  this  time.  This 
early  (Vaqueros)  sedimentation  was  followed  by  the  deposition  of 
sandstone,  volcanic  ash,  and  limestone,  and  a  great  thickness  of  diato- 
maceous  material  (Monterey).  "  It  was  an  age  of  diatoms.  These 
small  marine  plants  lived  in  extreme  abundance  in  the  sea  and  fell  in 
showers  with  their  siliceous  tests  to  add  to  the  accumulating  ooze 
of  the  ocean  bottom,  just  as  they  are  forming  ooze  at  the  present  day 
in  some  oceanic  waters.  It  is  well  known  that  diatoms  multiply  with 
extreme  rapidity.  It  has  been  calculated  that,  starting  with  a  single 
individual,  the  offspring  may  number  one  million  within  a  month. 
One  can  conceive  that  under  very  favorable  life  conditions  such 
as  must  have  existed,  the  diatom  frustules  may  have  accumulated 
rapidly  at  the  sea  bottom  and  aided  the  fine  siliceous  and  argillaceous 
sediments  in  the  quick  building-up  of  the  thick  deposits  of  the  Middle 
Miocene  time,  some  of  which  are  a  mile  through.  These  diatoma- 
ceous  shales  are  the  source  of  some  of  the  richest  petroleum  deposits 
of  California."  (Arnold.) 

During  much  of  the  early  portion  of  the  Miocene  and  continuing 
somewhat  later,  faulting,  folding,  and  volcanic  outbursts  of  consider- 
able magnitude  occurred.  Great  volcanoes  were  active  from  Wash- 
ington and  Oregon  along  the  Pacific  ranges  of  California,  almost  as 
far  south  as  Los  Angeles.  During  the  middle  of  the  period  mountain 
building  and  great  local  deformations  took  place,  the  effects  of  which 
were  felt  from  Puget  Sound  to  southern  California.  Extensive  fault- 
ing along  the  earthquake  rift  and  other  fault  zones  occurred,  while 
in  other  regions,  low,  broad  folds  were  formed.  The  combined  result 
was  the  uplift  of  the  Coast  Ranges  of  California  and  Oregon  to  an  alti- 
tude of  several  thousand  feet.  The  Cascades  of  Washington  were 
also  increased  in  height.  This  stage  of  diastrophism  was  followed  by 
subsidence  (Upper  Miocene),  as  a  result  of  which  the  northern  part  of 
the  Great  Valley  (Sacramento)  was  submerged  and  in  it  were  depos- 
ited sands  and  clays  and  beds  of  diatoms. 

By  the  close  of  the  Miocene  the  Klamath  and  Sierra  Nevada  moun- 
tains were  peneplained,  the  material  derived  from  them  having  been 
deposited  in  the  Great  Valley  and  coastal  belt  of  northern  California, 
forming  the  thick  Tertiary  strata  now  found  there.  These  strata  are 


582  HISTORICAL  GEOLOGY 

composed  of  8000  feet  of  sediments,  largely  belonging  to  the  Upper 
Miocene,  as  well  as  an  equal  amount  from  the  earlier  stages.  Volcanoes 
had  practically  ceased  to  be  active  over  a  large  portion  of  the  territory, 
but  were  probably  still  in  eruption  in  some  localities.  The  Miocene 
deposits  on  the  Pacific  coast  are  much  folded;  and  some  are  even 
overturned,  being  in  marked  contrast  in  this  particular  to  those  of  the 
Atlantic  and  Gulf  coasts,  which  are  nearly  in  the  position  they  had 
when  first  laid  down. 

In  addition  to  the  marine  sediments  just  discussed,  continental 
deposits,  consisting  of  sands  and  clays  with  some  iron  and  coal,  were 
being  laid  down  during  the  Lower  Miocene  in  the  northern  part  of 
the  Great  Valley.  From  the  western  flanks  of  the  Sierra  Nevadas 
auriferous  gravels  were  carried  down  by  the  streams  and  dropped  in 
their  beds  during  portions  of  the  period,  producing  the  "  deep  au- 
riferous gravels  "  (Fig.  359  B,  p.  374)  and  later  the  "  bench  gravels," 
some  of  which,  as  now,  were  buried  beneath  streams  of  lava  and  beds 
of  tuff. 

Mountain  Building.  —  Before  the  close  of  the  epoch  the  upheaval 
of  the  Coast  Ranges  of  California  and  Oregon  and  the  Cascades  of 
Washington  occurred ;  the  fault  along  the  east  of  the  Sierra  Nevadas 
was  made ;  the  growth  of  the  present  Sierra  Nevadas  was  begun  and, 
as  will  be  seen  later,  many  of  the  great  mountain  ranges  of  the  world 
were  elevated.  During  this  epoch,  too,  the  plateaus  of  Utah  and  Ari- 
zona were  raised  so  as  to  permit  the  Colorado  River  to  begin  the 
excavation  of  its  great  canyon.  The  rugged  scenery  so  characteristic 
of  the  west  is  the  result  of  elevation  which,  for  the  most  part,  began 
at  this  time. 

Basis  for  Separation  into  Periods.  —  In  the  discussion  of  eras  and  periods  attention 
has  frequently  been  called  to  the  fact  that  they  were  brought  to  a  close  by  deforma- 
tions, some  great  and  some  small,  which  produced  mountain  ranges,  or  raised  or 
lowered  large  areas  of  the  earth's  surface.  We  have  just  seen,  however,  that  one 
of  the  great  times  of  mountain  building  occurred,  not  at  the  end  of  an  era,  nor  the 
close  of  a  period,  but  in  the  midst  of  an  epoch.  It  should  also  be  remembered  that 
climatic  and  other  changes  thus  produced  had  little  effect  on  the  contemporary  life  of 
the  time.  In  other  words,  the  separation  of  the  history  of  the  earth  into  chapters 
should  be  based,  not  upon  the  unconformities,  however  great,  but  upon  the  changes 
which  the  life  has  experienced.  Fortunately,  as  should  be  expected,  because  of  the 
effect  of  the  physical  conditions  upon  animals  and  plants,  the  sediments  laid  down 
during  eras  and  periods  are  usually  to  be  separated,  not  only  by  the  rather  sudden 
extinction  of  many  species  and  the  appearance  of  new  ones,  but  by  unconformities  as 
well.  The  problem  is  not,  however,  a  simple  one.  When,  for  example,  a  continent 
has  been  isolated  for  long  ages,  the  animals  and  plants  living  on  it  may  be  largely 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


583 


of  forms  that  belong  to  a  previous  epoch  in  other  parts  of  the  earth,  just  as  in  an 
age  of  electricity  and  cement  some  isolated  Indian  tribes  are  still  living  in  the  Stone 
Age. 

Igneous  Activity.  —  Perhaps  no  other  period  in  the  history  of  the 
earth  since  Pre-Cambrian  times  displayed  such  extraordinary  volcan- 
ism  as  the  Tertiary,  and  of  the  four  epochs  of  the  period,  the  Miocene 
was  by  far  the  most  important  in  this  particular. 

It  has  already  been  seen  that  the  great  volcanic  outbursts  of  the 
Pacific  coast  occurred  during  the  Miocene  —  especially  during  the 
middle  of  that  epoch  —  covering 
that  region  of  North  America 
with  ash  which  furnished  the 
material  for  a  great  thickness  of 
sedimentary  deposits.  Not  only 
on  the  Pacific  coast,  but  perhaps 
in  every  state  west  of  the  Rocky 
Mountains,  some  evidence  of  the 
igneous  activity  of  this  time  can 
be  found.  It  was  during  this 
period  that  a  great  quantity  of 
lava  and  ash  was  poured  into 
the  basin  of  the  Yellowstone 
National  Park.  Some  of  the 
forests  that  were  buried  in  the 
ash  at  that  time  were  later  petri- 
fied and  have  been  partially  un- 
covered by  erosion.  Seventeen  -  c  .  f  ,  ,  r  - 
.;:  FIG.  531.  —  Section  of  the  north  face  of 

such  petrified  forests,  one  above  Amethyst  Mountain,  Yellowstone  National 
the  Other,  may  be  seen  in  one  Park.  Seventeen  or  more  successive  forests 

serfmn  (Vitr    c-21^  were  covered   with  volcanic  ash  and   the 

sect  on  \js  ig.  531;.  ^      logs  petrified     About  two  thousand  feet  of 

The   greatest    area   of   lava    in    strata  are  shown.     (After  W.  H.  Holmes.) 
North  America  covers  a  region  of 

between  200,000  and  300,000  square  miles  in  Washington,  Oregon, 
Idaho,  and  California  (Fig.  304,  p.  311),  and  is  known,  from  the  ex- 
posures on  faulted  and  tilted  blocks,  to  have  a  maximum  thickness  of 
at  least  5000  feet.  By  far  the  largest  bulk  of  this  was  outpoured 
during  the  Miocene.  This  enormous  mass  of  lava  was  built  up  by 
successive  lava  flows  averaging  about  75  feet  in  thickness.  On  the 
canyon  walls  some  of  the  sheets  are  seen  to  be  separated  by  old  soil 
beds,  showing  that  the  former  lava  surface  had  been  exposed  to  the 


584  HISTORICAL  GEOLOGY 

action  of  the  weather  so  long  as  to  be  disintegrated  to  great  depths 
before  the  overlying  lava  was  outpoured.  Lake  beds,  in  one  case 
1000  feet  thick,  also  rest  upon  one  sheet  and  are  covered  by  another. 
Although  the  Snake  and  Columbia  rivers  have  canyons  that  reach 
a  depth  of  several  thousand  feet,  they  have  not  yet  succeeded  in  cut- 
ting their  way  to  the  base,  except  where  they  encounter  the  summits 
of  the  mountains  buried  beneath  the  flood  of  molten  rock,  or  near  the 
margin  of  the  flow  where  it  is  thinnest. 

Near  the  edge  of  the  lava  plateau  water  is  sometimes  obtained 
from  artesian  wells,  which  have  been  sunk  to  the  sheets  of  sand  and 
gravel  spread  by  rivers  from  the  surrounding  mountains  upon  the 
earlier  lava  flows  whose  surfaces  were  afterwards  covered  by  late 
lavas.  However,  no  water  can  be  obtained  in  this  way  over  large 
areas,  because  the  porous  lava  permits  the  water  to  percolate  down 
to  great  depths,  where  it  appears  as  springs  far  down  in  the  canyons. 
Because  of  the  constant  and  uniform  supply  of  water  thus  obtained, 
the  volume  of  the  rivers  fluctuates  less  than  in  almost  any  other 
part  of  the  continent. 

Miocene  of  Other  Continents.  —  The  seas  that  overspread  Ger- 
many and  Belgium  in  the  Oligocene  were  withdrawn  during  the  Mio- 
cene, but  those  of  southern  Europe  not  only  remained  extensive,  but 
were  so  increased  in  size  as  to  make  that  region  an  archipelago.  With 
the  exception  of  bays  in  Portugal  and  France  and  the  submergence  of 
the  low  lands  bordering  the  North  Sea,  the  shores  of  western  Europe 
appear  to  have  extended  further  west  than  now.  Southern  Spain  was 
joined  to  Africa,  probably  by  a  wide  land  connection,  but  was,  in  turn, 
separated  from  northern  Spain  by  a  strait.  An  important  and  ex- 
tensive sea  stretched  from  Vienna  to  the  region  of  the  Black  and  Aral 
seas. 

The  Miocene  was  a  period  of  great  mountain  building  in  the  Old 
World  as  well  as  in  the  New.  The  Alps  were  upheaved  and  reached 
nearly  their  present  altitude  at  this  time.  The  elevation  which  pro- 
duced them  excluded  the  sea  and  formed  basins  in  which  rested  in- 
land seas  and  lakes  where  are  preserved  a  record  of  the  terrestrial  life 
of  the  time.  The  Apennines  were  reelevated  late  in  the  Miocene ; 
and  the  Caucasus,  on  which  Miocene  strata  occur  at  altitudes  of 
6000  feet,  also  date  from  this  epoch.  The  Himalayas  were  raised 
either  at  this  time  or  in  the  Eocene. 

Volcanism,  so  stupendous  in  North  America  at  this  time, 
seems  to  have  been  of  little  importance  in  Europe,  although  some 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


585 


of  the  movements   appear  to  have  been  accompanied  by  igneous 
activity. 

The  presence  of  extensive  Miocene  beds  in  Australia,  New  Zealand, 
north  Africa,  and  elsewhere  tell  their  story  of  submergence. 


PLIOCENE 

Atlantic  and  Gulf  Coasts.  —  With  a  few  exceptions,  the  eastern 
coast  of  North  America  had  practically  the  same  position  in  the  Plio- 
cene (Fig.  532)  as 
now.  The  Atlantic 
coast  from  New  York 
northward  extended 
farther  out  than 
at  present;  Florida 
was,  for  the  most 
part,  under  water; 
and  a  very  narrow 
belt  along  the  Gulf 
coast  from  Florida 
to  Texas  and  another 
in  Mexico  were  sub- 
merged. This  being 
the  case,  the  con- 
spicuous deposits  are 
naturally  those  laid 
down  upon  the  land, 
the  marine  sediments 
being  now  chiefly 
hidden  from  view 
beneath  the  sea.  The 
comparatively  wide 
distribution  of  these 
continental  sedi-  FIG.  532. -Map  showing  the  probable  outline  of 
,  .  ,  North  America  during  a  portion  of  the  fliocene.  Ine 
mentS  IS  due  in  the  continental  deposits  are  shown  in  solid  black.  (Modified 


first    place    to    their     after  Schuchert.) 

recent    age,    and    in 

the  second   place  to  the  fact  that  some  of  them   occupy  sites  of 

continued   deposition,  as,  for  example,  in  the  Great   Basin  region. 

These  deposits  have  an  origin  similar  to  those  of  previous  epochs 


586  HISTORICAL  GEOLOGY 

already  discussed.  Streams  debouching  from  mountainous  lands 
dropped  their  sediments  upon  reaching  a  low  gradient,  making  alluvial 
fans  and  plains.  On  account  of  the  reduction  of  their  volumes 
through  evaporation  and  seepage,  the  rivers  developed  great  flood 
plains.  Shallow  lakes  which  existed  at  that  time,  formed  either  by 
warping  or  by  the  choking  up  of  river  channels  by  deposits  of  sand 
and  gravel,  were  later  filled  with  sediments. 

Mention  should  be  made  of  a  series  of  deposits  (Lafayette)  of  Ter- 
tiary age,  the  exact  status  of  which  is  yet  in  doubt  (formerly  sup- 
posed to  be  Pliocene,  but  some  of  which  are  Oligocene)  which  have  an 
extensive  distribution,  occurring  in  many  places  on  the  Atlantic  and 
Gulf  coastal  plains  in  the  southern  portion  of  the  Mississippi  Valley 
up  to  southern  Illinois,  and  in  the  valleys  west  of  the  Appalachians. 
This  formation  (Lafayette  or  Orange  Sand)  commonly  has  a  thick- 
ness of  20  to  30  feet,  and  is  composed  of  gravel  and  sand  in  the  lower 
Mississippi  Valley  and  of  clay  and  silt  over  large  areas  of  the  uplands 
east  of  the  Mississippi  River.  It  was  derived  from  the  insoluble 
residue  of  older  formations  and  consists  of  chert,  quartz  pebbles,  and 
other  insoluble  materials.  The  color  varies,  but  is  often  red,  orange, 
or  yellow.  This  deposit  was,  probably,  formed  as  follows.  The  pene- 
planation  and  subsequent  weathering  of  the  land  surfaces  during  the 
early  stages  of  the  Tertiary  produced  a  layer  of  loose,  insoluble  mate- 
rial. In  the  Oligocene  an  upwarping  along  the  axis  of  the  Appalachians 
began  and  increased  during  the  epoch.  As  a  result,  the  rejuvenated 
streams  carried  much  detritus  and  dropped  a  part  of  it  upon  reaching 
the  lower  lands.  With  the  continued  rise  of  the  mountain  belt  and 
adjacent  regions,  the  streams  removed  the  sediments  first  laid  down, 
and  redeposited  them  farther  downstream.  The  sands  and  gravel 
deposited  not  only  filled  up  the  lower  portions  of  the  valleys,  but  also, 
to  some  extent,  covered  the  former  divides.  At  present,  much  of  the 
formation  has  disappeared  in  regions  of  strong  erosion  and,  seaward, 
is  more  or  less  concealed  by  younger  beds.  In  some  places  it  caps 
divides  but  is  absent  from  the  valleys. 

The  marine  deposits  have  a  very  limited  distribution  on  the  east 
coast  and  are  of  little  thickness,  being  most  important  in  Florida. 

Western  Interior.  —  The  Pliocene  deposits  of  the  western  interior 
are  widely  scattered  and  of  limited  extent.  Beds  of  this  epoch  have 
been  recognized  in  Kansas,  Nebraska,  Oregon,  and  the  Staked  Plain 
of  Texas.  As  already  stated,  it  is  probable  that  much  of  the  Great 
Basin  and  other  regions  is  underlain  by  Pliocene  deposits. 


CENOZOIC  ERA:    AGE  OF  MAMMALS  587 

Pacific  Coast.  —  Deposits  of  this  age  are  less  widespread  on  the 
Pacific  coast  than  those  of  the  Miocene.  A  change  from  marine  to 
fresh-water  conditions  in  a  portion  of  the  area  may  have  been  due  to 
a  raising  of  the  land  near  the  coast,  or  to  an  elevation  along  faults 
which  excluded  the  sea.  Volcanic  activity  took  place  during  the 
period  in  certain  portions  of  northern  and  central  California,  and  in 
the  Sierra  Nevadas  and  Cascades. 

Pliocene  Elevation.  —  The  deformation  of  the  peneplain  of  the 
Appalachian  region  raised  the  Coastal  Plain  and  shifted  the  coast  line 
to  the  east,  except  in  Florida,  where  there  was  a  slight  depression.  It 
is  possible  that  during  this  period  of  elevation  the  now  submerged 
valleys  of  the  St.  Lawrence,  Hudson,  Delaware,  and  Mississippi  were 
eroded. 

The  plateau  region  of  the  west  was  uplifted  at  various  times  prior 
to  the  Pliocene,  during  that  epoch,  and  later,  and  has  since  been  in- 
trenched to  form  the  great  canyons  for  which  it  is  famous. 

Near  the  close  of  the  Pliocene  the  Rocky  Mountains  and  the  Sierra 
Nevadas  began  a  period  of  growth  which  has  given  them  their  present 
altitude.  Instead  of  folding,  as  at  the  close  of  the  Mesozoic,  the  ele- 
vation was  chiefly  due  to  warping  and  faulting.  A  study  of  a  cross 
section  of  the  Sierra  Nevadas  brings  out  the  fact  that  the  slope  on 
the  west  is  long  and  gradual  and  deeply  intrenched  by  such  great 
valleys  as  the  Yosemite  and  Hetch  Hetchy,  while  on  the  east  it 
is  very  abrupt  and  short.  This  marked  difference  in  the  eastern  and 
western  slopes  is  due  to  a  profound  fault  on  the  east,  which  was  first 
formed  in  the  Miocene,  along  which  an  enormous  block  was  raised 
and  tilted  to  the  west,  leaving  its  eastern  edge  to  form  the  crest  of  the 
range.  The  movement  along  the  fault  plane  has  apparently  not  yet 
ceased,  as  is  shown  by  a  slip  of  25  feet  which  occurred  in  1872.  The 
raising  of  the  Sierra  Nevadas  inclosed  the  Great  Basin  region,  shutting 
off  the  moist  winds  of  the  Pacific  and  making  it  a  desert. 

During  the  Pliocene  the  Cascade  Mountains  seem  to  have  been 
peneplained,  the  mountain  mass  being  raised  shortly  before  or 
after  its  close.  The  rugged  scenery  of  these  mountains  is  the  result 
of  comparatively  recent  erosion.  Volcanic  activity  continued  in 
the  epoch  and  became  marked  at  the  end,  many  of  the  great  vol- 
canoes of  the  west  dating  from  the  close  of  this  epoch,  or  later. 

The  close  of  the  Pliocene  was  a  time  of  widespread  elevation,  the 
outline  of  the  continent  being  extended,  with  few  exceptions,  farther 
out  than  now.  So  marked  was  this  elevation  that  for  many  years 


5 88  HISTORICAL  GEOLOGY 

it  was  generally  believed  to  have  been  the  cause  of  the  accumulation 
of  ice  which  resulted  in  the  Glacial  Period. 

High  Plains  and  Bad  Lands.  —  The  great  sheets  of  clay,  sand, 
and  gravel  which  during  the  Tertiary  were  burying  the  eroded  sur- 
face of  the  Upper  Cretaceous  and  other  rocks  in  the  region  east  of 
the  Rocky  Mountains,  gradually  built  up  a  great  plain,  in  some 
places  500  feet  thick,  stretching  from  the  foothills  of  the  Rocky 
Mountains  for  hundreds  of  miles.  This  is  known  as  the  High  Plains 
region.  The  deposition  that  formed  the  Great  Plains  was  not  con- 
tinuous in  any  one  place  throughout  the  period,  but  shifted  from 
time  to  time,  being  local  and  contemporary  with  more  or  less  erosion. 
Eolian  deposits  (loess)  were  building  up  the  level,  grassed  surfaces 
(as,  indeed,  they  are  to-day)  and  constitute  a  not  inconsiderable  part 
of  the  formation.  In  recent  times,  however,  erosion  has  been  in 
excess  of  aggradation,  and  the  plain  is  being  cut  away.  Uneroded 
remnants  of  this  plain,  remarkable  for  their  level  surfaces,  remain  in 
western  Kansas,  Nebraska,  and  westward. 

Where  the  plain  has  been  dissected  by  canyons  and  ravines,  it  is 
seen  to  be  composed  of  unconsolidated  gravels,  sands,  and  clays. 
Since  the  region  has  a  scanty  rainfall,  although  with  occasional  heavy 
downpours  (cloud-bursts),  vegetation,  except  on  the  level  surfaces 
of  the  plain,  is  sparse.  The  scantiness  of  the  vegetation  on  the 
sides  of  the  ravines,  combined  with  the  looseness  of  the  sediments 
of  which  the  country  is  built,  affords  conditions  most  favorable  for 
rapid  erosion  when  the  torrents  of  water  from  the  occasional  heavy 
rains  rush  down  the  ravines.  As  a  result,  in  certain  places  along 
the  edges  of  the  High  Plains  we  find  a  maze  of  hills  and  ravines 
(Fig.  533)  with  almost  no  vegetation  except  on  the  tops  of  the  mesas 
(the  remnants  of  the  former  surface).  These  are  the  "  Bad  Lands," 
"  Mauvaises  Terres  "  of  the  early  French  explorers,  and  constitute  a 
scenery  as  weird  as  any  on  earth. 

Pliocene  of  Other  Continents.  —  The  emergent  condition  of 
Europe  during  the  Pliocene  was  in  contrast  to  the  widespread  seas 
of  the  previous  epoch.  In  the  north  of  Europe,  with  the  exception 
of  Belgium  and  a  little  of  northern  France,  the  seas  had  withdrawn. 
Great  Britain,  as  throughout  the  early  epochs  of  the  Tertiary,  had 
a  greater  land  area  than  now,  since  only  a  small  portion  of  the  south- 
ern part  was  beneath  the  sea  at  this  time,  while  England,  Ireland, 
and  Scotland  were  probably  connected;  and  the  northern  coast 
extended  farther  out  than  now. 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


589 


590  HISTORICAL  GEOLOGY 

The  Himalayas  were  being  eroded  during  the  Pliocene;  and 
thousands  of  feet  of  sandstones  and  conglomerates  were  deposited 
at  their  foot  before  its  close,  some  of  which,  however,  were  laid  down 
in  the  later  Miocene.  In  South  America  the  coasts  of  Argentina 
and  Patagonia  were  submerged,  and  the  last  upheaval  of  the  south- 
ern Andes  was  accomplished  at  this  time. 


REFERENCES  FOR  PALEOGEOGRAPHY 

BLACKWELDER,  E.,  —  Regional  Geology  of  the  United  States  of  North  America,  pp.  39-46. 
OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  84-93;  182-184;  244-246;  304-306. 
SCHUCHERT,  CHAS.,  —  P ale o geography  of  North  America:    Bull.  Geol.   Soc.   America, 

Vol.  20,  1910,  pp.  597-600. 
WILLIS  AND  SALISBURY,  —  Outlines  of  Geologic  History,  pp.  200-264. 


LIFE  OF  THE  TERTIARY 

Rise  of  Mammals.  —  In  the  present  imperfect  state  of  our  knowl- 
edge of  the  life  at  the  beginning  of  the  Cenozoic,  it  is,  perhaps,  even 
more  difficult  to  account  for  the  presence  of  highly  developed  mam- 
mals, as  soon  as  the  reptiles  became  extinct,  than  to  account  for  the 
disappearance  of  the  latter. 

It  is  improbable  that  the  mammals  on  the  earth  to-day  were  de- 
scended from  any  of  the  mammals  whose  remains  have  been  found 
in  the  later  Mesozoic  rocks.  Indeed,  it  is  even  doubted  that  the 
true  (Eutheria)  mammals  were  descended  from  the  marsupials 
(Metatheria)  mammals.  Two  theories  are  offered  to  explain 
their  sudden  appearance,  (i)  The  first  postulates  their  existence  in 
some  isolated  country,  in  the  Arctics  whose  climate  was  not  cold  at 
that  time,  or  elsewhere,  for  a  long  period  of  time  during  which  they 
had  been  developing  along  different  lines,  but  from  which  they  were 
prevented  from  spreading  because  of  some  barrier  to  their  move- 
ment, either  water  or  mountains.  When  this  barrier  was  removed, 
the  mammals  deployed  over  the  world,  and  finding  the  new  conditions 
favorable  for  their  existence,  rapidly  took  the  place  in  nature  formerly 
occupied  by  the  reptiles.  (2)  The  second  theory  (p.  549)  is  based 
upon  the  supposed  existence  of  mammals  on  the  uplands  of  the 
Mesozoic.  Since  practically  all  of  our  knowledge  of  the  life  of  that 
period  is  obtained  from  coastal  swamp  and  delta  deposits  in  which 
almost  no  forms  of  life  are  found  except  those  which  frequented 
marshes,  little  is  known  of  the  life  of  the  higher  and  more  extensive 


CENOZOIC  ERA:    AGE  OF  MAMMALS  591 

areas  of  the  earth's  surface.  It  is  to  be  noted,  too,  that  the  very 
earliest  Tertiary  upland  deposits  contain  a  rich  mammalian  fauna. 
It  does  not  seem  improbable,  therefore,  that  on  the  higher  land  of 
Asia  and  North  America  mammals  of  considerable  variety  were  in 
existence  during  the  later  days  of  the  Mesozoic ;  all  proof  of  which 
has  either  been  lost  by  the  wiping  out  of  the  upland  deposits  by 
erosion,  or  else  has  not  yet  been  discovered. 

Archaic  Mammals  of  Ancient  Ancestry.  —  In  the  earliest  known 
Eocene  beds  (Puerco)  the  remains  of  small,  archaic  mammals 
(marsupials)  associated  with  true  mammals  (Eutheria)  occur,  which 
clearly  belong  to  animals  whose  ancestors  lived  in  the  Mesozoic, 
some  of  which  (Plagiaulacidae)  date  back  even  to  the  Upper  Trias- 
sic.  These  animals  are  characterized  by  large  grinding  teeth  (Fig. 
534)  with  many  elevations  (multi- 
tuberculate),  and  elongated  front 
teeth  (incisors) ;  the  latter  being, 
in  some  cases,  chisel-shaped,  as 
in  the  rabbit,  and  in  others 
pointed.  The  "  back  teeth  "  of 
the  lower  jaw  are,  moreover, 
usually  very  different  from  those 
of  the  upper  jaw.  These  animals  FIG.  534.  — An  archaic  mammal,  Ptilo- 
were  all  small  or  of  moderate  *™  <UPPer  Cretaceous).  (After  Gidley.) 
•  "  i  ,  .  1  he  many-ridged  teeth  are  especially  to 

size,    the    largest   known   having    \^e  noticed. 

about    the    bulk    of    a    beaver. 

Judging  from  the  teeth,  it  seems  probable  that  some  were  gnawing 

animals    like   rabbits    or    mice    (but    not   true   rodents),  and   that 

others  were   either    fruit   eaters   (frugivores)   or  even  insect  eaters 

(insectivores). 

This  entire  class  became  extinct  before  the  close  of  the  Eocene 
and  should  be  considered  as  survivors  from  the  Mesozoic,  which 
lingered  for  a  time  in  the  Tertiary.  The  question  naturally  arises 
as  to  the  cause  of  their  extinction,  since  they  were  able  to  survive 
the  many  changes,  not  only  of  the  Mesozoic,  but  of  those  at  the  close 
of  the  era  as  well.  If  the  variety  of  their  fossils  in  the  Mesozoic 
formations  indicates  the  relative  abundance  of  these  archaic  mam- 
mals as  compared  with  other  forms  during  the  era,  conditions  of 
climate  or  of  food,  or  competition  with  the  dinosaurs  must  have  pre- 
vented their  increase.  If  competition  with  the  dinosaurs  prevented 
their  increase  in  the  Mesozoic,  it  would  have  been  surprising  had 

CLELAND   GEOL. — 38 


592 


HISTORICAL  GEOLOGY 


they  been  able  to  compete  with  the  true  mammals  which  appeared 
in  the  Eocene.  Although  mammals,  they  were  lowly  in  organiza- 
tion ;  and,  even  in  the  Upper  Cretaceous  where  the  vegetation  was 
of  the  modern  type,  they  were  not  abundant. 

Amblypoda  (Greek,  amblus,  blunt,  and  pous,  foot).  —  Along  with 
other  true   mammals   associated   with   the    above    archaic    ones    of 


FIG.  535.  —  Skeleton  and  restoration  showing  the  evolution  of  the  amblypods  in  the 
early  Eocene.  (Models  by  C.  R.  Knight,  under  the  direction  of  Prof.  H.  F.  Osborn.) 

Mesozoic  type,  there  appeared  a  group  of  heavy  creatures  (Ambly- 
poda), with  stout  limbs  ending  in  stumpy,  five-toed  feet.  These 
amblypods  (Fig.  535  A,  B)  were  conspicuous  in  North  America 
during  the  Eocene  but  became  extinct  before  its  close. 

The  early  representatives  (Fig.  535)  had  few  distinguishing  char- 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


593 


acters  except  their  heavy  build,  but  before  the  extinction  of  the 
race,  some  of  them  (Eobasileus)  not  only  took  on  greater  bulk, 
attaining  elephantine  proportions,  but  also  developed  a  peculiar 
head  (Fig.  536),  the  most  conspicuous  features  of  which  were  the 
three  pairs  of  knobs,  or  horns,  and  the  long,  saberlike  teeth  (canines) 
which  projected  several  inches  below  the  upper  jaw.  One  pair  of 
the  knobs  was  situated  on  the  nose,  a  larger  pair  over  the  eyes,  and 
the  third  pair  above  the  ears  at  the  back  of  the  skull.  It  is  not 
known  whether  the  protuberances  were  covered  with  horn  or  with 
callous  skin,  but  it  was  probably  the  latter.  The  use  to  which  the 
long,  saberlike  (canine)  teeth,  possessed  by  both  males  and  females, 
were  put  is  not  definitely  known,  but  it  seems  probable  that  they 
were  used  to  pull  down 
branches  from  the  trees, 
and  that  the  leaves  were 
then  stripped  off  into  the 
mouth  by  a  rapid  side 
motion  of  the  head. 
The  brain  was  smaller  in 
proportion  to  the  bulk 
of  the  animal  than  in 
any  other  mammal,  liv- 
ing or  extinct,  an  animal 

weighing  two  tons  hav-  FlG>  536.  — The  most  highly  specialized  of  the  am- 
ing  a  brain  no  larger  bly pods,  Eobasileus  (Upper  Eocene).  (After  Professor 
than  that  of  a  dog.  Osborn-) 

Moreover,  the  brain  was  smooth,  and  a  large  proportion  of  it  was 
formed  of  the  lobes  of  smell  (olfactory).  These  animals  seem  to 
have  reached  the  climax  of  brute  mass  as  compared  with  brain 
power  on  the  mammalian  stem,  and  are  to  be  compared  with  the 
massive,  small-brained  dinosaurs  of  the  reptilian  stem.  At  certain 
times  they  were  very  abundant,  as  is  shown  by  the  fact  that  two 
hundred  more  or  less  complete  skeletons  have  been  collected  by  one 
museum  alone. 

The  Lower  Eocene  amblypods  were  simpler  in  some  particulars 
than  the  later  ones,  being  smaller  and  hornless,  with  shorter  canine 
teeth,  although  the  grinding  teeth  differed  very  slightly  from  those 
of  their  massive  descendants.  In  other  words,  aside  from  increase 
in  size  and  the  ornamentation  of  the  skull,  the  evolution  of  the  race 
was  slight. 


594 


HISTORICAL  GEOLOGY 


It  is  interesting  to  speculate  on  the  causes  of  the  extinction  of  this 
race  which  may  have  had  an  existence  of  more  than  a  million  years. 
Its  fate  may  have  been  due  to  two  causes :  (i)  to  the  small  size  of 
the  brain,  and  (2)  to  the  poorness  of  the  grinding  teeth  which  were 
no  more  efficient  in  the  huge  forms  towards  the  close  of  the  period 
than  in  the  earlier  and  smaller  species.  The  low  brain  power  was 
of  disadvantage  to  them  in  their  competition  with  other  forms  and 
also  gave  them  little  ability  to  protect  their  young  from  the  more 
crafty  carnivores.  Bulk  is  a  disadvantage  under  changing  conditions 
(p.  550)  and  may  alone  have  been  responsible  for  the  disappearance 
of  the  race.  This  great  order  was  one  of  the  many  which,  for  a 
time,  took  a  prominent  place  among  the  animals  of  the  world,  but 
which  after  a  long  span  of  life  disappeared,  leaving  no  descendants. 


REFERENCES  FOR  AMBLYPODA 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3,  pp.  232-233. 
HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  249-260. 
SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  443-451. 
WOODWARD,  A.  S.,  —  Vertebrate  Paleontology,  pp.  292-299. 

Ancestors   of   the    Carnivores.  —  The   earliest  Eocene  carnivores 
(Creodonta)  are  so  generalized  (i.e.,  combine  characters  now  possessed 


-  537- — A  primitive  carnivorous  mammal,  creodont  (Middle  Eocene). 
(After  Professor  Osborn.) 

by  widely  different  groups  of  animals)  that  it  is  difficult  to  tell  even 
to  what  order  they  belong.  Their  teeth  are  rather  better  adapted 
for  cutting  food  than  for  grinding  it  (none,  however,  have  sectile 
teeth,  perfectly  adapted  for  flesh  eating)  ;  and  their  toes  are  provided 
with  curved  nails  that  are  rather  clawlike  but  are  not  the  sharp, 
retractile  claws  such  as  are  possessed  by  the  cat  to-day.  These 
creatures  were  descended  from  others  whose  feet  were  even  more 


CENOZOIC   ERA:    AGE  OF  MAMMALS 


595 


generalized,  which  also  gave  rise  to  the  hoofed  mammals,  such  as 
the  horse,  elephant,  and  ox.  They  were,  in  other  words,  mammals 
with  such  indefinite  characters  that,  by  the  modification  of  their 
organs,  their  descendants  could  be  developed  into  animals  widely 
different  in  form  and  habits,  such  as  the  lion  and  the  dog,  the  seal 
and  the  whale.  Some  of  the  members  of  the  generalized  carnivores 
(Creodonta)  were  larger,  others  smaller,  than  a  fox.  The  largest 
form  of  the  Eocene  (Pachyaena)  was  the  size  of  a  small  bear  and  had 
unusually  blunt  teeth,  which  are  thought  to  indicate  that  it  lived  on 
decaying  flesh. 

The  primitive  carnivores  (Creodonta)  (Fig.  537)  lived  through  the 
Eocene  into  the  Oligocene,  when  they  became  extinct.  Those  that 
passed  into  the  latter  epoch  attained  not  only  their  greatest  bodily 
size,  but  their  greatest  brain  capacity  as  well.  This  bears  out  the 
general  rule  that  the  brains  of  surviving  races  are,  upon  the  whole, 
larger  than  those  of  declining  races.  However,  we  shall  find  in  our 
later  study  that  certain  tribes  with  well-developed  brains,  as  for 
example  certain  rhinoceroses  (Teleoceras)  and  elephants  (Mastodon), 
failed  to  survive.  The  reason  for  such  extinction  is  usually,  though 
not  always,  to  be  found  in  the  failure  of  other  organs  to  develop  to 
meet  new  conditions. 

Marine  Mammals.  —  Perhaps  nothing  shows  the  rapid  evolution 
of  mammals  in  the  Tertiary  better  than  the  appearance  early  in 
the  Eocene  of  whales  perfectly  adapted  to  marine  existence,  which 
were  not  descended  from  the  marine  reptiles  of  the  Mesozoic,  but 
from  land  mammals.  Whether  mammals  gradually  acquired  an 
aquatic  habit  because  of  the  abundance  offish  which  they  voluntarily 
and  habitually  sought,  or  whether  they  were  forced  to  find  new  food 
on  account  of  the  competition  on  the  land,  it  is  not  possible  to  state 
(see  also  marine  reptiles,  p.  552),  but  probably  in  the  one  way  or  the 
other,  whales,  porpoises,  sea  lions,  and  other  animals  arose.  These 
marine  mammals  were  not  descended  from  a  common  ancestor,  but 
some  (manatee)  are  thought  to  have  been  derived  from  the  same 
stock  as  the  elephant,  some  (whales)  from  carnivores,  and  some 
(seals),  possibly,  from  the  same  stock  as  the  bear. 

Zeuglodon.  —  For  many  years  enormous  vertebrae  have  been 
found  in  the  Eocene  deposits  of  the  Gulf  coast,  the  largest  of  which 
measure  15  to  18  inches  in  length  and  weigh  50  to  60  pounds  in  the 
fossil  condition.  They  belong  to  marine  mammals  to  which  the 
name  Zeuglodon  (Greek,  zeugle,  yoke,  and  odont-,  tooth)  has  been  given 


596 


HISTORICAL  GEOLOGY 


because  of  the  double-rooted  back  teeth  which  present  the  appearance 
of  a  yoke.  The  head  of  Zeuglodon  was,  in  some  cases,  4  feet  long, 
the  length  of  the  body  10  feet,  while  the  tail  was  40  feet  long.  The 
animal  (Fig.  538  A,  B)  was  comparatively  slender,  an  individual 


FIG.  538.  —  Skeleton  and  restoration  of  Zeuglodon.     (Skeleton 

50  to  60  feet  long  having  a  thickness  of  only  6  to  8  feet.  The  teeth 
are  very  unlike  those  of  the  primitive  mammals,  having  been  modified 
for  grasping  and  cutting.  Back  of  the  head  were  two  short  paddles 
not  unlike  those  of  a  fur  seal,  but  the  hind  limbs  were  so  reduced 
that  they  were  retained  within  the  skin. 

The  zeuglodonts  were  divers  and  probably  lived  upon  squids,  as 
do  the  sperm  whales  to-day.  The  advantage  of  such  a  long  tail  in 
proportion  to  the  rest  of  the  body  has  led  to  two  suggestions  :  (i)  with 
it  the  animal  could  move  at  great  speed  through  the  water,  perhaps 
20  to  30  miles  an  hour;  and  (2),  as  far  as  definite  evidence  shows,  the 
tail  may  have  been  used  quite  as  much  for  the  storage  of  fat  as  for 
propulsion. 

The  ancestry  of  the  zeugolodonts  has  been  traced  back  to  a  small 
whale  (Protocetus)  with  a  skull  about  two  feet  long,  in  which  the 
teeth  show  a  surprising  resemblance  to  those  of  primitive  carnivorous 
land  mammals  (Creodonta).  This  whale  has  the  typical  number  of 
teeth  (44)  with  one,  two,  or  three  roots,  the  dogteeth  (canines)  pro- 
jecting beyond  the  others.  Following  these  whales  came  others 
(Eocetus)  differing  from  those  last  described,  in  the  fine,  saw-edged 
teeth.  Probably  descended  from  these  (Eocetus)  are  others  (Pro- 
zeuglodon)  in  which  the  teeth  depart  widely  from  those  of  land 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


597 


mammals  and  closely  approach  those  of  its  most  specialized  descend- 
ant, Zeuglodon.  It  is  thus  seen  that  the  Eocene  whales  were  not 
descended  from  the  Mesozoic  marine  reptiles,  but  from  the  land 
mammals,  just  as  the  ichthyosaurs  and  mosasaurs  (p.  555)  were 


after  Gilmore,  and  restoration  modified  after  Osborn.) 

descended  from  land  reptiles.  The  specialized  zeuglodonts  constitute 
a  side  branch  and  are  not  true  whales.  They  became  extinct  before 
the  close  of  the  Eocene. 

Ancestors  of  Existing  Whales.  —  The  earliest  known  ancestors  of  modern  sperm 
whales  are  believed  to  have  been  small,  Eocene  marine  mammals  (Microzeuglodon), 
whose  modified  descendants  in  the  Miocene  have  been  called  "shark-toothed"  whales 
because  they  had  teeth  somewhat  similar  in  appearance  to  those  of  a  shark.  The 
Miocene  whale  differs  from  its  Eocene  ancestor  (Microzeuglodon)  in  the  number  and 
simplicity  of  the  teeth  and  in  the  skull,  which  resembles  that  of  existing  toothed  whales. 
With  these  Miocene  shark-toothed  whales  (Squalodonta)  begins  an  almost  unbroken 
series  which  leads  to  the  sperm  whale.  By  one  investigator  it  is  stated  that  the  evolu- 
tion from  the  shark-toothed  whale  to  the  sperm  whale  is  sudden  and  almost  "  explosive," 
the  entire  evolution  being  completed  in  a  very  small  section  of  the  geological  time  of 
the  Upper  Miocene.  Dolphins  and  whalebone  whales  are  known  only  from  the 
Miocene,  but  probably  date  from  an  earlier  epoch.  Sea  cows  (Eosiren)  are  mingled 
with  the  remains  of  zeuglodonts  in  the  Eocene  deposits  of  Africa. 

REFERENCES  FOR  MARINE  MAMMALS 

ABEL,  O., —  The  Genealogical  History  of  the  Marine  Mammals:  Smithsonian  Rept., 

1907,  pp.  473-496. 

BEDDARD,  F.  E.,  —  The  Book  of  Whales. 
BEDDARD,  F.  E.,  —  Mammalia. 
LUCAS,  F.  A.,  —  Animals  of  the  Past,  pp.  58-64. 
OSBORN,  H.  F.,  —  Age  of  Mammals,  p.  171. 


598  HISTORICAL  GEOLOGY 

Ancestors  of  the  Hoofed  Mammals  (Ungulates).  —  Interest  in 
the  Eocene  centers  not  so  much  upon  such  groups  of  animals  as 
the  Amblypoda,  which,  though  the  largest  and  most  conspicuous 
of  their  time,  left  no  descendants,  as  upon  those  animals  that  are 
either  actually  the  ancestors  of  recent  mammals  or  so  closely  related 
to  them  that  they  help  us  to  understand  the  evolution  and  past 
history  of  the  mammals  living  to-day. 

Some  of  the  earliest  Eocene  herbivorous  mammals  (Condylarthra) 
are  so  generalized  that  many  groups  seem  to  converge  in  them,  even 
the  carnivores  and  herbivores  not  being  easily  distinguishable.  These 
ancestral  herbivores  were  small  or  of  moderate  size  and  walked  flat 
on  the  foot  (plantigrade)  and  not  on  the  toes  (digitigrade),  as  do 
the  horse  and  cow.  The  ends  of  the  toes  were  not  quite  in  the  form 

either  of  hoofs  or  of 
claws.  One  of  the 
best  known  forms 
(Phenacodus)  (Fig. 
539),  although  not  the 
direct  ancestor  of  any 
of  the  modern  mam- 

mals,  is  of  great  in- 
FIG.  539.  —  Phenacodus,  an  Eocene  mammal  which  in     t-~r^ct    cin^  ™k 

,.,.,.  .       ,  LClCaL     olllCC      1L      prOD" 

many    particulars    is    like    the    ancestor    of   the    hoofed        ,  ,  ,.—  , 

mammals  or  ungulates.     (After  Scott.)  ablY        differed        but 

slightly  from  those  in 

the  direct  line  of  descent.  It  resembles  the  carnivores  in  having 
an  arched  back,  strong  legs,  and  five  toes  on  its  feet.  It  walked 
somewhat  on  its  toes  and  the  toes  ended  in  a  flat  "  nail  "  which 
may  be  considered  as  the  beginnings  of  a  hoof.  The  teeth  were 
short-crowned  (that  portion  of  the  tooth  above  the  jaw  being  short) 
and  comparatively  simple,  showing  that  their  possessor  was  om- 
nivorous in  habit.  The  head  is  remarkably  small  and  the  nearly 
smooth  brain  is  small,  even  for  a  head  of  this  size.  It  apparently 
had  no  means  of  defense  and  sought  safety  in  flight.  Some  species 
of  the  genus  attained  the  size  of  a  sheep. 

It  was  from  some  such  animal,  so  simple  in  structure  that  it  might 
almost  equally  well  be  ancestral  to  the  carnivores  (the  dog  and  lion) 
and  to  the  hoofed  mammals  (ungulates,  —  horse,  ox,  camel),  that  the 
modern  hoofed  mammals,  such  as  the  horse,  ox,  rhinoceros,  and 
elephant  are  descended.  It  is  interesting  in  this  connection  to  note 
that  Huxley  and  Cope  had  independently  pictured  what  the 


an- 


CENOZOIC   ERA:    AGE  OF   MAMMALS 


599 


cestors  of  the  hoofed  mammals  would  be  like  when  discovered. 
This  prophecy  was  fulfilled  in  the  finding  of  Phenacodus,  although 
the  animal  has  proved  not  to  be  directly  ancestral  to  any  form,  but 
rather  to  stand  as  a  type. 

REFERENCE  FOR  PHENACODUS 
SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  456-458. 

Divergence  of  the  Even  and  Odd-toed  Hoofed  Mammals  (Ungu- 
lates).—  The  common  herbivorous  mammals1  of  the  present  are 
separated  into  two  great  divisions,  those  with  a  cloven  hoof  (Fig. 
540),  the  even-toed  ungulates  (Artiodactyla),  such  as  the  pig,  deer, 


A  BCD 

FIG.  540.  —  Evolution  of  the  foot  of  even-toed  mammals  (artiodactyls) ;  A,  hog; 
B,  roebuck;   C,  sheep;  Z),  camel. 

and  camel,  and  those  with  a  large  central  toe,  the  odd-toed  ungulates 
(Perissidactyla)  (Fig.  541),  such  as  the  horse  with  one  toe,  the 
rhinoceros  with  three,  and  the  tapir  with  four  toes  on  the  fore  foot 
and  three  on  the  hind  foot.  The  five-toed  ancestors  of  the  earliest 
Eocene  had  already  developed  feet  that  gave  promise  of  odd-toed 
and  even-toed  descendants;  even  Phenacodus,  the  most  generalized 
of  the  early  mammals,  has  a  foot  in  which  the  central  toe  is  rather 
larger  than  the  others,  and  should  be  placed  in  the  division  of  odd- 
toed  ungulates  (Perissidactyla). 

1  The  Proboscidea  (elephants)  constitute  a  third  great  group  of  hoofed  animals  with  five-toed 
feet. 


6oo 


HISTORICAL  GEOLOGY 


It  will  readily  be  seen  that,  if  the  weight  of  the  body  rested  prin- 
cipally upon  the  middle  or  third  toes,  and  if  the  animal  raised  the  heel 
from  the  ground,  the  thumb  or  first  finger  would  not  ordinarily  touch 
the  ground ;  and  if  this  habit  of  walking  on  the  toes  became  better 
developed  in  successive  generations,  not  only  the  first  toe  but  the 
fifth  as  well  might  become  of  no  use  to  the  animal  and  might  finally 
atrophy.  A  continuation  of  the  process,  accompanied  by  a  length- 
ening of  the  foot,  would  result  in  the  dropping  of  the  second  and 

fourth  toes  and  in  the  formation 
of  the  highly  specialized,  one-toed 
foot  of  the  horse  (p.  608). 

If  the  weight,  instead  of  being 
directly  on  the  middle  toe,  was 
between  the  third  and  fourth  toes, 
a  more  digitigrade  habit  (walking 
on  the  toes)  would  result  in  the 
reduction  and  later  dropping  of 
the  thumb  or  first  finger,  leaving 
A  B  C  a  four-toed  foot.  By  the  further 

FIG.  541.  — Evolution  of  the  foot  of    reduction  in  size  of  the  side  toes, 

odd-toed  mammals  (perissidactyls),illus-     a  foot  like  that  of  a  pig,  with  tWO 


dard.)  would    result.      When   these   side 

toes  disappeared,  the  animal  had 

but  two  toes  on  each  foot,  like  the  camel  and  sheep.  Judging  from 
the  abundance  of  the  even-toed  ungulates  (artiodactyls),  it  seems 
that,  as  a  whole,  this  type  of  foot  has  proved  to  be  the  best.  These 
modifications  in  foot  structure  apparently  were  the  result  of  a 
change  from  the  forest  conditions  of  the  Eocene,  where  soft  ground 
and  succulent  vegetation  were  the  rule,  to  the  plains  vegetation 
(p.  630)  of  the  later  times,  with  their  siliceous  grasses  where  a  short, 
spreading  foot  would  not  give  the  animal  the  speed  necessary  to 
move  long  distances  in  a  short  time,  for  food  and  water  (p.  610). 

FACTORS  IN  THE  EVOLUTION  or  MAMMALS 

In  the  course  of  the  history  of  the  mammals  to  be  studied  it  will  be 
found  that,  beginning  with  some  such  ancestor  as  Phenacodus,  which 
is  full  of  mechanical  imperfections,  the  skeletons  were  modified  chiefly 
in  four  particulars,  each  of  which  was  of  more  or  less  vital  impor- 
tance to  the  various  races  affected. 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


60 1 


(1)  That  race  was  more  likely  to  survive  whose  members  had  teeth 
enabling  their  possessors  to  grind  up  nutritious  food,  no  matter  how 
tough   and   hard.     Particularly  was  this  true  if  the  teeth  of   suc- 
cessive generations  developed   better  grinding   surfaces,  permitting 
their  possessors  to  take  advantage  of  new  food  or  food  that,  because 
of  inability  to  grind  it,  was  unsuited  to  their  ancestors.     The  efficient 
grinding  teeth  of  the  horse,  cow,  and  elephant,  as  will  be  seen,  are  the 
result  of  such  an  evolution. 

(2)  Since  the  swiftest   animals   are  more  likely  to  escape    their 
enemies,  those  that  possessed  limbs  constructed  for   rapid  motion 
were  most   likely  to 

survive.  The  leg 
best  suited  for  this 
purpose  is  one  in 
which  the  foot  is 
lengthened,  the  joints 
perfected,  and  the 
number  of  toes  re- 
duced. The  one-toed 
horse  may  be  con- 
sidered the  climax  of 
such  evolution. 

(3)  Since  more  sa- 
gacious   animals   are 
better    able    to    find 
food,     escape     their 
enemies,  and  care  for 

their  young,  it  natu-        ,-,  „    •      r      •      /     i  r  \ 

J  r  i  *IG'  S42-  —  Brains  of  ancient  (on  left)  compared  with 

rally      follows       that     modern    (on    right)    mammals:    A,   creodont;    B,    dog; 
those        with        large     C>  early  amblypod  ;  D,  rhinoceros ;  E,  highly  developed 
brains  (Fiff    $4.2)  were     amblypod  (Uintatheriuni) ;  F,  hippopotamus.     Olfactory 
\     6OT  /  lobes  (dots),  cerebral  hemispheres  (oblique  lines),  cerebel- 

more  likely  to  survive.     lum  and  medulla  (dashes).     (After  Osborn.) 
As  the  various  races 

of  mammals  are  discussed,  attention  will  be  called  to  the  increase  in 
the  size  of  the  brains,  and  any  exceptions  will  be  noted. 

(4)  Increased  bulk  and  the  strength  which  usually  accompanies 
size  is  often  a  protection  against  enemies  and,  in  the  case  of  males, 
results  in  the  destruction  of  the  smaller  and  weaker  members  of  the 
same  species.     As  a  consequence,  it  will  be  seen  that  the  surviving 
species  of  a  given  order  often  become  larger  in  the  course  of  their 


602 


HISTORICAL  GEOLOGY 


history.  The  disadvantage  of  great  size  has  been  discussed  (p.  550). 
Mammalian  Teeth.  —  The  typical  or  ancestral  number  of  teeth 
is  44,  a  number  which  is  seldom  found  in  living  forms,  since  some 
have  been  developed  at  the  expense  of  others  and  some  have 
been  dropped.  It  is  seldom  that  a  larger  number  occurs.  (The 
porpoise  has  246.)  The  primitive  type  of  grind- 
ing teeth  (molars),  from  which  the  highly  per- 
fected teeth  of  the  present  carnivorous  and 
herbivorous  mammals  were  derived,  had  a  grind- 
ing surface  merely  roughened  by  three  sharp- 
pointed  cones  arranged  in  the  form  of  a  triangle 
(tritubercular).  From  a  tooth  of  this  simple 
type  has  been  developed  the  complicated  and 
efficient  grinders  of  the  higher  herbivores  (Fig. 
543).  Another  notable  change,  in  such  races  as 
the  horse  and  the  elephant,  has  been  the  lengthen- 
ing  of  the  tooth,  adapting  it  to  the  nutritious 
grasses  (p.  610)  of  the  dry,  sandy  plains. 

Feet.  —  The  primitive  foot  (p.  598)  is  five- 
toed  with  the  sole  resting  flat  on  the  ground. 
From  such  a  foot,  the  extremely  effective  one  of 
the  higher  hoofed,  herbivorous  mammals  was 
developed.  This  was  accomplished  (i)  by  the 
raising  of  the  ankle  and  wrist  joints  which  lifted 
the  first  and  fifth  toes  from  the  ground  so  that 
these  toes  became  useless,  and  degeneration  set 
in  which  eventually,  as  in  the  case  of  the  horse, 
and  the  modern  horse  caused  au  except  the  middle  toe  to  disappear. 
(above).  Ihe  relative  /NTi  ...  r  ,..  ri 

size  and  efficiency  are    \2/  ^n  tne  primitive  root  the  joints  or  the  wrist 
shown.    (After  H.  F.    and    ankle  were   loose,   but  they  became  more 

Osborn,  Age  of  Mam-    efficient  by  the  development  of  the  "  tongue  and 
mals.)  .,  ...  rr       •      i 

groove  structure  which  very  effectively  pre- 
vented lateral  movement.  The  change,  in  general,  has  been  from  a 
loose-jointed  limb  with  "  ball-and-socket "  joints,  to  one  with  keeled 
joints;  from  walking  with  the  sole  of  the  foot  flat  on  the  ground 
(plantigrade),  to  walking  on  the  toes  with  the  heel  well  elevated 
above  the  ground  (digitigrade) ;  from  a  five-toed  foot  to  one  with 
a  smaller  number  of  functional  toes.  It  should  not  be  forgotten, 
however,  that  along  with  races  that  were  changing  in  structure, 
there  lived  others  that  have  been  little  modified. 


FIG.  543.  —  Corre- 
sponding grinding  teeth 
of  Eohippus  (below) 


CENOZOIC  ERA:    AGE  OF   MAMMALS  603 

The  feet  of  the  carnivores  seldom  show  a  reduction  in  the  number 
of  toes.  This  is  due  to  the  fact  that,  since  the  foot  must  be  adapted 
for  both  rending  and  tearing,  as  well  as  for  locomotion  over  both 
rough  and  smooth  ground,  it  would  not  be  of  advantage  to  the  animal 
to  have  the  number  of  toes  greatly  reduced.  The  principal  changes 
from  the  primitive  carnivores  (creodonts)  (Fig.  537)  to  the  modern 
forms,  as  far  as  foot  structure  is  concerned,  have  been  in  the  perfec- 
tion of  the  joints,  in  the  more  digitigrade  habit  of  walking  (walking 
on  the  toes),  and  in  the  formation  of  sharp,  retractile  claws. 

Limits  to  Evolution.  —  There  is  a  limit  to  evolution  after  funda- 
mental modifications  in  the  structure  have  occurred.  For  example, 
thus  far  no  mammal  is  known  to  have  been  transformed  from  an 
aquatic  to  a  land  type,  although  numerous  examples  of  the  reverse  are 
known.  No  swift-moving  types  have  retrogressed  into  slow-moving 
forms.  Animals  adapted  to  tree  life,  however,  are  believed  to  have 
taken  on  terrestrial  habits  and  to  have  become  modified  to  fit  them. 

Lost  parts  are  never  reacquired ;  as,  for  example,  if  the  number  of 
toes  of  an  animal  is  reduced,  its  descendants  never  have  more  than 
the  minimum  number  possessed  by  its  ancestors.  Each  part  that 
is  lost,  such  as  a  tooth  or  a  digit,  narrows  down  the  possibility  of 
future  changes  in  structure  to  meet  new  conditions.  A  specialized 
organ  can  never  again  become  generalized.  It  will  readily  be  de- 
duced from  the  above  that  animals  highly  specialized  to  meet  cer- 
tain conditions  will  be  more  likely  to  fail  to  meet  changed  conditions 
than  those  that  have  a  more  generalized  structure. 

REFERENCES  FOR  THE  EVOLUTION  OF  MAMMALS 

Os BORN,  H.  F.,  —  Age  of  Mammals,  pp.  18-35. 

SCOTT,  W.  B.,  —  A  History  of  the  Land  Mammal's  in  the  Western  Hemisphere,  pp.  645- 
655- 

ODD-TOED  MAMMALS  (PERISSIDACTYLS) 

This  division  of  the  mammals  was  in  the  past  more  important 
than  at  present,  being  now  represented  by  the  elephant,  tapir,  and 
horse. 

Titanotheres  (Greek,  titan,  a  giant,  and  therion,  a  beast).  — Of  the 
many  families  that,  for  a  time,  gave  promise  of  permanence  and 
later  became  extinct,  none  is  more  interesting  than  the  titanothere, 
a  tribe  distantly  related  to  the  rhinoceros,  which  is  first  known  in 


604 


HISTORICAL  GEOLOGY 


the  early  Eocene  (Wind  River)  and  became  extinct  with  apparent 
suddenness  in  the  early  Oligocene  (White  River),  just  as  it  had, 
perhaps,  reached  its  greatest  abundance  and  variety. 

Two  groups  of  titanotheres  are  represented  in  the  Lower  Eocene 
near  the  beginning  of  the  history  of  the  race;  one  abundant  genus 
(Lambdotherium)  had  slender  limbs  and  was  capable  of  swift  move- 
ment, indicating  that  it  was  adapted  to  the  open  basins  of  the  moun- 
tain regions ;  and  the  other  (represented  by  Eotitanops)  was  composed 


c 


FIG.  544.  —  Evolution  of  the  titanotheres.     (After  Scott.) 


of  larger  and  stockier  animals,  ancestors  of  those  in  the  Oligo- 
cene, that  grew  to  be  about  two  thirds  the  size  of  a  tapir.  The 
largest,  as  well  as  latest,  forms  (Brontotherium)  were  ambulatory 
creatures  with  an  elephantine  body  but  with  legs  less  massive  than 
those  of  an  elephant.  The  head  (Fig.  544)  was  saddle-shaped,  with 
a  pair  of  large  horns,  placed  side  by  side  and  branching  off  from 
the  end  of  the  nose,  and  which  were  probably  covered  with  a 
callous  skin.  The  brain  was  not  larger  than  the  fist  of  an  average 


CENOZOIC  ERA:    AGE  OF  MAMMALS  605 

man.  They  belong  to  the  odd-toed  division  of  mammals  (perissi- 
dactyls),  with  four  toes  on  the  fore  and  three  on  the  hind  foot,  the 
larger  middle  toe  of  the  fore  foot  showing  that,  like  the  living  tapir, 
the  titanotheres  belonged  to  the  odd-toed  division  of  mammals. 

No  sooner  had  the  titanotheres  reached  the  climax  of  their  evolu- 
tion than,  with  apparent  suddenness,  they  became  extinct.  This 
is  well  shown  in  the  Oligocene  deposits  of  the  Bad  Lands  of  South 
Dakota  (p.  588),  where  they  are  magnificently  represented  and 
undergo  their  entire  final  evolution  and  extinction  during  the 
time  taken  in  the  deposition  of  the  200  feet  of  sediments  in  which 
their  remains  are  embedded.  (Osborn.)  The  bulk  and  specialization 
of  the  animals  rendered  them  more  liable  to  extinction,  since  they 
were  at  a  disadvantage  with  the  smaller  and  more  active  true  rhinoc- 
eroses of  similar  food  habits  which  were,  perhaps,  able  to  make 
longer  journeys  between  water  and  feeding  grounds.  To  this  should 
be  added  a  growing  scarcity  of  food,  emphasized  by  drought  at 
certain  seasons.  It  is  not  improbable  that  the  competition  with 
the  camels  and  other  swift-moving  forms  with  teeth  better  adapted 
to  the  conditions  may  have  been  an  important  factor  in  causing  the 
scarcity  of  food  which  was  fatal  to  the  huge  titanotheres,  although 
not  so  to  the  less  bulky  rhinoceroses. 

From  the  Oligocene  on,  the  swifter,  grazing  forms  tended  to  replace 
the  slow-moving,  browsing  (feeding  on  the  leaves  of  shrubs  and  trees) 
forms,  although  some,  such  as  the  rhinoceros,  have  survived  to  the 
present. 

REFERENCES  FOR  TITANOTHERES 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters   and  Creatures  of   Other  Days,  (Brontops), 

p.  261. 

OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  134,  239-240. 
SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  308-319. 

Rhinoceroses.  —  The  history  of  this  great,  odd-toed,  hoofed 
(perissidactyl)  family  illustrates  two  points  to  which  attention 
will  be  directed  in  the  discussion  of  other  families:  (i)  the  presence 
in  abundance  in  North  America  of  members  of  a  family  that  has 
long  since  been  extinct  in  the  western  hemisphere  but  is  still  living 
elsewhere,  and  (2)  the  evolution  of  a  number  of  side  branches,  dif- 
fering widely  in  structure  and  habits.  The  rhinoceros  family  is 
now  confined  to  Africa,  southern  Asia,  and  a  few  of  the  large  islands 
of  the  Indian  Ocean,  but  in  the  Oligocene  and  Miocene  not  only 


6o6 


HISTORICAL  GEOLOGY 


did  rhinoceroses  that  are  ancestral  to  existing  genera  live  in  North 
America,  but  a  number  of  side  branches  were  also  developed  on  that 
continent  which  differed  widely  from  those  of  to-day,  some  of  which 
lived,  at  least  locally,  in  great  abundance.  In  a  remarkable  deposit 
of  the  Lower  Miocene  (Harrison  Beds,  near  Agate,  Nebraska)  a 
slab  of  rock  10  feet  by  40  feet  by  18  inches  was  uncovered  by  an 
American  Museum  party  in  1912,  in  which  are  75  skulls  of  a  species 
of  rhinoceros  (Diceratherium),  together  with  the  bones  of  these  and 
other  mammals.  This  deposit  is  without  doubt  exceptional,  but 
nevertheless  shows  that,  in  certain  localities,  these  rhinoceroses  were 
extremely  abundant.  In  the  Oligocene  of  North  America  three 
branches  of  the  family  are  known,  but  in  the  Miocene  they  had 
evolved  into  a  number  of  branches,  which,  however,  may  be  united 
into  three  groups. 

(i)  One  of  these  may,  for  convenience,  be  called  the  "  swimming 
rhinoceros "  because  of  the  spreading,  four-toed  foot  which  was 
doubtless  an  efficient  organ  for  swimming.  This  branch  (Metamyno- 
don)  was  apparently  semi-aquatic  and  was  fitted  for  life  in  the  lakes 
and  rivers  of  the  Oligocene.  It  was  stout  and  rhinoceros-like  in 
shape,  the  eyes  were  placed  high  on  the  head,  and  the  nostrils  opened 

upward  so  that  it  could  breathe 
when  the  head  was  partly  sub- 
merged. Its  canine  teeth  were 
elongated  into  tusks  and  were 
doubtless  used  for  uprooting  the 
plants  from  the  bottom  and  banks 
of  the  lakes  and  rivers  which  it 
~  frequented. 

FIG.  545.  —  A  running  rhinoceros  (2)  A  second  Oligocene  rhinoc- 
(Hyracodon} I  showing  the  modification  of  eros  branch  whose  career>  Hke 

that  of  the  swimming  rhinoceros, 
terminated  before  the  close  of  that 
epoch,  was  the  "  running  rhinoceros  "  (Hyracodon).  This  animal 
(Fig.  545)  did  not  have  the  appearance  which  is  usually  associated 
with  the  rhinoceros,  since  it  was  light-limbed  and  agile,  with 
horselike  shoulders  and  limbs.  It  had  three  toes  on  each  foot, 
very  similar  to  those  of  the  horse  of  its  time,  and  was  apparently 
adapted  to  the  hard,  dry  plains  of  the  Oligocene.  It  is  possible 
that,  had  this  animal  succeeded  in  adapting  itself  to  the  changing 
conditions  of  the  Tertiary  and  in  competing  with  the  other  grazing 


structure   for   plains   conditions. 
Prof.  H.  F.  Osborn.) 


(After 


CENOZOIC  ERA:    AGE  OF  MAMMALS  607 

mammals  of  the  time,  it  would  eventually  have  dropped  its  side 
toes  and  have  walked  on  one  toe  like  the  modern  horse  (p.  608). 

(3)  The  true  rhinoceroses  constitute  the  third  group,  but  with 
two  exceptions  (Diceratherium  and  Teleoceros),  none  of  the  North 
American  forms  had  horns.  None  of  this  family  are  known  to  have 
lived  in  North  America  after  the  Pliocene,  but  members  of  the  group 
were  able  to  adapt  themselves  to  the  vicissitudes  of  the  closing  days 
of  the  Tertiary  and  roamed  over  Europe  and  Asia,  some  (woolly 
rhinoceros)  being  adapted  even  to  the  cold  climate  of  northern  Asia, 
as  a  carcass  found  frozen  in  the  ice  of  northern  Siberia  shows. 

The  principal  changes  which  the  true  rhinoceros  group  underwent 
in  its  history  are  (i)  an  increase  in  bulk,  (2)  a  reduction  in  the 
toes  from  four  on  the  fore  foot  to  three  on  all  the  feet,  (3)  the  develop- 
ment of  horns,  and  (4)  the  development  of  somewhat  better  teeth. 

REFERENCES  FOR  RHINOCEROSES 

International  Encyclopedia,  —  Rhinoceros. 

OSBORN,  H.  F.,  —  Age  of  Mammals,  p.  272. 

OSBORN,  H.  F.,  —  Phytogeny  of  the  Rhinoceroses  of  Europe:  Bull.  Am.  Mus.  Nat. 
Hist.,  Vol.  13,  No.  5,  1900,  pp.  229-267. 

SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  326-353. 

(A  description  of  Arsinoitherium,  an  interesting  Eocene  animal  with  somewhat  the 
appearance  of  a  rhinoceros  but  unrelated,  is  to  be  found  in  OSBORN,  Age  of  Mam- 
mals, p.  202;  and  in  LANKESTER,  E.  R.,  Extinct  Animals,  pp.  152-154.) 

Tapirs.  —  It  is  interesting  to  find  an  animal  living  in  the  present 
which  still  retains  the  characters  of  animals  that  are  more  typical  of 
Eocene  and  Miocene  times  before  differentiation  became  marked. 

The  teeth  of  tapirs  are  short-crowned  and  differ  but  slightly  from 
those  of  their  Miocene  ancestors  of  Nebraska  and  South  Dakota. 
They  are  odd-toed  ungulates  (perissidactyls),  with  four  toes  (Fig. 
541  A)  on  the  fore  foot  (the  weight  being  on  the  third  toe),  and  three 
on  the  hind  foot,  the  fourth  toe  of  the  fore  foot  being  small.  The 
Eocene  ancestors  of  the  tapirs  graded  almost  insensibly  into  those 
of  the  horse  and  rhinoceros. 

Tapirs  live  in  marshes  or  dense  forests  in  proximity  to  water, 
occupying  a  place  in  nature  in  which  there  is  little  mammalian 
competition.  They  had  a  wide  distribution  in  the  past  and  are  an 
illustration  of  a  once  abundant  race  nearly  exterminated  but  still 
struggling  for  existence  where  competition  happens  to  be  least  severe 
in  their  particular  case.  Their  present  occurrence  only  in  South 

CLELAND   GEOL. — 39 


6o8 


HISTORICAL  GEOLOGY 


America  and  southern  Asia  seems  remarkable  unless  one  remembers 
that  during  the  Tertiary  tapirs  ranged  throughout  the  northern 
hemisphere,  making  their  way  to  South  America  late  in  the  Pliocene.1 
Horses.  —  Few  animals  have  a  family  history  which  goes  so  far 
back  into  the  past  and  is  at  the  same  time  so  well-known  as  that 
of  the  horse.  From  an  animal  less  than  a  foot  in  height,  with  a 
skeleton  more  like  that  of  a  carnivore  than  a  horse,  the  changes  in 
structure  and  size  have  been  traced  step  by  step  to  the  present. 
It  should  be  borne  in  mind,  however,  that  few  of  the  so-called  an- 
cestors are  truly  in  the  direct  line,  but  they  show  us  rather  what  the 
actual  forebears  were  like. 

Theoretically,  the  history  of  the  horse  begins  with  a  generalized, 
five-toed  animal  which  walked  with  the  sole  of  the  foot  on  the  ground 

(plantigrade),  or  with  the 
heel  but  slightly  raised ;  with 
the  normal  number  of  teeth 
(44) ;  with  an  arched  back 
somewhat  like  a  carnivore's, 
and  with  toes  covered  with 
nails  which  were  neither 
hoofs  nor  claws;  in  other 
words,  an  animal  similar  to 
Phenacodus  (p.  598). 

The  earliest  American  horse 
(Fig.  546)  (Eohippus ;  Greek, 
eos,  dawn,  and  hippos,  horse) 
of  which  we  have  a  record 
lived  in  the  early  Eocene  and 
was  a  small  and  unhorselike 
animal  about  the  size  of  a  fox.  It  still  retained  the  normal  number 
of  teeth  (44),  as  did  practically  all  of  the  animals  of  its  time,  the 
teeth  being  simple  with  very  short  crowns,  somewhat  resembling 
those  of  the  pig  and  monkey,  and  very  unlike  the  long,  complicated 
grinders  of  the  horse  of  to-day.  So  generalized  are  the  teeth  of 
this  early  horse  that  it  is  often  a  matter  of  great  difficulty  to  dis- 
tinguish them  from  those  of  the  ancestors  of  what  are  now  widely 
removed  orders  of  animals.  There  were  four  well-developed  toes 

1  If  South  America  were  raised  in  the  central  portion  so  as  to  permit  the  Amazon  to  deepen 
its  valley  and  drain  its  basin,  the  tapir  would  doubtless  become  extinct  in  the  New  World. 
The  lesson  is  an  important  one  when  the  extinction  of  other  animals  is  considered. 


FIG.  546.  —  Model  of  the  Eocene  horse 
Eohippus.  (Restoration  by  C.  R.  Knight, 
under  the  direction  of  Prof.  H.  F.  Osborn.) 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


609 


and  a  rudimentary  first  toe  on  the  fore  foot,  while  the  hind  foot 
had  three  toes  and  rudimentary  first  and  fifth  toes.  The  foot  was 
a  spreading  one,  enabling  the  animal  to  walk  on  fairly  soft  ground. 
From  this  earliest  known,  simple  form,  the  course  of  evolution 
consisted  largely  in  such  modifications  of  the  skeleton  as  rendered 
the  animal  better  fitted  to  secure  food  and  masticate  it  and  to 
escape  its  enemies.  This,  as  will  be  seen,  resulted  in  the  production 
of  a  very  perfect  grinding  apparatus.  The  necessity  for  speed  in 
seeking  safety  and  in  going  long  distances  for  food  and  water, 
resulted  in  the  remarkably  perfect  locomotive  apparatus. 


THE  EVOLUTION  OF  THE   HORSE. 


Hypothetical  Ancestors  with  Five  To*  on  Each  Fool 
and  Teeth  like  thaw  of  Monk™  etc. 


FIG.  547.  —  Table  showing  the  evolution  of  the  horse.     (After  W.  D.  Matthew.) 

The  next  horse  in  the  line  of  descent  (Protorohippus,  Fig.  547) 
appeared  in  the  Upper  Eocene  and  was  four  or  five  inches  higher 
than  the  early  Eocene  horse  (Eohippus),  with  longer  limbs,  which 
indicate  ability  for  increased  speed.  The  fore  foot  had  four  toes, 
but  lacked  the  rudimentary  toe,  or  splint,  of  the  Eohippus,  while 
a  shortening  of  the  outermost  toe  (the  fifth)  gave  promise  of  a  three- 
toed  foot  in  its  descendants.  The  hind  foot  had  three  toes  but  no 
splint. 

The  Oligocene  horse  (Mesohippus,  Fig.  547)  differs  from  the 
Eocene  one  (Proterohippus)  in  having  but  three  toes  on  the  fore  foot 
and  a  splint  which  represents  the  outermost  (or  fifth)  toe  of  the 


6io  HISTORICAL  GEOLOGY 

earlier  horses.  Besides  the  reduction  in  the  number  of  toes,  the  leg 
had  lengthened  as  the  body  thickened,  and  the  animal  stood  about 
1 8  inches  high.  The  teeth  were  still  short-crowned  and  lacked  the 
complicated  structure  of  the  later  forms. 

A  Miocene  horse  (Protohippus,  or  Hipparion,  Fig.  547)  shows  the 
next  stage  in  the  evolution  of  the  race.  In  these  animals  there  was 
one  large  toe  on  each  foot,  with  two  smaller  slender  toes,  one  on  each 
side  of  it,  which  were  of  no  use  to  the  animal,  as  they  did  not  reach 
the  ground  when  it  walked.  The  teeth  are  very  like  those  of  the 
modern  horse,  in  which  the  plates  of  enamel  form  curved,  complex, 
irregular  patterns  (p.  602),  but  are  shorter  and  probably  wore  out 
at  an  earlier  age.  The  average  height  of  the  animal  was  about  three 
feet.  Associated  with  this  more  highly  specialized  horse  (Proto- 
hippus) were  others  with  short-crowned  teeth  and  with  all  three  toes 
functional  (Parahippus  and  Hypohippus). 

The  stage  between  the  Miocene  horse  (Protohippus)  and  the  true 
horse  (Equus,  Fig.  547)  is  not  definitely  known,  but  was  doubtless 
represented  by  an  animal  with  a  large  central  toe  and  with  either 
very  diminutive  side  toes  or  large  splints,  and  longer  and  more  per- 
fect grinding  teeth. 

Summary  of  the  Evolution  of  the  Horse.  —  The  changes,  there- 
fore, which  took  place  in  the  horse  family  during  its  geological  his- 
tory are:  (i)  a  reduction  in  the  number  of  teeth  from  44  to  36, 
accompanied  by  a  lengthening  and  perfecting  of  the  grinding  teeth ; 
(2)  a  reduction  in  the  number  of  the  toes  from  five  to  one;  (3)  an 
improvement  of  the  joints  of  the  legs  by  means  of  which  motion  was 
permitted  in  but  two  directions,  forward  and  backward ;  (4)  a 
lengthening  of  the  limbs,  especially  in  the  lower  portions.  This 
has  left  the  center  of  gravity  high,  and  the  limb,  though  long,  moves 
quickly  like  a  short  pendulum,  combining  rapidity  of  movement 
with  a  lengthened  stride ;  (5)  an  increase  in  the  size  of  the  animal ; 
(6)  a  proportionally  greater  increase  in  the  size  of  the  brain  than 
the  body;  (7)  besides  the  above,  other  changes,  such  as  the  lengthen- 
ing of  the  neck  and  head  to  permit  the  animal  of  increased  height 
to  crop  grass  from  the  ground ;  (8)  the  gradual  perfection  of  the 
body ;  and  others  of  which  space  will  not  permit  mention. 

Probable  Cause  of  the  Evolution  of  the  Horse.  —  These  radical 
structural  changes  seem  to  be  the  indirect  result  of  a  modification 
of  the  climate  of  the  Great  Plains  region  of  North  America  and  the 
accompanying  change  in  the  character  of  the  vegetation.  Eohippus 


CENOZOIC  ERA:    AGE  OF  MAMMALS  6ll 

was  apparently  an  immigrant  from  Europe  by  way  of  Asia,  but  it 
was  in  America  that  the  race  developed,  although  from  time  to  time 
modified  representatives  migrated  back  to  Europe.  During  the 
Eocene  the  climate  was  moist,  forests  covered  the  lands,  and  lakes, 
marshes,  and  streams  were  abundant.  Under  conditions  such  as 
these  the  early  horses  lived.  During  the  Oligocene  the  conditions 
had  not  greatly  changed,  but  increasing  aridity  caused  a  drying  up 
of  the  streams  and  lakes  and  the  development  of  considerable  areas 
of  prairie  lands.  The  woodlands,  meadows,  and  dry  prairies  of  the 
time  favored  the  evolution  of  several  branches  adapted  to  the  dif- 
ferent environments,  and  the  horse  remains  of  the  epoch  show  that 
branches,  fitted  for  the  varied  conditions,  were  developed.  Some 
of  these  soon  became  extinct,  while  others  gave  rise  to  the  horses  of 
the  Miocene. 

The  great  expansion  of  the  prairies  and  diminution  of  the  forested 
areas  in  the  Miocene  favored  the  evolution  of  horses  fitted  for  rapid 
motion  on  the  dry,  hard  plains.  Two  explanations  for  the  increasing 
length  and  complexity  of  the  teeth  have  been  offered:  (i)  that,  as 
the  race  changed  from  a  habitat  of  forest  and  marsh  to  one  of  prairie, 
the  teeth  became  fitted  to  grind  up  the  hard,  nutritious  grasses  that 
covered  the  plains  (Osborn) ;  and  (2)  that  on  dry,  sandy  plains  where 
the  grass  was  short,  the  teeth  wore  out  rapidly  because  of  the  sand 
grains  which  were  necessarily  caught  up  with  the  grass  when  it  was 
cropped,  and  which  wore  away  the  teeth  even  more  rapidly  than 
hard  vegetation  would.  (Gidley.)  Those  who  hold  the  latter  view 
maintain  also  that  the  plains  grasses  were  and  are  actually  less  hard 
than  the  vegetation  of  the  marshes  and  forests,  and  that  consequently 
a  change  to  plains  vegetation  would  have  been  unimportant  had  it 
not  been  for  the  presence  of  sand  grains  in  the  food.  As  a  result  of 
the  above  causes,  we  find  the  Miocene  horses  fitted  for  plains  condi- 
tions increasing,  and  those  with  the  spreading  foot  and  short-crowned 
teeth  fitted  for  forest  conditions  becoming  extinct.  After  the  Mio- 
cene the  race  became  more  and  more  like  the  modern  horse.  By 
the  beginning  of  the  Glacial  Period  they  had  become  extraordinarily 
abundant,  but  at  its  close  they  had  entirely  disappeared  from  the 
western  hemisphere,  though  the  descendants  of  migrants  to  the 
Old  World  lived  on. 

Cause  of  the  Extinction  of  the  Horse  in  North  America.  —  It  is 
difficult  to  assign  a  reason  for  the  extinction  of  the  horses  in  America. 
The  cold  of  the  Glacial  Period  has  been  suggested,  but  is  hardly 


612  HISTORICAL  GEOLOGY 

adequate  since  the  climate  of  the  continent  south  of  the  ice  sheets 
was  not  unfavorable,  and  at  the  present  time  horses  on  the  western 
plains  survive  a  temperature  of  many  degrees  below  zero,  without 
shelter  or  any  food  other  than  that  which  they  can  obtain  for 
themselves,  being  able,  in  fact,  to  withstand  conditions  fatal  to 
cattle  and  sheep.  The  suggestion  that  the  extinction  was  due  to 
some  epidemic  receives  some  support  from  the  discovery  of  two 
species  of  tsetse  fly  in  the  Miocene  deposits  of  Colorado,  similar  to 
the  African  types  which,  in  that  country,  render  thousands  of  square 
miles  uninhabitable  by  horses.  Epidemics  such  as  that  carried  by 
the  tsetse  fly,  the  tick,  and  other  insects  are  most  prevalent  in  wet 
seasons.  The  moist  conditions  which  are  believed  to  have  prevailed 
in  North  America  during  glacial  times  would  favor  the  spread  of 
such  a  disease  over,  perhaps,  the  whole  of  the  New  World  and  might 
readily  wipe  out  of  existence  the  entire  race  of  horses. 


REFERENCES  FOR  HORSES 

BEDDARD,  F.  E.,  —  Mammalia. 

FLOWER,  SIR  W.  H.,  —  The  Horse. 

International  Encyclopedia,  —  Horse. 

LANKESTER,  E.  R.,  —  Extinct  Animals,  pp.  132-142. 

LUCAS,  F.  A.,  —  Animals  of  the  Past,  pp.  159-175. 

LULL,  R.  S.,  —  The  Evolution  of  the  Horse:  Am.  Jour.  Sci.,  Vol.  23,  1907,  pp.  161-182. 

MATTHEW,  W.  D.,  —  The  Evolution  of  the  Horse:  Am.  Museum  Jour.,  Vol.  3,  No.  i,  1903. 

OSBORN,  H.  F.,  —  The  Evolution  of  the  Horse  in  America:  Century  Mag.,  Vol.  69,  1905, 

PP.  3-17- 
SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  291-308. 

Elephants.  —  The  massive  body  and  legs,  the  long  tusks,  and 
flexible  trunk  of  the  elephant  combine  to  make  it  one  of  the  strangest 
of  animals  and,  from  external  appearance  alone,  one  which  might 
seem  least  likely  to  be  descended  from  the  generalized  mammals  of 
the  early  Eocene. 

The  earliest  known  fossil  elephants  (Mceritherium)  (Fig.  548) 
have  been  found  in  Upper  Eocene  deposits  of  Egypt.  They  were 
about  three  and  one  half  feet  high  and,  even  at  this  time,  were  of 
stocky  build,  although  they  would  hardly  be  recognized  as  belonging 
to  the  elephant  family  were  it  not  for  later  forms  which  became 
more  and  more  elephantlike.  The  structure  of  the  skull  shows 
that  a  flexible  upper  lip,  the  beginning  of  a  trunk,  was  present  in 
life.  The  teeth  had  already  been  reduced  to  36,  and  one  pair  of 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


613 


front  teeth  (incisors)  in  each  jaw  was  longer  than  the  others,  giving 
promise  of  the  great  tusks  of  the  elephant  and  mastodon.  Those 
of  the  upper  jaw  were  sharp-pointed  and  curved  downward,  while 
those  of  the  lower  jaw  were  directed  upward.  The  grinders  (molars) 


LOWER  PLIOCENE 
UPPER  MIOCENE 


MIDDLE  MIOCENE 
LOWER  MIOCENE 


Tefrabelodon 

[Shorten  my  ch/n) 


Tetrabe/odon 
(Sony  c/?/s?J 


Masfodon 
(short  chin) 


LOWER    OLIGOCENE 
UPPER    EOCENE 


MIDDLE  EOCENE 


LOWER  EOCENE       frfiorf  cfi/nj 
(ancestor  unknown/ 


Tefrsbe/odon 


Pa/eomasfodon 


Mo  e 


FIG.  548.  —  Chart  showing  the  evolution  of  the  elephant's  head  and  teeth. 
(Modified  after  Lull  and  Scott.) 

were  somewhat  ridged.     The  neck  was  of  sufficient  length  to  permit 
the  animal  to  reach  the  ground  in  feeding. 

The  next  elephant  in  the  line  of  descent  (Paleomastodon)  (Fig. 
548)  appeared  in  the  Upper  Eocene  and  was  larger  and  stockier, 
with  legs  similar  to  those  of  its  modern  relatives.  The  upper  and 


614  HISTORICAL  GEOLOGY 

lower  tusks  were  much  longer  than  in  the  earlier  form,  those  of  the 
upper  jaw  being  large,  with  a  slight  downward  curve.  This  en- 
largement of  the  tusks  was  accompanied  by  a  decrease  in  the  total 
number  of  teeth,  and  the  three-ridged  grinding  (molar)  teeth  were 
better  instruments  for  mastication  than  those  of  the  earlier  elephant. 
The  structure  of  the  skull  shows  that  the  upper  lip,  in  this  form,  had 
probably  been  developed  into  a  short  trunk  which,  however,  may 
not  have  extended  much  beyond  the  lower  tusks. 

An  elephant  from  the  Miocene  of  France  (Tetrabelodon),  smaller 
than  the  modern  Indian  elephant,  shows  a  great  advance  over  those 
of  the  Eocene.  This  is  to  be  expected,  since  no  Oligocene  elephants 
have  yet  been  found.  In  this  form  the  upper  tusks  are  long  and 
almost  straight,  while  the  lower  are  short,  but  since  they  are  set  in  a 
greatly  elongated  lower  jaw,  they  project  almost  as  far  as  the  upper. 
The  trunk  was  longer  than  in  the  earlier  members  of  the  family, 
resting  upon  the  lower  jaw,  and  could  only  be  raised  and  moved 
from  side  to  side.  As  the  trunk  lengthened  the  neck  shortened, 
since  the  animal  could  feed  without  the  mouth's  reaching  the  ground. 
Moreover,  as  the  tusks  and  trunk  became  heavier,  a  long  neck  would 
have  been  a  mechanical  disadvantage.  The  teeth  in  this  form  are 
quite  large  and  have  numerous  elevations  and  ridges.  This  genus 
spread  over  the  Old  World  and  North  America,  and  by  the  dropping 
of  the  lower  tusks  which  permitted  the  trunk  to  hang  straight  down, 
gave  rise  to  the  mastodon  (Dibelodon).  The  mastodon  differs  from 
the  true  elephant  in  two  principal  particulars:  (i)  in  the  teeth 
(Fig.  548),  which  are  composed  of  ridges  covered  with  enamel,  while 
the  grinding  surface  of  the  elephant's  tooth  is  made  up  of  vertical 
plates  of  enamel  and  dentine,  alternating  with  cement,  the  mastodon 
tooth  being  adapted  for  crushing  succulent  vegetation  such  as  leaves 
and  tender  twigs,  but  not  for  grinding  hard  grasses,  as  is  the  ele- 
phant's ;  and  (2)  in  the  greater  length  of  the  lower  jaw  which  in  true 
elephants  is  remarkably  short.  The  true  elephants  were  apparently 
derived  from  a  branch  (Stegodon)  that  lived  in  India  during  the 
Pliocene.  In  this  form  the  teeth  show  the  first  signs  of  developing 
cement  between  the  ridges,  which,  by  further  development,  formed 
the  remarkable  grinding  apparatus  of  the  mammoth  and  modern 
elephant. 

Both  the  mastodons  and  the  true  elephants  (mammoths)  spread  over 
North  America,  living  here  even  after  the  disappearance  of  the  last 
ice  sheet,  as  is  proved  by  the  presence  of  skeletons  in  the  bogs  that 


CENOZOIC  ERA:    AGE  OF  MAMMALS  615 

have  accumulated  in  the  depressions  of  the  latest  glacial  deposits. 
Paintings  and  carvings  on  ivory  of  mammoths,  made  by  prehistoric 
man  in  Europe,  prove  the  existence  of  elephants  after  the  appearance 
of  man  on  that  continent;  There  is,  however,  no  evidence  pointing 
to  their  presence  in  North  America  since  the  advent  of  man. 

Summary  of  the  Evolution  of  the  Elephant.  —  (i)  The  few  (three 
in  each  half  jaw)  long,  large,  many-ridged  teeth  of  the  elephant,  of 
which  not  more  than  one  and  one 
half  are  in  use  in  each  half  jaw  at 
one  time,  were  developed  from 
small,  short-crowned,  simple  teeth 
with  two  poorly  developed  ridges. 
(2)  The  tusks  are  greatly  elongated 
front  teeth  (incisors).  (3)  The 
trunk  began  as  a  short,  flexible  lip 
which,  as  the  lower  tusks  became 
longer,  gradually  lengthened  to  FIG.  549.  —  Restoration  of  a  Miocene 
1,1  i  i  i  elephant  (Dinotherium)  with  tusks  on 

enable    the    animal    to    reach    the    the  lower  jaw.    (After  H.  F.  Osborn.) 

ground   for  food  (Fig.  549).      The 

trunk  at  first  rested  upon  the  lower  tusks,  but  when  these  disap- 
peared in  the  course  of  the  evolution  of  the  race,  it  hung  straight 
down  and  because  of  this  new  position  soon  developed  its  present 
characteristics.  (4)  Accompanying  the  development  of  the  trunk 
and  tusks  was  a  shortening  of  the  neck.  (5)  The  bulk  and  height  of 
the  animal  increased  and  the  leg  straightened  to  support  the  greater 
weight. 

REFERENCES  FOR  ELEPHANTS 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  249-282. 
LANKESTER,  E.  R.,  —  Extinct  Animals,  pp.  125-132. 

LULL,  R.  S.,  —  The  Evolution  of  the  Elephant:  Am.  Jour.  Sci.,  Vol.  25,  1908,  pp.  169-212. 
SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  129-199; 
422-442. 

EVEN-TOED  HOOFED  MAMMALS  (ARTIODACTYLS) 

This  division  of  the  hoofed  mammals  (Fig.  540,  p.  599)  is  the  most 
important  at  present,  being  represented  by  such  animals  as  the  camel, 
deer,  sheep,  goat,  and  antelope. 

Camels.  —  Evidence  that  the  climate  over  large  areas  of  the 
western  interior  of  North  America  was  dry  for  long  periods  of  time 
is  shown,  as  has  been  indicated  (p.  611),  in  the  evolution  of  certain 


6i6 


HISTORICAL  GEOLOGY 


animals  —  an  evolution  especially  fitting  them  for  life  on  the  plains 
and  deserts.  The  camel,  as  is  well  known,  is  admirably  adapted 
for  arid  conditions.  Its  two-toed  foot  encased  in  a  single  pad  is 
efficient  in  traveling  over  desert  sand,  its  long,  well-nigh  structurally 
perfect  legs  enabling  it  to  move  rapidly  and  with  the  minimum  effort, 

and  its  capacity  for  carrying  water,  were 
the  results  of  a  long  evolution  under  such 
conditions. 

Although   not   as   well    known    as   the 
ancestry  of  the  horse,  the  history  of  the 
camel  is  fairly  complete  (Fig.  550)  from 
the  Eocene  to  the  present.     Following  a 
very  generalized  ancestor  (Trigonolestes) 
of  the  earlier  Eocene,  which  may  equally 
well    be    considered    ancestral    to    other 
families,    there    appeared     in    the    later 
Eocene  a  very  generalized  camel  (Proty- 
lopus),  a  little  larger 
than  a  jack  rabbit, 
which  is  possibly  an- 
cestral to  the  mod- 
ern    camels.       The 
points    in    which    it 
differs  most  notice- 
ably    from      living 
members    of    the 
family  are:   (i)  the 

size;  (2)  the  small, 
FIG.  550. -Evolution  of  the  camel's  foot  from  the  Eocene    sj        le  teeth   of  the 
to  the  present.     (After  Scott.) 

normal  number  (44) ; 

(3)  the  presence  of  two  side  toes  on  each  foot  in  addition  to  the  two 
useful  toes ;  and  (4)  the  separate  bones  of  the  forearm  (radius  and 
ulna),  which  did  not  grow  together  until  late  in  the  life  of  the  in- 
dividual. 

The  Oligocene  camel  (Poebrotherium)  was  of  slender  proportions, 
somewhat  resembling  a  llama,  though  with  a  shorter  neck.  In  this 
camel  evolution  had  progressed  in  two  principal  particulars :  the 
side  toes  were  absent  and  were  represented  by  splints,  and  the 
bones  of  the  forearm  were  joined  when  the  animal  was  still  very 
young. 


CENOZOIC   ERA:    AGE  OF  MAMMALS  617 

The  next  in  line  (Procamelus)  lived  in  the  Miocene  and  shows  a 
further  approach  to  the  modern  camel,  having  a  longer  neck  than 
the  preceding  and  more  camel-like  contour.  In  this  stage  the  bones 
of  the  forearm  were  united  before  birth,  and  certain  bones  of  the  foot 
(metapodials,  or  "  cannon  bones  ")  were  united  early  in  life.  The 
splints  were  entirely  absent.  The  animal  was  intermediate  in  size 
between  the  llama  and  camel.  Some  of  the  Pliocene  and  Pleistocene 
camels  attained  a  larger  size  than  any  existing  species.  Besides 
these  in  the  direct  line  of  descent,  a  number  of  side  branches  arose 
to  take  advantage  of  various  conditions  of  climate  and  food. 

The  camel  is,  in  the  even-toed  line  (artiodactyl),  what  the  horse 
is  in  the  odd-toed  line  (perissidactyl),  each  family  having  apparently 
progressed  almost  as  far  as  possible  in  the  perfection  of  its  foot 
structure. 

The  camel  family  lived  in  the  New  World  for  perhaps  3,000,000 
years  and  then  completely  disappeared  from  the  North  American 
continent,  where  its  evolution  had  taken  place.  The  camels  (llamas 
and  alpacas)  of  South  America  succeeded  in  living  on  in  that 
continent,  to  which  their  ancestors  had  migrated  in  the  Pliocene  or 
Pleistocene.  The  entire  family  was  confined  to  North  America  until 
the  Pliocene,  when  it  invaded  the  Old  World,  and  in  the  Pliocene  or 
Pleistocene  it  invaded  South  America.  The  dromedary  and  camel 
still  survive  in  Africa  and  Asia,  and  the  llama  and  vicuna  in  South 
America.  Here  again  is  a  great  family  which,  having  developed 
into  almost  its  present  form  in  North  America,  became  extinct  on 
this  continent,  although  still  living  on  in  others.  The  cause  of  the 
extinction  of  this  family  is  as  difficult  to  find  as  that  of  the  horse ; 
for,  as  in  the  case  of  the  horse,  the  conditions  in  portions  of  America 
to-day  (Arizona,  New  Mexico,  and  Mexico)  have  been  shown  by  ex- 
periment to  be  as  favorable  for  camels  as  those  in  their  African  and 
Asiatic  homes,  and  they  would  doubtless  be  used  in  arid  and  semi- 
arid  regions  of  western  North  America  if  economic  necessity  de- 
manded. Their  extinction  in  North  America  may  have  been  due 
to  the  same  cause,  or  causes,  that  produced  the  extinction  of  so  many 
species  at  the  close  of  the  Pliocene. 

REFERENCES  FOR  CAMELS 

BEDDARD,  F.  E.,  —  Mammalia. 

International  Encyclopedia,  —  Camel. 

SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  386-402. 


6i8 


HISTORICAL  GEOLOGY 


FIG. 

ancestral  deer  (Lower 
Oligocene).  (After  H.  F. 
Osborn.) 


Deer.  —  There  are  few,  if  any,  cases  in  which  the  changes  that 
the  individual  undergoes  in  his  growth  from  birth  to  old  age,  so  con- 
spicuously parallel  those  which  his  ancestors 
underwent  in  the  course  of  their  geological 
history  as  in  the  deer.  In  the  development  of 
the  existing  deer  the  males  and  females  are 
born  hornless ;  at  the  end  of  the  first  year  the 
551.  —  Hornless  male  acquires  a  simple,  one-pronged  antler; 
this  is  shed,  and  at  the  end  of  the  second  year 
a  two-pronged  antler  is  grown ;  in  the  next 
year  the  antlers  have  two  or  three  tines,  and 
so  on  until  the  maximum  number  for  the  species  has  been  reached. 
The  geological  history  of  the  deer  agrees  in  many  particulars  with 
the  facts  of  individual  development  from  year  to  year.  The  oldest 
known  members  of  the  tribe  (Leptomeryx)  (Fig.  551)  in  the  Oli- 
gocene have  no  horns,  as  is  true  of  their  surviving  relatives  in  Asia. 

The  earliest  deer  with  horns 
(Dicroceras)  (Fig.  552  A)  of 
which  there  is  any  record 
lived  in  the  Miocene  where 
the  antlers  were  two- 
pronged.  In  the  Upper 
Miocene  deer  with  three- 
pronged  antlers  (Fig.  552  B} 
begin,  and  in  the  Pliocene 
the  four-pronged  (Fig.  552 
C) ;  then  the  five-pronged ; 
and  finally,  near  the  close 
of  the  Pliocene,  a  deer  ap- 
pears in  which  the  antlers 
are  extremely  branched. 
The  deer  first  migrated  into 
America  after  the  two- 
pronged  stage  had  been  de- 
veloped. 

The    teeth    of    the    Oli- 
Ay  two-pronged  deer  of  the  Middle    gocene  deer  are  very  short- 


B  C 

FIG.  552.  —  Horns  of  deer,  showing  the  evolu- 


crowned,  but  in  the  course 


tion  of  horns. 

Miocene  (Dicroceras} ;   B,  the  three-pronged  horn 

of  the  Lower  Pliocene  deer ;   C,  the  four-pronged  ™ 

horn   of  the   Upper   Pliocene.     (Modified   after    ot  tne  A  ertiary  they  become 

Dawkins.)  longer  and  more  thoroughly 


CENOZOIC  ERA:    AGE  OF  MAMMALS  619 

adapted  to  the  mastication  of  plains  vegetation. 
Various  side  branches  appeared  and  became 
extinct  during  the  period.  Among  them  one 
unusual  form  (Protoceras)  which  was  about 
the  height  of  a  sheep  had  two  pairs  of  short, 

bony  horns   and  canine  tusks  (Fig.  553).      Its    .   FlG;  553-  — A  four- 

,    .          ••.••••          ,      ,     horned  deer  (Syndyoce- 
ancestry  is  unknown,  and  it  probably  reached    ras}  (Upper  oiigocene). 

North  America  by  immigration.  (After  H.  F.  Osborn.) 

REFERENCES  FOR  DEER 

MATTHEW,  W.  D.,  —  Osteology  of  Blastomeryx  and  Phytogeny  of  the  American  Cervidce: 

Bull.  Am.  Mus.  Nat.  Hist.,  Vol.  24,  1908,  pp.  535-562. 
ROMANES,  G.  J.,  —  Darwin  and  after  Darwin,  Vol.  i,  pp.  166-169. 
WOODWARD,  A.  S., . —  Vertebrate  Paleontology,  pp.  363-365. 

Cattle,  Sheep,  and  Goats.  —  True  cattle  first  appear,  as  far  as 
known,  in  the  early  Pliocene  deposits  of  Asia.  Concerning  the 
geological  history  of  cattle,  sheep,  and  goats  little  is  known,  since 
the  difference  in  the  ox,  antelope,  sheep,  and  goats  is  largely  a  matter 
of  the  curve  of  the  horn  and  the  build.  This  entire  family  (Bovidae) 
have  their  maximum  development  at  the  present  time. 

Swine  and  Related  Animals.  —  True  pigs  were  confined  to  the 
Old  World  until  brought  to  the  New  by  Europeans,  although  their 
relatives,  the  peccaries,  were  abundant  in  portions  of  both  North 
and  South  America.  The  entire  family  is  very  simple  in  structure, 
being  the  least  altered  of  the  descendants  of  the  early,  even-toed 
mammals  (artiodactyls),  and  dates  back  to  the  early  Eocene,  although 
the  oldest  species  of  the  true  pig  is  not  known  before  the  Miocene. 
Several  extinct  families,  distantly  related  to  the  pig,  played  an  im- 
portant part  in  the  life  of  the  Tertiary;  and  of  these  one  (Entelodon) 
is  especially  worthy  of  mention.  One  species  was  as  large  as  a 
rhinoceros,  with  a  head  four  feet  long.  One  peculiarity  of  the  skull 
consisted  in  a  prolongation  of  the  cheek  bones  on  either  side  of  the 
lower  jaw.  They  had  two-toed  feet,  being  rather  highly  specialized 
in  this  and  other  particulars. 

Another  generalized  animal  (Oreodon),  in  some  respects  combining  the  characters 
of  the  deer  and  hog  but  not  ancestral  to  them,  was  extremely  abundant  at  certain 
times  during  the  Oiigocene  and  Miocene  and  may  have  been  partially  responsible  for 
the  extinction  of  the  titanotheres.  It  was  not  larger  than  a  sheep,  with  a  long  tail 
and  four-toed  feet. 

A  Climbing  Ungulate.  —  An  interesting  example  of  a  modification  in  structure, 
fitting  an  animal  for  conditions  very  different  from  those  to  which  its  relatives  were 


620  HISTORICAL  GEOLOGY 

accustomed,  is  seen  in  a  hoofed  mammal,  a  member  of  the  oreodont  family  (Agrio- 
chaerus).  In  this  Oligocene  creature  the  feet  and  limbs  are  modified  in  such  a  way 
as  apparently  to  enable  it  to  climb  trees  as  easily  as  a  jaguar  or  other  large  cat.  The 
hoofs  are  so  narrow  as  to  be  actually  converted  into  a  sort  of  claw,  and  the  wrist  and 
ankle  joints  are  modified  in  such  a  way  as  to  make  the  wrists  and  ankles  as  flexible  as 
those  of  a  cat.1 

Insectivores.  —  This  primitive  group  occupied  an  important  place 
in  the  Eocene,  since  which  time  it  has  dwindled  in  numbers  and  is 
now  represented  by  a  few  survivors  inhabiting,  for  the  most  part, 
uncongenial  regions,  or  else  protected  by  spiny  armor,  or  of  sub- 
terranean habits.  It  is  represented  by  the  mole,  hedgehog  (not 
the  porcupine),  shrew,  and  other  small  animals  that  feed  largely 
on  insects  and  worms.  Insectivores  have  remained  simple  in  their 
organization  since  their  introduction  and  are  the  least  altered  of  the 
great  branches.  Among  their  generalized  characters  are  the  smooth 
brain,  five-clawed  toes,  the  habit  of  walking  with  the  whole  or  greater 
part  of  the  soles  to  the  ground  (plantigrade),  and  other  less  con- 
spicuous features.  The  group  is  so  generalized,  in  fact,  that  it  is 
difficult  to  characterize  it  without  a  too  technical  description.  Per- 
haps the  most  striking  feature  of  some  living  genera  is  the  elongated, 
or  proboscis-like,  nose.  It  is  possible  that  this  group  more  nearly 
represents  the  characters  and  habits  of  the  primitive  true  mammals 
(Eutheria)  than  any  other  now  living.  There  seems  to  be  little  doubt 
that  bats  are  descendants  of  primitive  members  of  this  group. 

Rodents  (Gnawing  Animals).  —  Rodents  are  first  known  from  the 
Eocene,  before  the  close  of  which  epoch  they  had  acquired  practically 
all  their  present  characteristics.  With  the  exception  of  their  powerful 
gnawing  teeth  (incisors),  rodents,  in  the  past  and  present,  have  been 
animals  of  simple  structure.  Their  brains  are  smooth,  and  the  race 
has  apparently  changed  in  no  essential  feature  since  Eocene  times, 
with  the  exception  of  their  teeth  which  have  been  slightly  reduced 
in  Dumber,  and  the  grinders  (molars  and  premolars),  in  some  species, 
have  become  perhaps  as  highly  developed  as  those  of  any  other  class. 
Before  the  close  of  the  Oligocene,  squirrels,  marmots,  beavers,  rabbits, 
pocket  gophers,  and  others  were  present. 

A  burrowing  rodent  with  horns,  which  appears  in  the  Miocene  and  early  Pliocene,  is 
interesting  as  showing  the  possibility  of  variation.  It  seems  to  have  been  much  better 
adapted  for  digging  than  existing  gophers,  but  of  what  use  the  horns  could  have  been 

1  Matthew,  W.  D.,  —  A  Tree-Climbing  Ruminant:  Am.  Museum  Jour.,  Vol.  n,  1911, 
pp.  162-163. 


CENOZOIC  ERA:    AGE  OF  MAMMALS  621 

to  a  burrowing  animal  it  is  difficult  to  imagine.     They  may  have  served  as  accessories 
to  the  strong  claws  in  digging,  or  they  may  prove  to  be  sexual  characters. 

In  the  Eocene  there  also  appeared  a  race  (Tillotherium)  (Figs.  554  A,  B)  similar  in 
habits  to  the  rodents,  although  not  of  this  tribe,  some  members  of  which  grew  to  be  of 
considerable  size,  one  species  being  half  as  large  as  the  tapir.  In  South  America  during 


B 

FIG.  554.  —  A,  skull ;  and  B,  restoration  of  the  head  of  Tillotherium,  a  peculiar 
rodent-like  creature  (Eocene).     (After  H.  F.  Osborn.) 

the  Miocene  rodents  were  abundant,  but  all  belonged  to  the  great  porcupine  group 
such  as  live  on  that  continent  to-day;  the  forms  common  in  North  America  at  that 
time,  the  rats,  mice,  squirrels,  beavers,  marmots,  hares,  and  rabbits,  being  absent. 

It  is  an  interesting  fact  that,  notwithstanding  their  failure  to 
develop  a  highly  complex  brain,  rodents  are,  at  present,  the  most 
abundant  of  mammals.  This  has  been  possible  because  of  their 
fecundity  and  adaptability  to  varying  conditions. 

REFERENCES  FOR  RODENTS 

GIDLEY,  J.  W.,  —  A  New  Horned  Rodent  from  the  Miocene  of  Kansas:  Proc.  U.  S.  Nat. 

Mus.,  Vol.  32,  No.  1554,  1913. 
WOODWARD,  A.  S.,  —  Vertebrate  Paleontology,  pp.  373-374. 

Edentates  (Latin,  edentatus,  toothless).  —  The  earliest  Eocene  edentates  (Ganodonta) 
were  so  similar  to  the  ancestral  herbivores  (Condylarthra)  and  carnivores  (Creodonta) 
that  it  seems  probable  that  the  three  orders  were  derived  from  a  common  ancestor 
only  a  short  time  before.  The  most  familiar  living  edentates  are  the  armadillos,  the 
anteaters,  and  the  sloths.  The  name,  edentate,  is  misleading,  since  most  of  the  order 
have  teeth  which  are  much  alike  and  are  without  enamel.  Teeth,  however,  are  lack- 
ing in  the  front  part  of  the  mouth,  and  it  was  from  this  character  that  the  name  was 
given.  One  modification  —  among  a  number  —  should  be  mentioned.  In  the  earliest 
forms  the  teeth  were  covered  with  enamel,  as  is  commonly  true  of  mammalian  teeth, 
and  had  definite  roots.  In  the  later  forms  the  teeth  become  rootless,  and  the  enamel 
disappears  except  in  narrow  bands.  It  is  rather  surprising  to  find  armadillos  (Meta- 
cheiromys)  as  far  back  as  the  Eocene,  similar  in  many  respects  to  the  smaller  armadillos 
of  to-day,  but  apparently  possessing  a  leathery  instead  of  a  bony  shield  and  with  dif- 
ferent teeth. 


622  HISTORICAL  GEOLOGY 

The  order  did  not  attain  great  importance  until  the  Pliocene  and  Pleistocene,  at 
which  time  (p.  670)  it  assumed  a  leading  role  in  South  America.  Some  of  the  South 
American  edentates  were  the  largest  creatures  on  that  continent.  The  description  of 
the  South  American  sloths  will  be  taken  up  in  the  discussion  of  Pleistocene  mammals. 

True  Carnivores.  —  When  traced  back,  it  is  found  that  such  dis- 
tinct families  as  the  dog,  hyena,  and  cat  become  less  and  less  easily 
distinguished,  until  they  converge  in  the  primitive  carnivores  (Creo- 
donta,  p.  594)  of  the  Eocene;  and  these,  in  turn,  have  affinities 
with  both  insectivores  and  ancestral  hoofed  mammals  (Condylarthra). 
Before  the  close  of  the  Oligocene  many  families  of  the  true  carnivores 
appeared  and  lived  in  competition  with  the  ancestral  carnivores, 
which  they  entirely  replaced  before  the  close  of  the  Oligocene. 

In  the  Eocene  and  Oligocene  primitive  representatives  of  the 
families  to  which  belong  the  dog,  weasel,  cat,  and  hyena  appeared ; 
but  the  families  were  more  clearly  differentiated  in  the  latter,  at 
which  time  ancestral  dogs,  raccoons,  and  weasels  were  common 
although  not  yet  of  a  distinctly  modern  type.  In  the  epoch  following 
(Miocene)  carnivores  were  abundant,  and  some  of  them  so  closely 
resemble  those  of  to-day  that  they  have  been  included  in  the  same 
genera  as  living  animals.  Wolves,  foxes,  panther-like  animals, 
saber-toothed  tigers,  ancestral  raccoons,  as  well  as  weasels  and  other 
like  forms  were  present.  In  Europe  the  bear  and  hyena  were  rep- 
resented as  well  as  the  above.  In  the  Pliocene  carnivores  flourished 
and  perhaps  gained  on  the  herbivores,  forcing  them  to  develop  greater 
speed,  sagacity,  and  powers  of  defense. 

In  South  America  during  the  Miocene  there  were  no  true  carni- 
vores ;  but  their  place  in  nature  was  taken  by  carnivorous  marsupials, 
such  as  live  in  Australia  to-day. 

Primates  (Monkeys,  Apes,  Lemurs). — This  order  of  mammals 
has  especial  interest  because  it  also  includes  man.  The  first  known 
members  (lemurs)  date  back  to  the  earliest  Eocene  deposits,  where 
their  remains  so  closely  resemble  those  of  the  generalized  insec- 
tivores of  that  early  time  that  it  is  difficult  to  distinguish  one  from 
the  other.  Monkeys  and  lemurs  lived  in  North  America  during  the 
Eocene,  but  disappeared  from  this  continent  at  the  beginning  of 
the  Miocene.  Primate  remains  from  the  Miocene  of  France  (Dryopi- 
thecus)  are  of  great  interest  because  of  the  similarity  in  some  respects 
to  the  skeleton  of  man,  and  also  because  of  the  possibility  that  these 
animals  were  able  to  make  rough  flint  implements  (eoliths,  p.  675). 
"  How  far  it  may  be  regarded  as  a  stem  from  which  on  the  one  side 


CENOZOIC   ERA:    AGE  OF   MAMMALS  623 

the  line  led  to  the  human  race  and  from  the  other  to  the  living  anthro- 
poids, namely  the  chimpanzee,  orang,  gibbon,  and  gorilla  cannot  be 
certainly  determined."  (Osborn.)  The  discussion  of  the  so-called 
man-ape  (Pithecanthropus)  and  others  will  be  taken  up  in  connection 
with  the  evolution  of  man  in  the  next  period. 


REFERENCES  FOR  PRIMATES 

BEDDARD,  F.  E.,  —  Mammalia. 

WOODWARD,  A.  S.,  —  Vertebrate  Paleontology,  pp.  403-410. 

Birds.  —  The  presence  of  birds  in  the  Lower  Eocene,  typically 
modern  in  structure  and  in  no  sense  intermediate  between  the  Meso- 
zoic  toothed  birds  with  vertebrated  tails  and  the  birds  of  to-day, 
makes  the  question  as  to  the  origin  of  the  typical  Tertiary  life  (birds 
and  mammals)  exceptionally  difficult  in  the  present  state  of  our 
knowledge.  (The  discussion  under  Rise  of  Mammals,  p.  590,  should 
be  considered  in  this  connection.) 

Although  few  of  the  birds  of  the  Eocene  can  be  referred  to  living 
genera,  they  are  modern  in  all  essential  features.  Even  at  this  early 
date  there  were  living  relatives  of  the  vultures,  storks,  secretary  birds, 
sandpipers,  Old  World  quail,  sand  grouse,  cuckoos,  swifts,  herons, 
and  pelicans.  The  appearance  of  a  great,  flightless  bird  (Gastornis) 
as  large  as  an  ostrich  but  apparently  unrelated,  presenting  affinities 
to  wading  and  aquatic  birds,  is  interesting  as  showing  the  advanced 
stage  of  evolution  at  this  early  time. 

The  bird  life  of  the  Eocene  of  Europe  (Quercy,  France)  gives  a 
clue  to  the  climate  and  environment  of  portions,  at  least,  of  Europe 
at  that  time,  since  it  was  fitted  to  inhabit  great,  warm  plains,  scattered 
over  with  groves.  The  assemblage  is  a  tropical  one  and  approaches 
that  now  found  in  tropical  Africa  and  South  America,  although,  as 
in  the  case  of  the  vegetation  (p.  634),  tropical  forms  are  associated 
with  others  that  are  now  typical  of  temperate  regions. 

Most  of  the  bird  fossils  are  from  the  Miocene  and  later  formations, 
and  belong  to  existing  families  and  often  to  existing  genera.  A 
remarkable  bird  (Phororhachos)  from  the  Miocene  deposits  of 
Patagonia  shows  the  extreme  to  which  bird  evolution  may  be  carried. 
It  stood  about  seven  feet  high  and  had  a  head  as  long  as  that  of  a 
horse,  armed  with  a  pick-like  projection.  Its  habits  have  been 
variously  conjectured.  The  loss  of  wings  indicates  a  semiarid 
condition. 

CLELAND  GEOL. — 40 


624  HISTORICAL  GEOLOGY 

In  general,  it  can  be  said  that  the  earliest  Tertiary  birds  are  typi- 
cal modern  birds,  modified  for  various  conditions  of  life,  some  being 
aquatic,  some  waders,  and  some  land  birds;  and  that  the  changes 
which  took  place  during  the  period  resulted  in  the  production  of 
modern  genera  and  species.  Although  12,000  species  of  birds  are 
living  to-day,  less  than  500  are  known  from  the  Tertiary.  The 
reason  for  the  rareness  of  bird  remains,  as  has  already  been  discussed 
(p.  563),  is  probably  due  to  their  lightness,  which  causes  them  to 
float  and  thus  exposes  their  carcasses,  often  for  many  days,  to  fish 
and  other  carnivorous  animals. 

REFERENCES  FOR  BIRDS 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  220-224. 

LUCAS,  F.  A.,  —  Animals  of  the  Past,  pp.  138-158. 

OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  151,  152,  195,  257,  450. 

WOODWARD,  A.  S.,  —  Vertebrate  Paleontology,  p.  244. 

ZITTEL-EASTMAN,  —  Textbook  of  Paleontology,  Vol.  2,  pp.  268-278. 

Reptiles  and  Amphibians.  —  The  usual  practice  at  present  is  to 
include  in  the  North  American  Tertiary  no  formations  containing 
dinosaur  remains.  This  is  the  custom  even  though  a  great  uncon- 
formity exists  in  the  Laramie  (p.  517)  of  the  western  interior  of 
North  America,  which,  were  it  not  for  the  presence  of  dinosaur  fossils 
in  the  formation  (Lance)  overlying  the  unconformity,  would  doubt- 
less be  considered  the  dividing  line  between  the  Mesozoic  and  Ter- 
tiary. The  most  conspicuous  reptilian  survivors  of  the  Mesozoic 
were  the  turtles,  crocodiles,  and  large  river  lizards  (Champsosaurus), 
the  last,  however,  disappearing  early  in  the  period.  Snakes  began 
in  the  Cretaceous,  having  doubtless  been  derived  in  the  Mesozoi 
from  lizards,  by  the  degeneration  of  their  limbs.  A  number  of  species 
have  been  found  in  the  Tertiary,  few  of  which,  however,  are  to  be 
distinguished  from  those  now  living,  and  the  majority  belong  to  the 
non-poisonous  varieties.  Some  of  the  early  sea  snakes  attained  a 
length  of  about  20  feet. 

Among  amphibians,  salamanders,  newts,  frogs,  and  toads  occur  in 
Oligocene  deposits ;  and  numerous  impressions  of  tadpoles  have 
been  preserved. 

Deductions  as  to  the  climate  of  the  Tertiary  can  be  made  from  the 
reptilian  life  as  well  as  from  the  vegetation  (p.  634).  Crocodiles, 
large  and  small,  and  of  several  genera,  lived  in  abundance  in  the 
Middle  Eocene  (Bridger)  of  the  western  interior  and  suggest  a 


; 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


625 


climate  not  unlike  that  of  Florida  to-day,  and  a  country  similar  to 
the  bayou  region  of  the  Mississippi  delta.  The  rivers  of  this  time 
swarmed  with  turtles,  but  the  presence  of  land  tortoises,  some  of 
which  were  three  feet  long,  indicates  that  these  swampy  areas  were 
bordered  by  extensive  stretches  of  dry  land.  Numerous  land  tor- 
toises in  the  Oligocene  deposits  of  the  Great  Plains  show  that  dry- 
land conditions  were  widespread.  Since  spiny  lizards  are  largely 
confined  to-day  to  arid  regions,  the  presence  of  numerous  lizards 
(Glyptosaurus)  with  skulls  covered  with  spiny,  bony  plates  is  in- 
dicative of  dry  conditions  in  Montana  during  a  portion,  at  least,  of 
the  Oligocene. 

REFERENCES  FOR  REPTILES  AND  AMPHIBIANS 

CUNNINGHAM,  J.  T.,  —  Reptiles,  Amphibians,  and  Fishes. 
OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  208-209. 

Fishes.  —  The  fish  of  the  Tertiary  were  abundant  and  very  similar 
to  those  of  the  present  seas.  Ganoids  were  represented  by  a  few 
species,  and  teleosts 
were  very  much  as 
at  present,  both  in 
numbers  and  in  ap- 
pearance. The  most 
noted  deposit  of 
fossil  fish  in  America 
is  that  of  the  Green 
River  (Eocene) 
shales  of  Wyoming, 
where  thousands  of 
beautifully  pre- 
served specimens 
have  been  quarried, 
examples  of  which 
can  be  seen  in  almost 
any  museum.  The 
great  number  of  sharks'  teeth  (Fig.  555  A-D}  in  the  Tertiary  deposits 
of  the  Atlantic  Coastal  Plain  of  North  America  and  elsewhere  show 
that  sharks  were  very  abundant  in  this  period.  Some  of  them  must 
have  been  of  great  size,  judging  from  the  teeth  some  of  which  are  six 
and  one  half  inches  long  and  six  inches  broad.  A  close  living  relative, 
the  great  white  shark,  has  teeth  one  and  one  fourth  inches  long.  If 


FIG.  555.  —  Tertiary  shark  teeth:  A,  Odontaspis  cus- 
pidata;  B,  Carcharodon  megalodon;  C,  Hemipristis  serra; 
D,  Odontaspis  elegans.  (After  Maryland  Geol.  Surv.) 


626 


HISTORICAL  GEOLOGY 


the  proportion  of  size  of  teeth  to  length  of  body  holds  true  in  the 
two  species,  the  giant  Tertiary  shark  (Carcharodon  megalodon)  attained 
a  length  of  70  to  80  feet  and  possessed  jaws  five  to  six  feet  across. 
No  actual  measurements  of  these  sharks  have  been  made,  since 
their  skeletons,  being  cartilaginous,  have  not  been  preserved. 

REFERENCES   FOR   FISHES 

DEAN,  B.,  —  Fishes  Living  and  Fossil. ' 

OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  136,  160,  216,  339-340. 


INVERTEBRATES 

During  the  Tertiary,  limestone  strata  several  thousands  of  feet 
thick  were  built  up  by  the  accumulation  of  the  remains  of  inverte- 
brates. This  is  in  marked  contrast  to  the  deposits  formed  of  verte- 
brate remains,  which  are  never  of  great  thickness,  and  seldom  form 

even  thin  beds  of  great  extent. 
Limestone,  locally  of  enormous 
thickness  and  extent,  covering 
areas  in  the  Pyrenees  and  the  Alps 
mountains,  in  Greece,  northern 
Africa,  Persia,  China,  and  Japan, 
is  often  made  up  chiefly  of  the 
shells  of  Foraminifera  named 
Nummulites  (Latin,  nummus,  a 
Coin)  (Fig.  556)  from  the  shape 
Perhaps  at  no 

time  in  the  entire  history  of  the 
world  did  an  organism  of  similar 
size  live  in  greater  abundance.  Other  limestones  in  Europe  and  on 
the  Gulf  Coast  of  North  America  were  formed  in  large  part  of  other 
forms  of  Foraminifera. 

Brachiopods  and  crinoids  were  rare  throughout  the  period  and 
may  be  considered  as  races  about  to  become  extinct.  Sea  urchins 
(Fig-  557)  continued  to  be  abundant. 

Coral  reefs  are  rare  in  the  Eocene  and  had  a  distribution  different 
from  that  of  to-day,  well-developed  reefs  occurring  on  the  north  and 
south  flanks  cf  the  Alps  and  Pyrenees.  In  the  Miocene  and  Pliocene 
they  had  almost  their  present  distribution. 

Gastropods  and  pelecypods  (Figs.  558,  559)  were  abundant  through- 


FIG.  556. — TertiaryForammifera:  Num- 
mulites.     An  enlargement  of  a  portion  of     ancl  size  (p.  577). 
the  shell  is  seen  in  the  upper  left-hand 
corner. 


CENOZOIC  ERA:    AGE  OF  MAMMALS 


627 


out  the  Tertiary,  being,  as  now,  the  most  numerous  of  the  larger 
invertebrates.  During  the  period  they  became  more  and  more  like 
the  living  forms,  until  in  the  Pliocene  they  were  nearly  identical 
with  those  of  to-day.  Certain  species  are  characteristic  of  the  various 
formations  of  the  epoch  and  are  consequently  of  great  stratigraphic 
value.  Some  Miocene 
pelecypods  attained  a 
large  size;  oysters  13 
inches  long,  8  inches 
wide,  and  6  inches  thick, 
as  well  as  pectens  (Fig. 
558  /)  9  inches  in  diam- 
eter are  known.  Large 
size  was  not,  however, 
characteristic  of  the  pe- 
lecypods of  the  period. 
Larger  pelecypods  are 
living  to-day  than  in  any 
previous  period. 

The  cephalopods  were 
represented  by  the  nau- 
tilus and  squid,  the 
former  having  a  wider 
distribution  than  now. 

Insects.  —  At  the  present  about  400,000  living  species  of  insects 
have  been  described,  but  less  than  8000  fossil  species  are  known.  The 
small  number  of  fossil  insects  as  compared  with  those  of  the  present 
does  not  indicate  that  this  class  is  more  numerous  to-day  than  at 
certain  times  in  the  Tertiary,  but  rather  that  few  of  the  Tertiary 
species  have  been  preserved.  It  has  even  been  suggested  that  owing 
to  the  warmer  climate  and  more  luxuriant  vegetation,  and  judging 
from  the  proportion  of  species,  the  total  insect  fauna  of  the  Miocene 
of  Europe  may  have  been  greater,  in  some  respects,  than  it  is  now 
in  any  part  of  that  continent.  The  greater  number  of  Tertiary 
insects  have  been  preserved  in  amber,  in  which  they  were  entrapped 
when  the  gum  of  the  trees  on  which  they  were  crawling  was  first 
exuded  and  was  soft  and  sticky.  About  2000  species  have  been  thus 
preserved,  some  of  the  specimens  of  which  are  in  an  almost  perfect 
state  of  preservation,  all  of  the  external  characters  being  as  well 
shown  as  in  life ;  others  are  preserved  in  peat ;  and  still  others  ?n 


FIG.  557.  — Tertiary  echinoid  :   Scutella  aberti 
(Miocene).      (Maryland  Geol.  Surv.) 


FIG.  558.  —  Tertiary  invertebrates :  A,  Venericardia  planticosta  (Eocene) ;  B, 
Ostrea  sellcejormis  (Eocene) ;  C,  Turritella  humerosa  (Eocene) ;  D,  Hercoglossa  tuomeyi 
(Eocene) ;  E,  Turritella  mortoni  (Eocene) ;  F,  Ecphora  quadricostata  (Miocene) ;  G, 
fylutilithes  petrosus  (Eocene) ;  H,  Orthaulax  gabbi  (Miocene) ;  /,  Pecten  madisonius 
(Miocene);  /,  Turritella  variabilis  (Miocene). 


CENOZOIC  ERA:    AGE  OF   MAMMALS 


629 


mud.  The  finely  laminated  shales  of  Oeningen,  on  the  Lake  of  Con- 
stance, Florissant,  in  Colorado,  and  elsewhere  have  yielded  hundreds 
of  specimens. 

Horseflies,  Tsetse  Flies,  and  Ants.  —  The  presence  of  horseflies 
very  similar  to  living  forms  in  the  Miocene  deposits  (Florissant, 
Colorado)  is  interesting,  as  it  shows  that,  even  at  that  early  date, 


FIG.   559.  —  Tertiary  pelecypods  :    A,  Melina  (Pernd)  maxillata  (Miocene) ; 
B,  Arcoptera  avicultfformis  (Pliocene).     (Maryland  Geol.  Surv.) 

the  horse  was  probably  tormented  by  this  insect.  Although  the 
horse  has  changed  radically,  the  flies  have  remained  practically  the 
same.  In  the  same  deposits  (Florissant,  Colorado)  the  tsetse  flies 
occur.  '  The  exquisitely  preserved  ants  of  the  Baltic  amber,  be- 
longing to  the  Lower  Oligocene  formation,  are  in  all  respects  like 
existing  ants.  All  of  them  belong  to  existing  subfamilies,  most  of 
them  even  to  existing  genera,  and  a  few  of  them  are  practically  indis- 


630  HISTORICAL  GEOLOGY 

tinguishable  from  species  inhabiting  Europe  to-day.  That  some  of 
them  were  herders  of  plant  lice  is  proved  by  blocks  of  amber  con- 
taining masses  of  ants  mingled  with  the  plant  hce  which  they  were 
attending  when  the  liquid  resin  of  the  Oligocene  pines  flowed  over 
and  embedded  them.  Possibly  the  soldier  cast  is  a  recent  innovation, 
but  the  differentiation  of  the  males,  queens,  and  workers  was  as 
extreme,  and  precisely  of  the  same  character  as  now."  (W.  M. 
Wheeler.) 

The  insects  of  these  Colorado  Miocene  deposits  indicate  that  the 
locality  was  doubtless  an  upland  or  mountain  with  a  warm,  moist, 
but  not  tropical  climate.  The  presence  of  such  a  climate  in  the 
higher  lands  of  this  epoch  does  not  preclude  the  possibility  of  arid 
conditions  on  the  Great  Plains  and  in  Texas  (p.  635). 

There  appears  to  have  been  but  little  important  change  in  the 
insect  world  since  the  middle  of  the  Eocene  or  earlier,  almost  no  new 
orders  or  even  families  having  appeared,  although  the  genera  and 
species  have  changed.  This  is  perhaps  not  surprising,  since  insects 
of  the  modern  type  made  their  appearance  soon  after  flowering  plants 
became  widespread,  and  had  consequently  perfected  their  structure 
and  organs  previous  to  the  Tertiary,  during  the  long  Cretaceous 
Period. 

REFERENCES  FOR  INSECTS 

OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  263,  450. 
ZITTEL-EASTMAN,  —  Textbook  of  Paleontology,  Vol.  i,  pp.  794-821. 

VEGETATION 

The  vegetable  kingdom  reached  its  culmination  before  the  animal 
kingdom ;  and  as  far  as  plant  evolution  is  concerned,  it  is  almost 
arbitrary  to  separate  the  Tertiary  from  the  Mesozoic  or  from  the 
Pleistocene.  Even  at  the  beginning  of  the  Tertiary,  the  general 
aspect  of  the  forests  was  not  very  different  from  that  of  to-day,  as 
the  presence  of  maples,  poplars,  sycamores,  walnuts,  hazelnuts, 
elms,  yews,  cedars,  and  sequoias  (redwoods)  shows.  The  associa- 
tion, however,  is  rather  remarkable  since  with  the  above  are  found 
figs  and  palms.  The  presence  in  Greenland,  Iceland,  and  Spitz- 
bergen  of  trees  such  as  now  grow  in  the  United  States  indicates  a 
warmer  climate  in  the  polar  regions. 

Grasses.  —  There  was,  however,  one  very  important  element  of 
the  vegetation  —  the  grasses  —  which  at  the  beginning  of  the  period 


CENOZOIC  ERA:    AGE  OF  MAMMALS  631 

apparently  occupied  a  very  subordinate  position,  but  which  before 
its  close  became  widespread.  Because  of  the  part  played  by  this 
type  of  vegetation  in  the  evolution  of  the  most  important  branch 
of  mammals  (ungulates,  hoofed  mammals)  it  will  be  well  to  discuss 
the  evidence  upon  which  its  presence  is  based.  "  If  we  observe  the 
conditions  of  the  preservation  of  plant  remains  along  existing  ponds, 
river  borders,  or  swamps,  we  see  at  once  that  they  are  as  favorable 
for  the  preservation  of  deciduous  leaves  as  they  are  unfavorable  for 
the  preservation  of  grasses.  Grasses  are  firmly  attached  to  their 
roots  and  are  not  swept  away  either  by  water  or  wind.  Leaf  deposits, 
therefore,  abound  everywhere  and  give  us  sure  indications  of  the 
forest  flora,  while  we  know  but  little  of  the  field  and  meadow  flora, 
which  is  of  great  importance  in  connection  with  the  evolution  of  the 
grazing  herbivorous  ungulates  especially.  In  fact,  the  evidence  as 
to  grasses  is  very  limited  throughout  the  entire  Age  of  Mammals.  The 
number  of  kinds  of  grasses  (Graminae)  found  in  the  whole  Cenozoic  of 
Europe  is  comparatively  small,  and  it  is  difficult  to  draw  conclusions 
from  fossil  plant  remains  alone  as  to  their  relative  or  absolute  im- 
portance. At  what  period  grasses  began  to  assume  anything  like 
their  present  dominance  it  is  impossible  to  determine.  The  absence 
of  native  grasses  in  Australia  is  indirect  evidence  of  their  late  geological 
development."  (Osborn.)  The  indirect  evidence  of  the  history  of 
grasses,  derived  from  the  adaptation  of  the  teeth  of  the  hoofed  mam- 
mals, "  disposes  us  to  adopt  the  opinion  that  grasses  attained  wide 
distribution  in  both  hemispheres  only  toward  the  close  of  the  Eocene. 
Their  evolution  on  favorable  forestless  regions  was  certainly  a  pro- 
longed one,  beginning  in  Mesozoic  times."  (Osborn.)  Proof  that 
grasses  were  widespread  in  the  Miocene  is  based  upon  the  structure 
of  the  mammals ;  omnivorous  forms  were  becoming  grass  eaters ; 
the  method  of  chewing  was  changing  from  a  vertical,  biting  move- 
ment to  a  horizontal,  grinding  one;  and  the  teeth  were  becoming 
more  durable  and  better  suited  for  grinding  hard  food.  This  then 
was  apparently  the  time  at  which  the  grassy  plains  began. 

Daemonhelix.  —  Certain  fossils  occurring  abundantly  in  the  Mio- 
cene deposits  in  a  restricted  area  of  western  Nebraska  have  given  rise 
to  some  speculation.  They  consist  of  spirals  (Fig.  560)  of  harder 
rock,  held  together  by  fibrous  calcareous  material  which  sometimes 
shows  a  vegetable  structure,  and  because  of  their  shape  and  size 
they  have  received  the  name  Daemonhelix  (Devil's  Corkscrew). 
Some  of  them  are  ten  feet  or  more  in  height  and  a  foot  in  diameter; 


632 


HISTORICAL  GEOLOGY 


FIG.    560.  —  Daemonhelix,  Nebraska. 
(Redrawn  after  E.  H.  Barbour.) 


and  since  they  resist  erosion 
somewhat  better  than  the  sur- 
rounding rock,  they  often  stand 
out  prominently  against  the 
bluffs.  They  have  been  con- 
sidered the  burrows  of  extinct 
rodents  and  also  fossil  algae,  but 
the  proof  of  the  latter  seems 
now  to  be  well  established. 

Geological  History  of  Se- 
quoias. —  The  history  of  the 
sequoias  (big  trees  and  red- 
woods) of  California  is  a  strik- 
ing example  of  the  fate  of  many 
animals  and  plants,  and  illustrates  the  difficulty  of  finding  the 
cause  of  extinction  in  plants  as  well  as  animals.  These  big  trees, 
which  sometimes  grow  to  a  height  of  325  feet,  have  a  girth  of  50 
or  60  feet,  and  live  to  be  5000  years  old,  are  now  confined  to  the 
mountain  slopes  of  California.  In  the  Tertiary  the  sequoias  were 
common  trees  in  the  northern  hemisphere,  extending  from  Spitz- 
bergen  (78°  north  latitude)  as  far  south  as  the  middle  of  Italy,  in 
Asia  to  the  Sea  of  Japan,  and  over  a  large  part  of  North  America. 
The  sequoias  date  back  to  the  Cretaceous,  where  they  were  un- 
doubtedly represented  by  several  species.  "  This  is  perhaps  the  most 
remarkable  record  in  the  whole  history  of  vegetation.  The  sequoias 
are  the  giants  of  the  conifers,  the  grandest  representatives  of  the  family, 
and  the  fact  that,  after  spreading  over  the  whole  northern  hemisphere 
and  attaining  to  more  than  20  specific  forms,  their  decaying  remnant 
should  now  be  confined  to  one  limited  region  in  western  America  and 
to  two  species  constitutes  a  sad  memento  of  departed  greatness. 
The  small  remnant  of  Sequoia  gigantea  (the  big  trees)  still,  how- 
ever, towers  above  all  competitors,  as  eminently  the  "  big  trees," 
but,  had  they  and  the  allied  species  failed  to  escape  the  Tertiary  con- 
tinental submergences  and  the  disasters  of  the  Glacial  Period,  this 
grand  genus  would  have  been  to  us  an  extinct  type."  (Dawson.)  It 
is  stated  that  the  sequoias  were  so  abundant  in  northwest  Canada 
as  to  furnish  much  of  the  material  for  the  great  lignite  beds  of  that 
region.  That  the  climate  of  other  parts  of  the  world  is  still  suited 
to  the  growth  of  these  trees  is  shown  by  the  fact  that  sequoias  are 
now  growing  in  England  and  around  Lake  Geneva  in  Switzerland  from 


CENOZOIC  ERA:    AGE  OF  MAMMALS  633 

seeds  carried  there  from  California.  Their  extinction  becomes  more 
remarkable  since  it  is  known  that  "  under  the  most  favorable  condi- 
tions these  giants  probably  live  5000  years  or  more,  though  few  of 
the  larger  trees  are  more  than  half  as  old."  One  careful  observer 
says,  "  I  never  saw  a  big  tree  that  had  died  a  natural  death  :  barring 
accidents  they  seem  to  be  immortal,  being  exempt  from  all  the  diseases 
that  afflict  and  kill  other  trees.  Unless  destroyed  by  man,  they  live 
on  indefinitely  until  burned,  smashed  by  lightning,  or  cast  down  by 
storms,  or  by  the  giving  way  of  the  ground  on  which  they  stand." 
(John  Muir.) 

Diatoms.  —  The  earliest  specimens  known  with  certainty  to  be 
diatoms  are  from  the  Jurassic.  This  group  did  not  become  common 
until  the  Cretaceous,  or  abundant  until  the  Tertiary.  A  stratum 
50  feet  thick  in  Bohemia  is  almost  entirely  composed  of  diatoms ; 
about  Richmond,  Virginia,  there  is  a  deposit  30  feet  thick  and  many 
miles  in  extent ;  and  deposits  of  diatomaceous  earth  are  common  in 
other  parts  of  the  Coastal  Plain.  Thick  beds  also  occur  in  the  Cali- 
fornia Tertiary  (p.  581).  Diatom  deposits  are  now  forming  in  the 
Yellowstone  National  Park,  where  "they  cover  many  square  miles  in 
the  vicinity  of  active  and  extinct  hot  spring  vents  of  the  park,  and 
are  often  3  feet,  4  feet,  and  sometimes  5  to  6  feet  thick."  In  the  Ter- 
tiary, diatom  deposits  were  formed  in  sluggish  streams,  in  lakes,  on 
the  sea  bottom,  and  in  hot  springs,  as  they  are  now  in  Nevada  and 
California.  The  Cretaceous  and  Tertiary  species  very  closely  re- 
semble living  forms. 

Exceptional  Preservation  of  Plants.  —  The  preservation  of  such 
delicate  parts  of  plants  as  flowers,  catkins  of  the  oak,  pollen  grains, 
as  well  as  fungi,  in  the  amber  of  Oligocene  trees,  gives  us  almost 
as  definite  knowledge  of  some  of  the  Oligocene  plants  as  if  they  were 
living  to-day. 

In  certain  parts  of  Europe  Oligocene  mineral  springs  covered  with 
their  deposits  whatever  organic  remains  they  touched  and  thus 
buried  them.  After  a  time  the  organic  matter  decayed,  sometimes 
leaving  perfect  molds  of  their  forms.  When  these  molds  are  properly 
filled,  casts  even  of  fossil  flowers  and  insects  are  sometimes  obtained. 

Plant  Localities  in  North  America.  —  About  500  species  of  Miocene 
plants  are  known  in  North  America,  but  the  deposits  in  which  they 
are  found  are  small  and  widely  separated.  One  at  Brandon,  Ver- 
mont, consists  of  a  small,  pocket-like  deposit  of  lignite,  at  one  time 
worked  for  fuel,  which  has  yielded  a  large  number  of  fossil  fruits, 


634  HISTORICAL  GEOLOGY 

among  which  walnuts  and  hickory  nuts  have  been  identified,  al- 
though most  of  them  are  of  unknown  or  doubtful  affinity.  In  Colo- 
rado (Florissant)  the  deposits  of  a  Miocene  lake  (p.  580)  have  afforded 
large  numbers  of  plants,  such  as  alders,  oaks,  narrow-leafed  cotton- 
woods,  pines,  roses,  thistles,  asters,  and  Virginia  creepers.  Mixed 
with  these  are  others  of  a  more  southern  type,  such  as  the  holly, 
smoke  tree,  sweet  gum,  and  persimmon.  The  vegetation  of  the 
Pliocene  was  probably  almost  identical  with  that  in  the  adjoining 
regions  to-day,  but  so  few  plant  fossils  have  been  found  that  no 
definite  statements  can  be  made. 


CLIMATE 

Difficulty  in  Determining  Tertiary  Climates.  —  The  determina- 
tion of  the  climates  of  the  Tertiary  is  complicated  by  a  mingling  of 
plants  whose  relatives  are  no  longer  found  associated,  some  being  at 
present  restricted  to  tropical  or  subtropical  regions  and  others  to 
temperate  zones.  Since  closely  related  modern  species  sometimes 
live  under  very  different  climatic  conditions,  it  will  readily  be  seen 
that  the  presence  of  plants  in  the  Tertiary  related  to  species  now  living 
only  in  subtropical  regions,  for  example,  does  not  necessarily  prove 
that  these  early  species  lived  under  similar  conditions,  but  is  certainly 
strong  evidence  in  favor  of  such  a  supposition ;  especially  when  it 
can  be  shown  that  the  plant  associations  were  then  the  same  as  now; 
e.g.,  breadfruit  trees,  cycads,  and  many  ferns  grew  in  association  in 
Greenland  (72°  N.)  as  they  now  do  in  the  tropics.  This  mingling  of 
what  are  now  tropical  and  temperate  region  types  has  led  investi- 
gators to  very  different  conclusions.  For  example,  the  Miocene 
climate  of  Colorado,  because  of  the  presence  of  genera  now  living  in 
Colorado,  is  believed  by  one  investigator  (Cockerell)  to  have  been  in 
no  sense  tropical,  while  another  (Knowlton),  because  of  the  presence 
of  West  Indian  genera,  believes  that  the  climate  was  not  unlike  that 
of  certain  parts  of  the  West  Indies  of  to-day.  Certain  trees,  such  as 
palms,  are  generally  agreed  to  indicate  warm,  tropical,  or  subtropical 
conditions,  even  when  they  grew  in  forests  with  the  maple,  elm,  and 
other  temperate  region  plants.  When,  however,  the  remains  of 
insects,  reptiles,  or  mammals  occur  in  deposits  with  plants,  the  total 
evidence  often  becomes  conclusive. 

Eocene.  —  The  vegetation  suffered  so  little  change  between  the 
Upper  Cretaceous  and  the  Eocene  that  it  is  evident  the  climates  of  the 


CENOZOIC  ERA:    AGE  OF   MAMMALS  635 

two  epochs  were  not  unlike.  In  the  early  Eocene  (Fort  Union),  a  cool 
to  mild  temperate  climate,  with  a  much  greater  rainfall  than  now,  pre- 
vailed over  the  Dakotas,  Wyoming,  Montana,  and  as  far  north  as 
the  Mackenzie  River  in  Canada,  as  is  shown  by  the  remains  of  the 
walnut,  hickory,  viburnum,  grape,  elm,  poplar,  sequoia,  and  yew, 
and  by  the  presence  of  numerous,  often  thick,  beds  of  lignite.  In 
the  earliest  Eocene  the  vegetation  of  Greenland,  Iceland,  and  Spitz- 
bergen  included  alders,  magnolias,  lindens,  poplars,  and  birches, 
indicating  a  climate  similar  to  that  of  south  temperate  France  and 
California  at  the  present  time.  In  the  later  Eocene  palms  flourished 
in  southern  England ;  and  the  waters  were  tenanted  by  crocodiles 
and  giant  sea  snakes,  indicating  a  climate  like  that  of  tropical  America 
to-day,  and  warmer  than  in  the  western  interior  of  North  America. 

Oligocene.  —  The  climate  of  the  Oligocene  in  Europe  appears  to 
have  been  slightly  cooler  than  during  the  Eocene.  The  occurrence  of 
palms  in  the  Baltic  region,  however,  indicates  a  temperature  such  as 
now  prevails  in  Spain  and  Italy.  Nothing  is  known  of  the  grasses, 
and  the  development  of  the  teeth  of  the  mammals  does  not  afford  a 
positive  proof  of  their  presence.  The  presence  of  crocodiles  in  the 
Oligocene  deposits  of  South  Dakota  implies  a  climate  such  as  is  now 
found  in  Florida. 

Miocene.  —  Although  the  vegetation  is  similar  to  that  of  the 
Oligocene,  there  is  evidence  of  a  gradual  lowering  of  the  temperature 
in  the  Miocene ;  palms  ceased  to  exist  north  of  the  Alps ;  and  towards 
the  end  of  the  epoch  there  was  a  lowering  of  the  temperature  in  the 
Arctics.  In  Colorado  Miocene  deposits  no  palms  have  been  found, 
but  the  presence  of  figs,  which  now  do  not  grow  north  of  the  coast 
of  the  southern  states,  and  of  two  genera  of  trees  which  are  now  con- 
fined to  the  tropics  (Weinmania)  indicates  that  the  climate  of  this 
mountainous  region  was  more  equable,  moister,  and  somewhat  warmer 
than  now,  although  it  is  probable  that  on  the  Great  Plains  arid  con- 
ditions prevailed.  A  layer  of  fan-palm  leaves  a  foot  in  thickness  in  a 
formation  of  this  epoch  in  northern  Washington  points  to  almost 
tropical  conditions  in  that  region.  The  occurrence  of  breadfruit 
trees  associated  with  temperate  region  trees  in  the  Middle  Miocene 
of  Oregon  indicates  a  somewhat  warmer  climate  there  than  now. 

Pliocene. — The  gradual  cooling  of  the  climate  of  Europe  con- 
tinued in  the  Pliocene,  during  which  epoch  there  was  a  slow  south- 
ward movement  of  the  northern  forest  trees  and  a  disappearance 
of  delicate  tropical  types.  Towards  the  very  end  of  the  Pliocene  there 


636  HISTORICAL  GEOLOGY 

was  a  marked  lowering  of  the  temperature  and  perhaps  the  beginning 
of  glaciation  on  the  higher  mountains.  In  the  English  Pliocene  the 
proportion  of  Arctic  shells  rises  from  five  per  cent,  in  the  oldest  to 
over  sixty  per  cent,  in  the  youngest  beds. 

The  disappearance  of  rhinoceroses  and  the  browsing  types  of  horses 
and  camels  (those  that  lived  largely  on  the  leaves  of  shrubs  and  trees), 
as  well  as  the  existence  of  great  herds  of  land  tortoises  on  the  Great 
Plains  of  North  America,  is  perhaps  proof  of  arid  conditions  in  the 
western  interior.  The  disappearance  on  the  Pacific  coast  of  warm 
temperate  plants,  as  well  as  the  character  of  the  marine  and  fresh 
water  invertebrates,  indicates  a  change  to  colder  conditions.  Evi- 
dence is  at  hand  showing  that  Japan  was  colder  during  the  Pliocene 
than  during  the  Glacial  Period  (Yokohama).  This  has  again  sug- 
gested the  possibility  of  a  "  wandering  pole  "  (p.  660)  to  account  for 
the  Glacial  Period. 

REFERENCES  FOR  FLORA  AND   CLIMATE 

KNOWLTON,  F.  H.,  —  The  Relation  of  Pale  obotany  to  Geology:  Am.  Naturalist,  Vol.  46, 
1912,  pp.  207-215. 

KNOWLTON,  F.  H.,  —  Succession  and  Range  of  Mesozoic  and  Tertiary  Floras:  Jour. 
Geol.,  Vol.  18,  1910,  pp.  105-116. 

OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  104;  117;  184-185;  242-244;  282-285; 
306;  342-343. 

SCHUCHERT,  CHAS.,  —  Climates  of  Geologic  Time:  Carnegie  Institution  of  Washing- 
ton, Publication  No.  192,  1914,  pp.  263-298. 


EFFECTS  OF  ISOLATION  AND  MIGRATION 

During  the  Age  of  Mammals  the  seas  were  at  times  so  expanded  as 
to  isolate  large  areas  of  land,  while  at  others  they  were  so  restricted 
that  continents  now  separated  were  then  united  (Figs.  561,  562,  563). 
The  mammalian  life  of  Cuba  seems  to  have  been  derived  from  a  few 
species  that  were  carried  there  on  natural  rafts.  Other  islands  were 
doubtless  populated  in  the  same  way.  When  the  isolation  was  pro- 
longed, the  evolution  of  the  animal  life  of  the  various  continents  took 
place  independently.  When  the  lands  were  again  united,  widespread 
migrations  took  place.  This  isolation  and  later  establishment  of 
land  connections  occurred  several  times  during  the  Tertiary.  The 
proof  of  the  separation  or  reunion  of  great  land  areas  is  based  chiefly 
upon  the  dissimilarity  in  the  one  case,  and  the  similarity  in  the  other, 
of  the  life  of  the  past. 


CENOZOIC  ERA:   AGE  OF  MAMMALS 


637 


MIDDLE     EOCENE: 


FIG.  561.  —  Map  of  the  world  in  the  Middle  Eocene,  showing  the  isolation  of  the 
continents  and  the  conditions  favorable  for  the  development  of  provincial  faunas. 
(After  W.  D.  Matthew.) 


OLIGOCENE 


FIG.  562.  —  The  separation  of  Africa  and  South  America  from  their  neighboring 
continents  caused  the  animals  of  the  former  to  develop  independently,  while  the  union 
of  Asia  to  North  America  permitted  intermigration  between  these  continents.  (After 
W.  D.  Matthew.) 


638 


HISTORICAL  GEOLOGY 


The  effect  of  isolation  upon  the  animal  life  depends  somewhat  upon 
the  size  of  the  region  and  the  diversity  of  the  topography.  If  the 
topography,  climate,  and  vegetation  are  varied,  a  diversified  mam- 
malian fauna  will  arise  to  take  advantage  of  every  opportunity  of 
securing  food ;  and  the  body,  limbs,  and  feet  will  become  adapted  to 
a  great  variety  of  conditions ;  some  will  become  adapted  for  burrow- 
ing, some  for  life  in  the  water,  some  for  rapid  motion,  and  some  for 
tree  life.  The  larger  the  region  and  the  more  diverse  the  conditions, 
the  greater  will  be  the  variety  of  mammals  that  will  result.  When 


MIOCENE 


FIG.  563.  —  The  continents,  with  the  exception  of  South  America,  were  broadly 
united  during  the  Middle  and  Upper  Miocene,  permitting  widespread  migrations. 
(After  W.  D.  Matthew.) 

after  long  periods  of  isolation  animals  which,  because  of  the  physical 
conditions  under  which  they  lived  or  because  of  the  fierceness  of  the 
competition  with  other  forms,  had  become  especially  fitted  for  life, 
were  able  to  migrate  to  other  regions  where  for  various  reasons 
evolution  had  not  been  so  effective  in  producing  such  successful  types  ; 
the  better  fitted  quickly  possessed  the  new  regions,  either  forcing  the 
former  inhabitants  into  subordinate  positions  or  causing  their  extinc- 
tion. 

No  better  example  of  the  effect  of  such  isolation  can  be  found  than 
in  Australia  to-day,  where  only  mammals  of  a  low  type  (marsupials) 


CENOZOIC  ERA:    AGE  OF   MAMMALS  639 

occur.  These  Australian  mammals  are  very  different  from  those  of 
other  parts  of  the  world,  but  are  related  to  those  that  lived  in  Europe 
in  the  Mesozoic ;  types  which  with  a  few  exceptions  have  long  since 
been  extinct  in  other  continents.  The  natural  inference  is  that  Aus- 
tralia was  isolated  during  the  whole  of  the  Tertiary  Period  and  that, 
because  of  the  non-interference  of  the  higher  mammals,  the  mar- 
supials have  been  able  to  develop  there  along  their  own  lines,  produc- 
ing the  kangaroo,  the  wombat,  and  the  other  animals  peculiar  to  that 
continent. 

The  effect  of  isolation  and  migration  on  animal  life  is  especially 
well  shown  in  the  history  of  Tertiary  mammals. 

Eocene  Invasion.  —  The  first,  and  perhaps  most  important 
Tertiary  migration  occurred  at  the  very  beginning  of  that  period, 
as  is  shown  (i)  by  the  sudden  appearance  of  true  mammals  which, 
though  simple  in  structure,  were  already  somewhat  diversified,  and 
(2)  by  their  similarity  in  all  parts  of  the  world  where  found.  The 
land  connection,  or  connections,  which  permitted  this  migration 
from  a  center  whose  location  is  at  present  unknown,  disappeared, 
probably  by  subsidence,  and  the  continents  of  the  Old  and  the  New 
World  were  apparently  again  separated  by  broad  seas  for  a  long 
period  of  time  (Fig.  561),  permitting  the  life  of  the  isolated  regions  to 
develop  independently  during  the  Eocene.  A  comparison  of  the  life 
of  the  Middle  and  Upper  Eocene  shows  that  the  odd-toed,  hoofed 
mammals  (perissidactyls)  of  Europe  and  North  America  differed  in 
many  respects,  and  that,  although  horses  developed  on  the  two  conti- 
nents, they  were  markedly  dissimilar.  The  same  dissimilarity  is  shown 
in  the  carnivores  and  rodents.  During  this  period  of  isolation  three 
new  families  made  their  appearance  in  America,  the  camels  (p.  615), 
oreodonts  (p.  619),  and  armadillos  (p.  621) ;  and  the  most  striking 
of  the  Rocky  Mountain  Eocene  mammals  (Amblypoda)  were  prob- 
ably extinct  in  Europe  before  the  Upper  Eocene,  but  did  not  become 
extinct  in  America  until  near  the  close  of  the  Upper  Eocene.  The 
resemblance  that  existed  between  the  mammals  of  the  two  continents 
is  only  that  of  descent  from  similar  ancestors. 

Oligocene  Invasion.  —  The  simultaneous  appearance  in  Europe 
and  North  America  (Fig.  562)  of  new  families  of  mammals  of  a  de- 
cidedly more  modern  type  than  those  of  the  Eocene,  points  strongly 
to  their  evolution  in  some  region  separated  from  both  continents  for 
a  long  period,  and  united  to  them  at  approximately  the  same  time  by 
renewed  land  connections.  It  should  not  be  inferred  from  the  above 

CLELAND  GEOL.  —  41 


640  HISTORICAL  GEOLOGY 

that  the  intermigration  was  so  great  as  to  make  the  life  of  the  two 
continents  identical  at  the  beginning  of  the  Oligocene,  for  this  was 
far  from  being  the  case. 

Following  the  period  of  land  connections  at  the  beginning  of  the 
Oligocene,  the  Old  and  the  New  World  were  again  isolated  and  inde- 
pendent evolution  was  permitted. 

Miocene  African  Invasion.  —  The  similarity  of  the  life  of  Europe 
and  of  North  America  indicates  that  these  continents,  as  well  as  the 
East  Indies,  were  united  during  the  Miocene  (Fig.  563) ;  but  the  dis- 
similarity of  that  of  South  America  and  Australia  shows  that  these 
southern  continents  were  separated  from  the  others.  The  appear- 
ance in  the  Lower  Miocene  of  Africa  and  Europe  of  ancestral  elephants 
whose  development  had  been  taking  place  in  Africa  or  some  adjoining 
region,  during  the  earlier  portion  of  the  Tertiary,  shows  that  these 
continents  of  the  Old  World  were  then  united,  for  the  first  time 
perhaps,  since  the  early  Eocene.  The  mastodons  and  rhinoceroses 
migrated  to  America  at  this  time,  and  other  tribes  unquestionably 
came  with  them.  They  are  believed  to  have  reached  here  by  way  of 
the  Alaskan  land  connection.  These  strangers  had  little  effect  on 
the  life  of  the  New  World,  as  is  shown  by  the  development  in  America 
of  its  own  pigs,  oreodonts  (p.  619),  deer  (p.  618),  antelopes  (p.  619), 
camels  (p.  615),  horses  (p.  608),  etc.  Although  the  continental  con- 
nections were  well-established  between  Europe,  Asia,  Africa,  and 
North  America  in  the  Miocene  and  continued  throughout  the  epoch, 
permitting  the  spread  into  North  America  of  mastodons,  rhinoceroses, 
and  probably  other  mammals,  yet  many  of  the  important  races  of 
the  New  World  —  such  as  the  camels,  llamas,  ancestral  horses,  and 
ancestral  American  deer  —  were  confined  to  this  continent ;  and 
animals  equally  characteristic  of  the  Old  World  —  true  rhinoceroses, 
African  and  Asiatic  monkeys,  bears,  lynxes,  foxes,  hyenas,  true 
antelopes  —  are  not  known  to  have  migrated  to  North  America 
at  this  time. 

Pliocene  South  American  Invasion  and  Intermigration  between 
the  Old  and  New  Worlds.  —  With  the  connection  of  South  America 
and  North  America  in  the  Pliocene,  all  of  the  continents  of  the  world 
with  the  exception  of  Australia  were  united,  and  widespread  migra- 
tion occurred. 

South  America  seems  to  have  been  isolated  from  the  rest  of  the 
world  from  early  Eocene  times  until  the  Pliocene.  Its  fauna,  before 
the  Pliocene  connection  with  North  America  was  established,  was 


CENOZOIC  ERA:    AGE  OF  MAMMALS  641 

comprised  of  (i)  marsupials  resembling,  in  a  marked  degree,  those  of 
Australia  to-day,  and  (2)  true  mammals  differing  greatly  from  those 
that  had  been  developing  in  North  America.  The  explanation  of 
this  peculiar  fauna  is  probably  to  be  found  (i)  in  the  absence  of  all 
true  carnivores  (the  cat  and  dog  family  having,  so  far  as  is  known, 
failed  to  send  any  Eocene  representatives  there),  and  (2)  in  the  small 
variety  of  ancestral  forms  from  which  the  fauna  developed.  This 
small  number  of  ancestral  true  mammals  indicates  that  the  Eocene 
Central  American  connection  had  been  of  brief  duration.  These 
South  American  ancestral  forms  came  from  North  America,  or  from 
Australia  by  way  of  the  Antarctic  Continent,  or  from  both. 

During  the  period  of  separation,  several  families  of  strange  hoofed 
mammals  were  evolved  to  take  advantage  of  the  varied  physical 
conditions,  some  of  which  (Litopterna)  were  odd-toed,  with  bodily 
proportions  resembling  those  of  the  horse  and  llama.  In  this 
family  the  third  toe  was  always  the  largest,  and  in  some  species  the 
evolution  of  the  foot  had  been  carried  to  the  one-toed  stage,  producing 
a  foot  similar  to  that  of  the  horse.  They  were,  however,  inferior  in 
brain  and  teeth  to  the  even  and  odd-toed  herbivores  of  North  America. 

The  most  remarkable  development  occurred  in  the  sloth  tribe 
(edentates)  (p.  670),  in  which  huge  forms,  elephantine  in  size  but  of 
different  proportions,  some  of  which  were  covered  with  armor,  were 
numerous  and  conspicuous. 

Rodents  of  the  porcupine  type  and  monkeys  were  abundant  during 
the  Tertiary  and  are  living  in  South  America  to-day. 

With  the  joining  of  North  America  and  South  America,  an  inter- 
migration  of  animals  from  the  two  continents  began.  Horses, 
llamas,  deer,  mastodons,  tapirs,  members  of  the  cat  and  dog  family, 
and  others  invaded  South  America ;  and  at  the  same  time  the  giant 
sloths  and  other  South  American  forms  moved  north.  The  result 
was  to  be  expected.  Not  all  of  the  families  of  North  American  mam- 
mals found  a  home  in  South  America,  either  because  they  did  not 
migrate  there  or  because  the  conditions  were  unfavorable  for  their 
existence ;  but,  as  a  rule,  these  immigrants  from  the  north  in  which 
brain  and  limb  had  been  highly  developed  as  a  result  of  the  severe 
struggle  with  highly  specialized  carnivorous  enemies,  as  well  as  be- 
cause of  the  competition  with  other  herbivorous  forms,  soon  crowded 
out  the  more  conspicuous  but  less  highly  developed  indigenous 
animals.  To-day  the  most  conspicuous  South  American  animals 
are  those  whose  ancestors  reached  that  continent  in  the  Pliocene, 


642  HISTORICAL  GEOLOGY 

although  many  characteristic  ancient  forms,  such  as  the  armadillo, 
are  numerous. 

Not  only  did  the  North  American  mammals  invade  South  America, 
but  a  similar  invasion  in  the  opposite  direction  was  going  on  at  the 
same  time.  These  immigrants,  however,  failed  to  establish  them- 
selves permanently,  although  they  probably  lived  on  in  their  new 
home  for  some  time,  as  the  presence  of  their  remains  as  far  north  as 
Oregon  shows.  Their  extinction  before  the  Pleistocene  was  due 
either  to  the  competition  with  the  higher  forms  or  to  the  cold  of  the 
Glacial  Period,  but  probably  to  the  former. 

Duration  of  the  Tertiary.  —  Because  of  the  fact  that  the  Tertiary 
rocks  are,  with  the  exception  of  the  Pleistocene,  the  last  deposited, 
some  evidence  of  the  duration  of  the  period  is  at  hand  which  is  not 
available  in  estimating  the  length  of  former  periods.  The  most  im- 
portant means  which  can  be  employed  are  the  following : 

(1)  Biologists  are  generally  agreed  that  the  time  necessary  for  the 
evolution  of  modern  mammals  from  the  generalized  ancestors  of  the 
early  Eocene  was  very  long.     For  example,  the   highly   specialized 
modern  horse  could  not  have  been  evolved  from  the  little  Eohippus 
with  his  four-toed  foot,  simple  teeth,  and  carnivorous-like  body,  in 
tens  of  thousands,  or  hundreds  of  thousands  of  years;  but  a  much 
greater  length  of  time  must  have  been  required. 

(2)  Again,  as  in  other  periods,  we  can  gain  some  idea  of  the  vast- 
ness  of  Tertiary  time  by  a  consideration  of  the  mountain  ranges  which 
had  their  birth  and  principal  growth  during  the    period.     At  the 
beginning  of  the  Tertiary,  Switzerland   was    probably   a   compara- 
tively flat  plain  where  the  lofty  peaks  of  the  Alps  now  stand ;    and 
the  grandest   mountains   of  the  world,    the    Himalayas,   were    not 
raised  until  about  the  middle  of  the  Miocene.     It  is  impossible  to 
state  how  long,  in  years,  this  great  deformation  required,  but  it  is 
evident  that  an  almost  inconceivable  length  of  time  was  necessary. 

(3)  Since  the  close  of  the  Eocene  the  Grand  Canyon  region  has 
been  elevated  11,000  feet;    and  the  Colorado  River  has  been  able  to 
carve  its  way  through  limestone,  sandstone,  and  granite  to  a  depth 
of  6500  feet. 

(4)  The  most  approved  method  of  estimating  geological  time  is 
by  the  maximum  thickness  of  the  sediments  deposited  during  the 
period  (p.  417).     Upon  this  basis  the  duration  of  the  period  has  been 
variously  estimated  at  from  3,000,000  to  4,000,000  years,  with  the 
former  estimate  more  generally  accepted  than  the  latter. 


CHAPTER  XXII 
QUATERNARY 

THE  last  great  period  of  the  earth's  history  —  the  Quaternary  — 
may  be  considered  as  beginning  with  the  initiation  of  extensive  sheets 
of  ice  in  the  northern  hemisphere.  It  is  divided  as  follows  : 


Quaternary 


Recent,  Post-Glacial  or  Human.  Since  the  disappearance 
of  the  continental  ice  sheets. 

Pleistocene  (Greek,  pleistos,  most,  and  kainos,  recent)  or 
Glacial.  Extending  from  the  beginning  of  glaciation 
until  the  final  disappearance  of  continental  glaciers. 


CHANGES  AT  THE  CLOSE  OF  THE  TERTIARY 

Three  important  changes  at  the  close  of  the  Tertiary  should  be 
noted. 

(i)  Elevation.  The  later  Pliocene  and  early  Quaternary  was 
a  time  of  elevation,  during  which  the  continents  stood  higher  than 
now,  and  broad  land  connections  existed,  permitting  migration  be- 
tween the  continents.  On  the  Atlantic  coast  the  Pleistocene  eleva- 
tion has  been  variously  estimated  at  from  a  few  hundred  to  a  few 
thousand  feet.  Evidence  is  at  hand  of  an  elevation  of  1500  feet 
in  southern  California,  of  3000  to  6000  feet  in  the  Sierra  Nevadas, 
1500  feet  in  Oregon,  and  of  an  even  greater  raising  of  the  land  in 
British  Columbia.  In  the  West  Indies,  Panama,  and  South  America, 
observations  point  to  a  higher  level  of  the  land  than  now  during  por- 
tions of  the  epoch.  An  upward  movement  increased  the  height  and 
extent  of  Europe,  so  that  at  the  time  of  maximum  elevation  Great 
Britain  was  a  portion  of  Europe,  and  Europe  was  united  to  Africa  by 
broad  land  connections  across  Gibraltar  and  through  Italy  by  way 
of  Sicily.  After  a  time  elevation  ceased,  and  a  subsidence  began 
which  first  separated  Ireland  from  Wales  and  later  from  Scotland,  and 
finally  isolated  Great  Britain.  The  land  connections  with  Sicily, 

643 


644  HISTORICAL  GEOLOGY 

which  joined  Europe  and  Africa,  and  that  at  Gibraltar  also  disap- 
peared. It  seems  probable  that  the  separation  of  Japan  and  the 
Philippine  archipelago  occurred  in  post-glacial  times. 

(2)  Glaciation.  —  The  gradual  refrigeration  of  the  climate  at  the 
close  of  the  Tertiary  culminated  in  the  Pleistocene.     The  cause,  or 
causes,  which  produced  this  marked  decrease  in  temperature  will  be 
discussed  later  (p.  660). 

The  lowering  of  the  temperature  resulted  in  the  accumulation  of 
snow  and  ice  to  form  great  ice  sheets,  several  hundreds  to  several 
thousands  of  feet  thick,  which  spread  over  6,000,000  to  8,000,000 
square  miles  of  the  earth's  surface,  especially  in  the  northern  hemi- 
sphere. Although  this  is  an  important  event  in  the  world's  history, 
it  should  be  remembered  that  it  is  not  unique,  since  extensive  glacia- 
tion  occurred  in  the  Permian  (p.  505)  as  well  as  in  Pre-Cambrian  times 
(p.  398),  and  possibly  at  other  periods.  It  should  also  be  noted  that 
these  earlier  ice  invasions  have  not  been  considered  of  sufficient  im- 
portance to  form  a  basis  for  a  further  subdivision  of  these  earlier 
periods.  Why,  then,  is  the  separation  into  Tertiary  and  Quaternary 
made  ?  It  is  because  the  event  was  so  recent  (geologically)  that 
the  evidences  of  glaciation  are  widespread  and  conspicuous,  since 
sufficient  time  has  not  yet  elapsed  to  obliterate  them  by  erosion  and 
weathering,  and  also  because  the  indirect  effect  upon  man  has  been 
of  great  importance  (p.  662). 

(3)  Changes  in  Life.  —  The  least  important  change  between  the 
periods  is  in  the  life.     As  far  as  this  is  concerned,  the  Quaternary 
might  almost  equally  well  be  considered  a  continuation  of  the  Plio- 
cene.    At  the  beginning  of  the  period  nearly  all  living  species  of 
mollusks  were  in  existence,  and  most  of  the  species  of  living  mammals, 
but  during  the  Pleistocene  there  was  a  gradual  disappearance  of  many 
mammals,  such,  for  example,  as  the  mammoth,  mastodon,  woolly 
rhinoceros,  and  saber-toothed  tiger.     After  the  Tertiary  there  was  no 
longer  a  mingling  of  tropical  and  subtropical  plants  with  temperate 
and  Arctic  plants  (p.  634) ;   but,  apparently  as  a  result  of  the  migra- 
tions forced  on  them  by  the  climate,  they  became  adapted  to  special 
habitats. 


REFERENCES  FOR  CHANGES  AT  THE  CLOSE  OF  THE  TERTIARY 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3,  pp.  483-490. 
WILLIS  AND  SALISBURY,  —  Outlines  of  Geologic  History,  pp.  265-275. 


QUATERNARY 


DISTRIBUTION  OF  THE  ICE  SHEETS 


645 


The  great  event  of  the  Pleistocene  was  the  accumulation  of  vast 
continental  glaciers. 

i.  Other  Continents. — As  a  result  of  the  increasing  cold  the 
whole  of  northern  Europe  (Fig.  564)  was  buried  under  an  ice  sheet  of 
great  thickness,  which  filled  up  the  basins  of  the  Baltic  and  North 
seas  and  spread  over  Scotland  and  the  greater  part  of  England. 


IP 

Prague*    "^•Jii.V  ^$3$? 

•;.-.  .*:*.    ..!?   .  Cracow       Bel**' 

^ 

•SMfOiV.*!  \-\-'l't'l  :  ~jS          * 


FIG.  564.  —  Map  of  Europe  during  glacial  times.  The  area  covered  by  glaciers 
is  shaded,  and  the  direction  of  the  movement  of  the  ice  is  indicated  by  arrows.  (After 
J.  Geikie.) 

The  Alps  and  Pyrenees  were  covered  by  great  snow  fields  and  glaciers 
which  stretched  over  the  neighboring  lowlands,  and  even  the  island 
of  Corsica  had  glaciers.  In  the  southern  hemisphere  and  in  the  tropics 
glaciers  appear  to  have  existed  where  none  are  now,  and  our  present- 
day  glaciers  are  insignificant  remnants  of  those  of  Pleistocene  times. 
One  interesting  exception  to  the  general  glaciation  of  the  northern 
portion  of  the  Old  World  was  the  absence  of  glaciers  in  Siberia,  where, 
even  at  the  present,  a  portion  of  the  country  has  a  mean  temperature 
of  five  degrees,  the  soil  is  permanently  frozen  to  a  depth  of  several 
hundreds  of  feet,  and  Arctic  conditions  prevail  over  large  areas. 
The  absence  of  glaciers  is  now,  and  probably  was  then,  due  to  the  de- 


646 


HISTORICAL  GEOLOGY 


ficient  precipitation,  which  makes  an  accumulation  of  snow  impos- 
sible. 

2.  North  America.  —  North  America  (Fig.  565)  was  more  exten- 
sively affected  by  glaciation  than  any  other  part  of  the  world,  about 
4,000,000  square  miles  being  covered  by  ice  at  the  time  of  its  greatest 
extension.  Two  peculiar  features  are  especially  striking  in  the  dis- 
tribution of  the  North  American  ice  sheets  :  (i)  the  greatest  extent 
of  ice  was  in  the  low  regions  of  the  northeast,  instead  of  in  the  high 


FIG.  565.  —  Map  of  North  America,  showing  the  area  covered  by  ice  at  the  stage  of 
maximum  glaciation,  and  the  centers  from  which  the  ice  moved.  The  arrows  show  the 
directions  of  ice  movement. 

mountains  of  the  west  and  northwest;  and  (2)  the  northeastern 
portion  of  the  continent,  rather  than  the  northern,  was  the  scene  of 
maximum  glaciation,  even  Alaska  being  largely  free  from  ice. 

The  great  ice  sheets  moved  in  all  directions  from  three  great  centers 
and  probably  to  some  extent  from  other  smaller  ones,  as  is  proved  by 
the  direction  of  the  striations  on  the  underlying  rocks,  and  the  courses 
along  which  the  bowlders  were  carried.  These  three  great  centers  of 
radiation  were:  (i)  that  situated  in  Labrador  (the  Labradorean) ; 
(2)  that  just  west  of  Hudson  Bay  (the  Keewatin) ;  and  (3)  that  in 
the  western  mountains  (the  Cordilleran).  The  greatest  extent  of 


QUATERNARY  647 

the  Labradorean  ice  sheet  was  to  the  southeast,  where  it  stretched 
1600  miles  south  of  the  center.  There  was  also  a  movement  north 
from  this  center,  but  it  is  not  known  to  have  been  nearly  so  extensive. 
This  ice  sheet,  in  its  greatest  expansion,  crossed  the  Ohio  River  into 
Kentucky,  and  extended  into  southern  Illinois. 

The  Keewatin  ice  sheet  extended  almost  as  far  southward  as  the 
Labradorean,  its  front  at  one  time  being  in  Kansas  and  Michigan, 
about  1500  miles  from  its  center.  The  movement  of  the  Keewatin 
ice  sheet  is  remarkable  since,  beginning  in  a  low,  flat  region,  which 
is  now  semiarid,  the  ice  moved  upgrade  into  the  United  States. 
This  is  more  astonishing  when  we  find  that  the  Cordilleran  sheet, 
starting  from  the  lofty  mountains  of  western  North  America,  ap- 
parently failed  to  move  beyond  their  foothills.  The  Cordilleran  ice 
sheet  should,  perhaps,  be  considered  as  the  product  of  the  confluence 
of  mountain  glaciers,  spreading  out  as  they  reached  lower  and  less 
rugged  ground,  much  as  do  some  of  the  Alaskan  glaciers  to-day. 

Besides  these  great  centers  of  ice  movement,  large  local  glaciers 
accumulated  on  the  mountains  of  the  western  United  States,  where 
they  were  vigorous  for  many  years,  as  is  shown  by  the  cirques  (p.  143), 
moraines,  rock  basins,  and  other  evidences  of  glaciation  seen  through- 
out the  high  mountains  of  the  west.  This  is  well  seen  on  the  topo- 
graphic maps  of  Montana,  Wyoming,  Colorado,  and  neighboring 
states. 

REFERENCES  FOR  GLACIATION  IN  NORTH  AMERICA 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3. 

GEIKIE,  A.,  —  Textbook  of  Geology. 

GEIKIE,  J.,  —  The  Great  Ice  Age. 

WRIGHT,  G.  F.,  —  The  Ice  Age  in  North  America. 

WRIGHT,  W.  B.,  —  The  Quaternary  Ice  Age. 

DEVELOPMENT  OF  THE  ICE  SHEETS 

The  great  ice  sheets  (with  the  probable  exception  of  the  Cordilleran 
center)  did  not  begin  as  mountain  glaciers  which  by  their  coalescence 
became  one  great  glacier,  but  were  the  result  of  the  gradual  accumu- 
lation of  snow  in  the  north,  due  to  the  lowering  of  the  temperature 
(p.  660). 

Thickness  of  Ice  Sheets  at  Center.  —  It  has  been  held  that  the 
great  ice  sheets  were  several  miles  thick  at  the  various  centers,  from 
which  points  they  gradually  thinned  toward  the  margins.  A  study  of 
existing  Greenland  and  Antarctic  glaciers  shows  that  such  is  not  now 


648  HISTORICAL  GEOLOGY 

the  case,  but  that  the  thickness  of  the  ice  not  far  from  the  margin 
is  practically  the  same  as  that  of  the  interior,  the  surface  of  which  is 
a  comparatively  level  plain. 

The  slope  of  the  sides  of  the  tongues  of  ice  that  reached  down 
ravines  of  the  Allegheny  River  somewhat  beyond  the  margin  of  the 
main  sheet  varied  from  100  to  130  feet  a  mile,  and  the  average  slope 
of  the  ice  lobe  of  the  Hudson  valley  has  been  estimated  to  have  been 
25  to  30  feet  a  mile.  Even  the  smaller  of  these  figures  would  make  an 
enormous  thickness  for  the  ice  sheets  at  the  centers,  if  the  slopes  were 
uniform.  Since  the  ice  in  Illinois  is  known  to  have  reached  1500  to 
1600  miles  south  of  the  center  of  accumulation,  an  average  slope  of 
25  feet  a  mile  would,  on  this  basis,  give  a  thickness  of  about  eight 
miles  at  the  center.  It  is  probable,  however,  that  the  slope  was  not 
nearly  so  steep  some  distance  back  from  the  margin.  When  this  is 
taken  in  connection  with  the  fact  that  the  thickness  of  the  ice  near 
the  margin  was  probably  approximately  the  same  as  that  at  the  center, 
a  much  less  depth  is  obtained  than  on  the  former  estimates.  Upon 
any  basis,  however,  the  thickness  must  have  been  great.  In  New 
England,  for  example,  the  ice  was  so  thick  that  it  passed  over  the 
Green  Mountains  where  they  are  3000  to  5000  feet  high,  in  a  course 
diagonal  to  their  general  direction,  showing  that  such  a  mountain 
chain  made  "  scarcely  a  ripple  on  the  surface."  (Tarr.) 

GLACIAL  AND  INTERGLACIAL  STAGES 

The  Glacial  Period  was  made  up  of  a  number  of  advances  of  the 
ice  and  corresponding  recessions  when  the  ice  either  entirely,  or 
largely,  disappeared  from  the  northern  hemisphere.  The  duration 
of  the  various  glacial  stages  was  long,  and  that  of  the  interglacial 
stages  so  extended  as  to  permit  trees  and  plants  to  clothe  again  the 
glaciated  regions.  The  proofs  of  distinct  ice  sheets  (Fig.  566), 
separated  by  long  intervals,  when  the  land  was  free  from  glaciers  and 


FIG.  566.  —  Diagram  showing  the  proof  of  successive  ice  sheets.  Resting  upon  the 
lower  till  sheet  and  underlying  the  upper  are  ancient  peat  bogs  (solid  black).  The 
erosion  of  the  older  (lower)  drift  where  exposed  is  much  further  advanced  than  that 
of  the  younger  (upper).  The  composition  of  the  drift  sheets  differs  also. 


QUATERNARY 


649 


even  warmer  than  at  present  in  the  same  regions,  are  conclusive. 
The  drift  of  the  earlier  glacial  stages,  where  it  has  not  been  covered 
by  more  recent  drift,  differs  from  the  latter  in  a  number  of  particulars : 
(i)  the  drainage  of  the  surface  is  mature,  being  in  this  respect  in 
marked  contrast  to  the  immature  drainage  of  the  most  recent  drift, 
with  its  lakes  and  marshes ;  (2)  the  bowlders  of  the  older  drift  sheets 
are  found  to  be  different  from  those  of  later  drifts,  showing  that  they 
were  brought  by  ice  sheets  that  moved  in  a  somewhat  different  direc- 
tion ;  (3)  the  bowlders  of  the  older  drift  are,  moreover,  much  weath- 
ered, so  that  even  granite  bowlders  can  sometimes  be  crumbled  in 
the  hand,  and  the  clay  is  deeply  oxidized  and  the  lime  largely  leached 
out.  (4)  Deeply  eroded  surfaces,  covered  with  peat  or  ancient 
soils  (Fig.  567)  occur,  underlying  more  recent  drift.  The  most  satisfy- 
ing proof  is  found  in  two  sections,  one  in  Iowa  and  the  other  near 
Toronto,  in  which  stratified  deposits  containing  fossils  rest  upon  old, 
weathered  till  and  are,  in  turn,  overlain  by  younger  drift. 
The  recognized  glacial  stages  in  America  and  Europe  are : 


NORTH  AMERICA 

GERMANY 

Postglacial 

Iron 
Bronze 
Neolithic 

Wisconsin 

Later 
Fourth  Interglacial 
Earlier 

Wiirm 

(  ?)  Iowa 

(  ?)  Interglacial  (Peorian) 
(There  is  some  doubt  as  to 
the  interpretation  of  the 
lowan.) 
Third  Interglacial 
(Sangamon) 

Third  Interglacial 
Paleolithic  man  in  Europe 

Illinoian 

Riss 

Second  Interglacial 
(Yarmouth) 

Second  Interglacial 
Earliest  remains  of  Pale- 
olithic man  in  Europe 

Kansan 

Mindel 
Heidelberg  man  (Europe) 

First  Interglacial 
(Aftonian) 

First  Interglacial 

Subaftonian 

(Jerseyan) 

Giinz 

6$° 


HISTORICAL  GEOLOGY 


Characteristics  of  Former  Drift  Sheets.  —  It  will  not  be  advisable  to 
discuss  these  glacial  stages  in  detail  (see  references).  The  different 
stages  varied  in  duration,  in  the  character  of  the  material  deposited, 


Moraines  of  the  Wisconsin 
Ice  Sheet  and  the  limit  of 
maximum  glaciation 


FIG.  567.  —  Map  showing  the  moraines  and  the  direction  of  the  ice  movement 
(shown  by  arrows)  of  the  last  continental  ice  sheet.  The  lobate  character  of  the  mo- 
raines is  very  pronounced.  The  "driftless  area"  was  not  covered  by  the  ice.  (Modi- 
fied after  Taylor.) 

and  in  the  extent  of  the  glaciation.  The  deposits  of  the  Kansan 
ice  sheet,  for  example,  contain  little  stratified  drift,  indicating  that 
stream  action  was  of  little  importance.  This  is  surprising,  since  the 
melting  of  great  masses  of  ice  naturally  carries  with  it  the  idea  of 
flooded  streams  and  great  deposits  of  stratified  drift,  such  as  resulted 


QUATERNARY  651 

from  the  last  ice  sheet.  The  effect  of  the  last  recrudescence  of  glacia- 
tion  in  the  Wisconsin  stage  is  best  known,  since  its  deposits  cover 
the  earlier  drifts.  It  seems  probable,  moreover,  that  the  surface  of 
the  last  drift  had  a  relief  stronger,  originally,  than  that  of  any  of  the 
former  ice  sheets.  One  marked  feature  is  the  lobate  form  of  the  mo- 
raines which  occur  in  a  succession  of  crescentic  belts  (Fig.  567). 

HISTORY  OF  THE  GREAT  LAKES 

There  is  general  agreement  that  the  Great  Lakes  were  not  in  exist- 
ence immediately  previous  to  glacial  times.  This  belief  is  based 
upon  the  fact  that  the  region  now  occupied  by  them  had  been  sub- 
jected to  erosion  so  long  that  any  lakes  which  might  at  some  time 
have  been  in  existence  must  have  been  destroyed  by  filling  or  the 
cutting  down  of  their  outlets.  It  is  probable  that  in  preglacial  times 
the  region  in  the  vicinity  of  the  Great  Lakes  was  not  unlike  that 
of  central  and  eastern  Tennessee,  Kentucky,  and  southern  Indiana, 
where  the  weak  rock  formations  are  marked  by  lowlands,  and  the 
more  resistant  by  highlands. 

Preglacial  Drainage.  —  The  precise  course  of  the  preglacial  drain- 
age of  this  region  is  yet  to  be  determined,  but  the  evidence  indicates 
that  the  St.  Lawrence  River  was  not  as  now  the  channel  through 
which  it  flowed  to  the  ocean.  At  that  time,  indeed,  the  head  of  the 
St.  Lawrence  River  may  have  been  in  the  vicinity  of  the  Thousand 
Islands.  Well  borings  in  Michigan  and  Indiana  have  revealed  the 
fact  that  ancient,  drift-filled  valleys  of  great  depth  lead  towards  the 
south,  giving  strong  support  to  the  supposition  that  the  preglacial 
drainage  was  southward  to  the  Ohio  and  Mississippi,  instead  of  to 
the  east.  In  fact,  at  the  present  time  a  lowering  of  the  land  a  few 
feet  at  the  southern  end  of  Lake  Michigan  would  turn  the  drainage 
to  the  Mississippi  Valley. 

Origin  of  the  Basins. — The  basins  of  the  Great  Lakes  are  lowlands 
which  have  been  modified  in  several  ways:  (i)  by  drift  deposition 
which  has  not  only  blocked  up  the  valleys  leading  south,  but  has 
also  increased  the  height  of  the  divides  by  terminal  moraines ;  (2)  by 
glacial  erosion,  though  whether  the  glaciers  accomplished  more  than 
the  removal  of  the  weathered  rock  and  soil  is  a  mooted  question ; 
all  will  concede,  however,  a  deepening  to  this  extent  at  least;  (3)  by 
a  depression  of  the  region  at  the  north  as  a  result  of  the  weight  of  the 
ice.  Since  the  disappearance  of  the  ice  sheets  the  land  at  the  north 


652 


HISTORICAL  GEOLOGY 


has  been  slowly  rising,  as  is  shown  by  the  beach  lines  of  the  ancient 
lakes  which  are  now  higher  at  the  north  than  they  are  further  south  — 
in  some  cases  400  feet  or  more.  The  surfaces  of  the  lakes  are  held 
up  by  rock  and  drift  barriers  to  levels  several  hundreds  of  feet  above 
their  rock  beds,  while  the  bottoms  of  all  the  lakes,  except  Lake  Erie, 
are  below  sea  level. 

Great  Lakes  Stages.1  —  The  Great  Lakes  had  their  inception  when 
the  ice  sheet  had  retreated  across  the  higher  land  which  turns  some  of 


LEGEND 
Areas  covered  by  the  Lakes. 

Lake  outlets - 

Glacial  drainage 

Successive  ice  borders 7 

Lers  in  New  York  largely  conjectural 
Correlative  Moraines  from  Wisconsin  .-: 

westward  not  determined.  -•',: 

CALE   OF  MILES 

ifcfl 


FIG.  568.  —  An  early  (first)  stage  in  the  history  of  the  Great  Lakes. 
(After  Taylor  and  Leverett.) 

the  water  to  the  north  and  some  to  the  south.  The  melting  waters 
being  prevented  from  flowing  north,  accumulated  between  it  and  the 
ice  front,  gradually  enlarging  upon  the  further  recession  of  the  ice. 
At  first  there  were  doubtless  many  small  lakes  which  had  temporary 
outlets  to  the  south.  As  the  ice  retreated  still  further  these  lakes 
coalesced  into  larger  ones.  The  brief  and  incomplete  history  of  the 
Great  Lakes  which  is  given  below  has  been  learned  from  a  study  of 
the  beaches,  sand  bars,  deltas,  and  outlets  made  at  former  lake  levels. 
The  history  of  the  Great  Lakes  may  be  considered  as  beginning 
after  the  ice  had  retreated  to  such  an  extent  that  a  large  lake  (glacial 

1  Taylor,  F.  B.,  —  The  Glacial  and  Postglacial  Lakes  of  the  Great  Lake  Region :  Smithsonian 
Rept.,  1912,  pp.  291-327. 


QUATERNARY 


653 


Lake  Maumee)  came  into  existence  near  the  western  end  of  what  is 
now  Lake  Erie,  and  which  emptied  through  the  Wabash  River  into 
the  Mississippi  River.  This  may,  for  convenience,  be  called  the 
first  stage  (Fig.  568)  in  the  history.  In  the  second  stage,  the  ice 
front  had  retreated  to  such  an  extent  that  a  greatly  expanded  Lake 
Erie  (glacial  Lake  Warren)  was  formed  which  emptied  through 
the  Illinois  River  into  the  Mississippi  River.  Upon  a  further  retreat 
of  the  ice,  a  third  stage  (Fig.  569)  was  inaugurated,  with  the  resulting 


LEGEND 

Areas  covered  by  ihe  Lakes Y//////A 

(Shore  lines  incompletely  traced) 


FIG.  569.  —  A  (third)  stage,  in  which  three  Great  Lakes  with  separate  outlets*were 
formed  by  the  further  retreat  of  the  ice  front.     (After  Taylor  and  Leverett.) 

formation  of  three  Great  Lakes  with  three  outlets ;  Lake  Superior 
(glacial  Lake  Duluth)  discharging  over  the  divide  at  Duluth  through 
the  St.  Croix  into  the  Mississippi  River,  Lake  Michigan  (glacial  Lake 
Chicago)  through  the  Illinois  River  as  before,  and  lakes  Erie  and 
Huron  (glacial  Lake  Lundy)  through  the  Mohawk  River  in  New 
York  to  the  Hudson  River.  In  the  fourth  stage  (Fig.  570)  the 
region  of  the  Great  Lakes  was  entirely  uncovered  with  the  exception 
of  the  St.  Lawrence  River ;  at  this  time  the  present  lakes  Michigan, 
Superior,  and  Huron  were  greatly  expanded  to  form  a  lake  which 
covered  a  greater  area  than  that  occupied  by  all  of  the  present  Great 
Lakes.  This  lake  (named  Lake  Algonquin)  discharged  through  the 
Mohawk  River  to  the  Hudson  and  probably  also  for  a  time  through 


654 


HISTORICAL  GEOLOGY 


FIG.  570.  —  In  this  (fourth}  stage  the  Great  Lakes  had  their  greatest  area,  forming 
Lake  Algonquin.  The  outlets  were  the  Mohawk  and  Illinois  rivers.  (Modified  after 
Taylor  and  Leverett.) 


LEGEND 
Area,  covered  by  the  Lakes 

(Shore  line  of  Champlain  Se» 
not  yet  determined) 


FIG.  571.  —  A  (fifth)  stage  with  the  drainage  through  the  Ottawa  River.  A  low- 
ering of  the  region  resulted  in  the  extension  of  the  sea  into  the  St.  Lawrence  valley 
and  Lake  Champlain.  (After  Taylor  and  Leverett.) 


QUATERNARY  655 

the  old  Chicago  outlet.  During  an  early  part  of  the  stage  Lake  Huron 
probably  emptied  into  Lake  Erie,  but  later,  when  the  ice  front  had 
melted  back  farther  to  the  north,  drained  through  the  Trent  River  in 
Canada  to  Lake  Ontario  (named  Lake  Iroquois),  reducing  Lake  Erie  to 
such  an  extent  that  the  amount  of  water  flowing  over  Niagara  falls  was 
probably  not  greater  than  that  now  pouring  over  the  American  falls. 
As  the  land  was  uplifted  in  the  north  (as  the  weight  of  the  ice  was  re- 
moved), the  drainage  of  Lake  Huron  was  again  discharged  over 
Niagara  Falls.  The  fifth  stage  (Fig.  571)  began  with  the  opening  of 
an  eastern  passage  along  the  ice  border  into  the  Ottawa  valley,  which 
lowered  the  surface  of  the  lakes  (forming  Lake  Nipissing).  The 
drainage  passed  through  this  outlet  until  the  elevation  of  the  land 
on  the  north  was  sufficient  to  send  the  waters  into  their  present  course. 
The  beach  lines  of  this  fifth  stage  rise  at  the  north  (are  higher  at  the 
north  than  at  the  south),  showing  an  uplift  of  100  feet  at  the  head  of 
the  Ottawa  River  since  it  was  abandoned. 

There  seems  little  doubt  that  at  each  advance  and  retreat  of  the 
ice,  during  the  different  stages,  lakes  were  formed  in  somewhat  the 
same  position  as  at  the  close  of  the  last  ice  (Wisconsin)  invasion. 
The  proof  of  such  lakes  is  not  abundant,  but  is  indicated  by  the 
sandy  character  of  the  drift  in  the  moraines  at  the  south  end  of  Lake 
Michigan,  the  sand  having  probably  been  obtained  by  the  ice  from 
the  deposits  of  a  former  Lake  Michigan. 


REFERENCES  ON  THE  HISTORY  OF  THE   GREAT  LAKES 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3. 
TARR,  R.  S.,  —  Physical  Geography  of  New  York  State. 

TAYLOR,  F.  B.,  —  The  Glacial  and  Postglacial  Lakes  of  the  Great  Lake  Region:   Smith- 
sonian Rept.,  1912,  pp.  291-327. 

The  Champlain  Subsidence. — The  fifth  stage  (Fig.  571)  of  the 
Great  Lakes  was  coincident  with  a  great  subsidence  of  the  north- 
eastern Atlantic  coast,  which  permitted  the  sea  to  spread  over  the 
St.  Lawrence  valley,  Lake  Ontario,  Lake  Champlain,  and  the  Hudson 
River,  thus  making  New  England  an  island. 

The  sediments  carried  into  these  bodies  of  water  at  that  time  contain 
marine  shells  and  even  the  skeletons  of  whales,  one  of  which  was  found 
in  a  Lake  Champlain  terrace,  and  another  in  the  Ottawa  valley. 
The  terraces  near  Montreal  which  are  600  feet  above  the  sea,  those  of 
Lake  Champlain  which  are  500  feet  at  the  northern  end  and  400  feet 

CLELAND    GEOL.— 42 


656 


HISTORICAL  GEOLOGY 


or  less  at  the  southern  end,  and  those  of  Maine  which  are  200  or  more 
feet  in  height,  show  both  the  amount  of  sinking  in  that  epoch  and  the 
differential  uplift  since  then. 


OTHER  PLEISTOCENE  LAKES 

Lake  Agassiz.  —  The  Red  River  valley  of  Manitoba,  North 
Dakota,  and  Minnesota,  so  remarkable  for  its  fertility  as  well  as  for 
its  flatness,  is  the  result  of  a  glacial  dam  which  prevented  the  usual 
drainage  through  the  Red  River  to  the  north,  and  produced  a  great 
lake  (Lake  Agassiz)  which  discharged  by  way  of  the  Minnesota  River 
into  the  Mississippi.  On  its  bottom  the  silts  carried  in  by  the  streams 
were  deposited,  making  a  surface  as  flat  perhaps  as  any  on  earth. 
Upon  the  retreat  of  the  ice,  drainage  to  the  north  was  permitted,  and 
the  lake  disappeared.  At  its  greatest  extent  this  lake  had  a  larger 
area  than  that  of  the  present  Great  Lakes  combined. 

Lake  Bascom.  —  In  rugged  regions  the  ice  sheet  formed  many 
temporary  lakes,  a  rather  remarkable  example  of  which  is  to  be  found 
in  northwestern  Massachusetts,  where  a  lake  (Lake  Bascom)  first 
stood  at  an  elevation  of  noo  feet  above  the  sea,  the  level  of  the  lake 
being  determined  by  the  pass  (col)  through  which  the  water  was  dis- 
charged. As  the  ice  retreated,  lower  passes,  approximately  1000, 

900,  700,  and  600 
feet  above  the  sea, 
were  found ;  and 
the  lake  was  finally 
drained  when  the 
present  outlet  — 
the  Hoosic  River 
—  was  uncovered. 
Great  Basin 
Lakes.  —  In  some 
semiarid  regions 
not  covered  by  the 
ice  sheet  the  cli- 
mate of  the  Pleis- 
tocene seems  to 
have  been  moister 
than  at  present 
and  previous  to 


FIG.  572.  —  Map  showing  the  position  of  Lake  Bonneville 
on  the  east  and  Lake  Lahontan  on  the  west.  The  relative 
size  of  Great  Salt  Lake  is  indicated. 


QUATERNARY  657 

glacial  times.  This  is  brought  out  by  a  study  of  the  Great  Basin 
region  of  Utah  and  Nevada.  Great  Salt  Lake  is  a  small  remnant 
of  a  Pleistocene  lake  (Lake  Bonneville),  which  was  many  times 
larger  (Fig.  572),  discharging  at  one  stage  by  a  northern  outlet  to 
the  Pacific.  This  lake  (Lake  Bonneville)  at  its  maximum  was 
1000  feet  deep  and  covered  an  area  of  17,000  square  miles.  Terraces 
marking  former  levels  of  the  lake  are  conspicuous  features  of  the  land- 
scape, as  seen  from  Salt  Lake  City  and  other  portions  of  the  basin. 
A  return  to  an  arid  climate  caused  the  shrinkage  of  the  lake  to  its 
present  area  and  maximum  depth  of  only  50  feet.  The  salt  lake 
which  doubtless  formerly  existed  there  in  preglacial  times  was  grad- 
ually freshened  as  the  lake  level  was  raised,  and  became  entirely  fresh 
when  the  excess  water  flowed  through  the  outlet.  When  the  climate 
again  became  drier,  the  lake  shrank,  and  all  of  the  soluble  salts  of  the 
larger  lake,  as  well  as  those  brought  into  the  basin  since  that  time, 
have  accumulated  to  form  the  present  exceedingly  saline  waters. 

Further  west  in  the  same  basin  were  other  lakes  (Lake  Lahontan) 
which,  however,  were  not  as  large  as  Lake  Bonneville,  although  of 
considerable  extent. 

REFERENCES   ON   PLEISTOCENE   LAKES 

BERLIN-GREYLOCK  AND  HOOSICK-BENNINGTON  Folios,  U.  S.  Geol.  Surv. 
GILBERT,  G.  K.,  —  Lake  Bonneville:   Mon.  U.  S.  Geol.  Surv.,  Vol.  I,  1890. 
Textbooks  of  Geology. 
UPHAM,  W.,  —  The  Glacial  Lake  Agassiz:  Mon.  U.  S.  Geol.  Surv.,  Vol.  25,  1896. 

LOESS 

In  the  Mississippi  basin,  especially  in  Illinois,  Iowa,  Nebraska, 
and  states  to  the  south,  are  extensive  areas  of  a  deposit  (loess)  in- 
termediate between  fine  sand  and  clay  (p.  52).  The  fact  that  it 
contains  angular,  undecomposed  particles  of  calcite,  dolomite,  feld- 
spar, hornblende,  mica,  and  magnetite  indicates  that  loess  was  de- 
rived from  the  finely  ground  rock  flour  of  the  glaciers.  Pebbles, 
with  the  exception  of  lime  and  iron  concretions,  are  absent,  except 
at  the  base  of  the  deposits.  The  most  striking  characteristic  of  loess 
is  its  ability  to  form  vertical  cliffs,  a  feature  which  can  best  be  seen 
in  railroad  cuts  and  along  stream  courses. 

One  peculiarity  of  its  distribution  is  its  independence  of  topography. 
Its  thickness  seldom  exceeds  50  feet,  while  10  feet  is  more  common. 
Loess  occurs  on  the  drift,  between  drift  sheets,  and  even  beyond  the 
limit  of  the  drift  sheets. 


658  HISTORICAL  GEOLOGY 

The  question  of  the  origin  of  this  widespread  deposit  has  given  rise 
to  much  discussion,  but  to  a  large  extent  the  aqueous  theory,  i.e.,  that 
loess  is  a  deposit  that  was  laid  down  in  standing  water,  has  been  re- 
placed by  the  eolian.  According  to  the  latter,  glacial  streams,  heavily 
loaded  with  rock  flour,  spread  silt  upon  their  flood  plains,  exposing 
it  to  the  action  of  the  winds,  which  caught  it  up  and  redeposited  it 
on  the  adjacent  uplands,  where  after  its  deposition  it  was  held  by  the 
vegetation.  A  similar  deposit  is  forming  to-day  on  the  western  plains, 
where  loess-like  dust  is  held  by  the  grasses  and  is  slowly  building  up 
portions  of  the  surface.  If  the  above  explanation  is  correct,  the 
presence  of  such  extensive  areas  of  loess  indicates  aridity  during  some 
of  the  glacial  or  interglacial  stages,  since  if  the  climate  was  moist, 
the  action  of  the  winds  would  be  inconsiderable. 

REFERENCES   ON   LOESS 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3  ;  (for  references  to  literature). 
WRIGHT,  G.  F.,  —  The  Ice  Age  in  North  America,  pp.  359-371. 

DURATION 

The  difficulty  of  arriving  ""at  a  definite  conclusion  as  to  the  length 
of  the  Glacial  Period,  as  expressed  in  years,  is  seen  when  the  ele- 
ments upon  which  such  estimates  must  be  based  are  analyzed. 
These  are  (i)  the  weathering  and  erosion  of  drift;  (2)  the  time 
necessary  for  the  climatic  changes  between  the  glacial  and  interglacial 
stages;  (3)  the  amount  of  vegetable  growth  in  interglacial  stages; 
(4)  the  time  necessary  for  the  immigration  of  plants  and  animals. 
These  all  show  that  a  long  period  of  time  must  have  elapsed,  but 
afford  little  basis  for  an  estimate  in  terms  of  years.  (5)  The  time 
required  for  the  advance  and  retreat  of  the  ice  sheets,  however, 
affords  something  of  a  clue,  since  the  rate  of  the  advance  and  retreat 
of  existing  glaciers  is  known ;  but  such  estimates,  at  best,  are  subject 
to  wide  variations,  depending  upon  the  rate  used  as  a  basis.  This  is 
well  shown  in  the  figures  given  by  different  investigators,  which  vary 
from  100,000  years  (Upham),  to  500,000  to  1,000,000  years  (Penck). 
The  former  estimate,  however,  is  evidently  much  too  small. 

The  time  which  has  elapsed  since  the  beginning  of  the  retreat  of 
the  last  ice  sheet  is  better  known,  because  of  other  lines  of  evidence. 
These  are  (i)  the  time  required  for  the  retreat  of  the  ice,  and  (2)  the 
time  necessary  for  the  excavation  of  the  Niagara  (Fig.  573)  and  St. 
Anthony  gorges,  the  present  rate  of  the  recession  of  these  falls  being 


QUATERNARY 


659 


TABLE  ROCK   HOUSED 


known.  When  allowances  are  made  for  fluctuation  in  the  volume  of 
the  rivers  at  different  times  after  the  ice  had  retreated  so  as  to  permit 
the  streams  to  flow  over  their  new  courses,  it  is  seen  that  10,000  to 
50,000  years  must  have  elapsed  since  the  cutting  of  the  Niagara  gorge 
began.  For  the  recession  of  the  St.  Anthony  Falls  between  12,000 
and  16,000  years  seem  necessary.  To  these  estimates  must  be  added 
the  time  required  for  the 
retreat  of  the  ice  from  its 
terminal  moraine  to  the 
Niagara  and  Minnesota 
rivers.  For  this  10,000 
to  30,000  years  more 
should  be  added.  This, 
then,  would  give  20,000 
to  80,000  years  since  the 
beginning  of  the  retreat 
of  the  last  ice  sheet.  It 
is  evident  upon  compar- 
ing the  weathering  of  the 
Wisconsin  drift  with  that 
of  older  drifts  that  the 
time  which  has  elapsed 
since  the  last  ice  sheet  is 
a  small  fraction  of  some 
of  the  interglacial  stages. 
This  has  led  to  the  sug- 
gestion that  perhaps  we 
are  now  in  an  interglacial 
stage.  It  is  interesting 
in  this  connection  to  note 
that  the  extent  of  the 
area  *at  present  covered  by  glaciers  is  one  tenth  that  of  the  maxi- 
mum glaciation. 

Marine  postglacial  clays  in  Sweden  have  furnished  an  interesting 
basis  for  determining  the  length  of  postglacial  time.  These  clays 
have  been  deposited  in  'regular  layers,  with  different  colors  and 
composition,  the  same  succession  being  repeated  time  after  time. 
The  layers  laid  down  in  summer  are  brown,  due  to  oxidation,  and 
thicker;  those  laid  down  in  the  autumn  are  darker  as  a  result  of  the 
greater  amount  of  organic  matter,  and  thinner.  Counting  these 


FIG.  573.  — Outline  map  of  a  portion  of  the  crest 
line  of  Niagara  Falls,  showing  the  recession  of  the 
brink  during  various  intervals  since  1842.  (After 
Taylor.) 


66o  HISTORICAL  GEOLOGY 

layers  (much  as  the  age  of  a  tree  is  determined  by  the  rings), 
it  is  estimated  that  Stockholm  was  covered  with  ice  only  nine 
thousand  years  ago,  and  that  the  glaciers  withdrew  at  a  rate  of  800 
feet  a  year.  The  ice  is  believed  to  have  receded  from  Ragunda, 
Sweden,  only  7000  years  ago. 

CAUSES  OF  GLACIATION 

Numerous  theories  have  been  offered  to  account  for  the  refrigera- 
tion of  the  climate  which  resulted  in  the  Glacial  Period,  each  of 
which  has  elements  of  probability,  but  none  of  which,  as  at  present 
worked  out,  is  perfect.  In  the  consideration  of  all  of  the  theories 
discussed  below,  it  should  be  borne  in  mind  that  the  appearance  of 
an  ice  sheet  does  not  necessarily  imply  an  extremely  low  average 
temperature.  It  has  been  estimated  that  a  fall  of  3°  F.  in  the  average 
temperature  of  the  Scottish  Highlands,  and  a  fall  of  12°  in  the  Lauren- 
tian  region  of  Canada  would  result  in  a  glacial  period  for  these  re- 
gions. 

1.  Elevation.  —  The    explanation    of   glaciation    which    naturally 
suggests  itself  is  that  the  refrigeration  was  due  to  a  great  elevation 
of  the  land  in  the  northern  hemisphere,  which  so  reduced  the  tem- 
perature that  snow  accumulated  to  form  glaciers  as  it  does  now  on 
high  mountains.     Those  who  hold  this  theory  point  to  the  evidence 
of  an  elevation  of  several  thousands  of  feet  as  shown  by  the   fiords 
on  northern  coasts.     The  objections  to  the  theory  are  (i)  that  it  is 
probable  maximum  elevation  and  maximum  glaciation  did  not  coin- 
cide in  time;  (2)  that  the  elevation  was  not  as  great  as  once  sup- 
posed; (3)  that  glaciation  not  only  occurred  in  the  northern  hemi- 
sphere, but  that  mountain  glaciers  throughout  the  world  were  more 
extensive  than  now.     (4)  Moreover,  this  hypothesis  would  require 
a  great  elevation  for  the  glacial  stages  and  a  corresponding  depression 
for  the  interglacial. 

2.  Astronomical.  —  A  theory  which  at  one  time  had  wide  accept- 
ance was  offered  by  Croll  and  is  known  as  "  Croll's  hypothesis." 
It  is  based  upon  (i)  the  variation  in  the  eccentricity  of  the  earth's 
orbit  as  a  result  of  which  the  relative  length  of  the  summer   and 
winter  seasons  changes  (Fig.  574).     When  the  eccentricity  is  great- 
est the  earth  is  14,000,000  miles  farther  from  the  sun  in  the  one 
season  than  in  the  other.     (2)   By  means  of  the  precession  of  the 
equinoxes  the  winter  of  the  northern  hemisphere,  which  now  occurs 
when   the  sun  is   nearest    the    earth  (perihelion)    (Fig.    574   A),   is 


QUATERNARY 


661 


gradually,  in  10,500  years,  brought  around  so  as  to  occur  when 
the  earth  is  farthest  from  the  sun  (aphelion)  (Fig.  574  B).  The  com- 
bined effect  of  (i)  maximum  eccentricity  and  (2)  the  precession  of 
the  equinoxes  is  to  make  the  winters  22  days  longer  and  20°  colder, 
and  the  summers  22  days  shorter  and  hotter  than  now.  The  cold  of 
the  northern  hemisphere  would  be  further  intensified  by  the  divert- 
ing of  some  of  the  ocean  currents  to  the  south  as  the  "  heat  equator  " 
moved  south.  In  the  Atlantic  Ocean,  if  the  heat  equator  were 


WINTER 


N.P. 


N.R 


SUMMER 


FIG.  574.  —  Diagram  illustrating  the  astronomical  theory  of  glaciation.  A,  dia- 
gram showing  the  relative  positions  of  the  earth  and  sun  when  the  northern  summer 
occurs  in  aphelion.  This  is  the  condition  now.  B,  diagram  showing  the  relative 
positions  of  the  sun  and  earth  when  the  northern  summer  occurs  in  perihelion.  This 
condition  favors  glaciation,  since  the  winters  are  longer  and  colder. 

farther  south,  the  equatorial  current  would  be  turned  southward  by 
the  wedge-shaped  eastern  coast  of  South  America.  The  lowering  of 
the  temperature  would  be  further  increased  if  elevation  occurred  at 
the  same  time. 

Some  of  the  objections  to  the  theory  are  (i)  that  the  various  ice 
invasions  were  not  of  equal  duration  as  the  theory  requires ;  (2)  that 
the  duration  of  each  glacial  stage  was  greater  than  10,500  years,  in 
some  cases  many  times  greater;  (3)  that  during  the  Pleistocene 
glaciation  was  greater  in  equatorial  regions  than  now,  where,  according 
to  the  theory,  there  should  have  been  little  change  of  temperature. 

3.  Atmospheric  Hypothesis.  —  An  hypothesis  based  upon  the  vary- 
ing amounts  of  carbon  dioxide  and  water  vapor  in  the  atmosphere 


662  HISTORICAL  GEOLOGY 

has  been  favorably  received,  especially  in  America.  Carbon  di- 
oxide and  water  vapor  act  much  as  does  the  glass  in  a  greenhouse, 
i.e.,  they  form  a  thermal  blanket  which  prevents  the  radiation  of  much 
of  the  heat  derived  from  the  sun.  When  the  amount  of  one  or  both 
of  these  gases  is  diminished,  the  radiation  increases,  and  the  climate 
becomes  colder.  During  periods  of  great  elevation  and  continental 
extension,  erosion  would  be  greatly  increased,  and  the  withdrawal  of 
carbon  dioxide  from  the  air  would  be  rapid.  This  would  be  the  case 
since  carbon  dioxide  is  consumed  in  large  quantities  by  rocks  in 
weathering.  Such  consumption,  under  conditions  favorable  to  great 
erosion,  may  be  in  excess  of  the  supply.  Also,  at  times  of  great  land 
extension  the  water  surfaces  are  relatively  small,  and  since  less  evap- 
oration occurs  there  is  diminution  of  the  water  vapor  in  the  air. 
The  elevation  of  the  land  at  the  close  of  the  Tertiary  favored  the  con- 
sumption of  carbon  dioxide,  and  the  contraction  of  the  oceans  fur- 
nished less  water  vapor  to  the  atmosphere  than  formerly.  "  By 
variations  in  the  consumption  of  carbon  dioxide,  especially  in  its 
absorption  and  escape  from  the  ocean,  the  hypothesis  attempts  to 
explain  the  periodicity  of  glaciation.  Localization  is  attributed  to 
the  two  great  areas  of  permanent  low  pressure  in  proximity  to  which 
the  ice  sheet  developed."  (Chamberlin  and  Salisbury.) 

REFERENCES  FOR  CAUSES  OF  GLACIATION 

CHAMBERLIN,  T.  C.,  —  An  Attempt  to  Frame  a  Working  Hypothesis  of  the  Cause  of 
Glacial  Periods  on  an  Atmospheric  Basis:  Jour.  Geol.,  Vol.  7,  1899,  pp.  545-584; 
667-685;  75I-787- 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3,  pp.  424-446. 

CLARKE,  F.  W.,  —  Data  of  Geochemistry:  Bull.  U.  S.  Geol.  Surv.  No.  491,  pp.  46, 
47,  and  135. 

CROLL,  JAMES,  —  Climate  and  Time  and  their  Geological  Relations. 

DANA,  J.  D.,  —  Manual  of  Geology,  p.  978. 

LfiCoNTE,  J.,  —  Elements  of  Geology,  5th  ed.,  pp.  612-619. 

EFFECTS  OF  GLACIATION 

Glaciation  benefited  some  regions  and  was  harmful  to  others,  but 
the  former  effects  were,  on  the  whole,  greater  than  the  latter. 
Among  other  benefits  may  be  mentioned  (i)  waterfalls  and  rapids, 
which  afford  valuable  water  power ;  (2)  lakes  which  not  only  afford 
means  of  transportation,  but  so  ameliorate  the  climate  as  to  per- 
mit the  raising  of  fruits,  such  as  peaches  and  grapes,  which  otherwise 
would  not  thrive.  In  addition  to  this,  they  beautify  the  region 


QUATERNARY  663 

where  they  occur,  affording  for  the  city  dwellers  many  attractive 
places  for  rest  and  recreation.  (3)  The  bays  of  New  England 
have  for  the  most  part  been  modified  by  glaciation.  (4)  Kames, 
eskers,  and  delta  deposits  furnish  gravel  for  roads  and  for  concrete 
where  rocks  suitable  for  these  purposes  were  absent  in  preglacial 
times.  (5)  Deposits  of  clay  suitable  for  the  manufacture  of  brick 
are  abundant  in  old  glacial  lakes  and  valleys.  (6)  Soils  are  some- 
times more  fertile  and  sometimes  more  sterile  as  a  result  of  glaciation, 
but  the  balance  is  in  favor  of  the  former.  The  mixing  of  soils  from 
different  regions  by  glaciers  is  often  beneficial,  especially  when 
they  contain  fine  fragments  of  fresh  rock  which,  upon  weathering, 
furnish  a  constant  supply  of  plant  food.  On  the  other  hand,  large 
areas  overspread  by  glacial  sand  and  gravel  are  comparatively 
worthless,  and  hilly  regions  are  often  covered  with  bowlders.  Regions 
such  as  central  Kentucky  and  the  valley  of  Virginia,  for  example, 
would  probably  be  injured  were  an  ice  sheet  to  pass  over  them,  while 
others,  such  as  the  Piedmont  of  Virginia,  might  be  benefited. 

REFERENCES   FOR  THE  EFFECTS  OF  GLACIATION 

CHAMBERLIN  AND  SALISBURY,  —  Geology,  Vol.  3,  pp.  358-412. 

VON  ENGELN,  O.  D.,  —  Effects  of  Continental  Glaciation  on  Agriculture:    Bull.  Am. 
Geog.  Soc.,  Vol.  46,  1914,  pp.  241-264;  336-355- 

LIFE  OF  THE  PLEISTOCENE 

As  the  dinosaurs  culminated  in  the  Jurassic  and  Cretaceous,  so 
the  mammals  attained  their  greatest  size  and  variety  in  the  Pliocene 
and  Pleistocene.  "  The  early  and  mid-Pleistocene  life  of  North 
America  is  the  grandest  and  most  varied  assemblage  of  the  entire 
Cenozoic  Period  on  our  continent.  It  lacks  the  rhinoceroses  of 
Europe,  but  possesses  the  mastodons,  in  addition  to  an  array  of  ele- 
phants more  varied  and  quite  as  majestic  as  those  of  the  Old  World. 
Great  herds  of  large  llamas  and  camels  are  interspersed  with  enor- 
mous troops  of  horses.  Tapirs  roam  through  the  forests.  True 
cattle  (Bos)  are  not  present,  but  imposing  and  varied  species  of  bison 
are  widely  distributed.  An  element  entirely  lacking  in  Europe  is 
that  of  the  varied  types  of  giant  sloths,  which  were  scattered  all  over 
the  country,  as  well  as  the  great  armored  glyptodonts  in  the  south. 
Preying  upon  these  animals  are  not  only  saber-toothed  cats,  but  true 
cats,  rivaling  the  modern  lion  and  tiger  in  size."  (Osborn.) 

Our  record  of  the  life  of  the  Tertiary  and  previous  periods  has 


664 


HISTORICAL  GEOLOGY 


been  obtained  largely  from  marine,  lake,  and  blown-sand  deposits. 
Deposits  of  these  kinds  also  contain  Pleistocene  fossils ;  but,  since 
Pleistocene  animals  were  in  existence  but  a  comparatively  short  time 
ago,  their  remains  are  also  found  in  superficial  deposits,  such  as  river 
terraces,  peat  bogs,  frozen  soils,  ice  cliffs,  and  cave  deposits  (Fig. 
575),  which,  being  easily  destroyed  by  erosion,  are  seldom  found  in 
the  older  formations. 

Marine  Pleistocene  deposits  are  not  common,  since  the  subsidence 
which  followed  the  emergent  condition  of  that  epoch  buried  most  of 

the  sediments  of  the 
time  beneath  the  sea. 
This  is  unfortunate, 
since,  if  a  complete 
marine  record  were 
extant,  we  should 
have,  as  the  ice 
sheet  advanced  and 
retreated,  a  succes- 
sion of  faunas  and 
floras;  the  temperate 
life  changing  to  arctic 
as  the  ice  sheet  ad- 
vanced, and  this,  in  turn,  being  replaced  by  the  temperate,  or  even 
subtropical  life  (if  the  climate  changed  to  that  extent),  when  the  ice 
again  retreated.  During  each  advance  and  retreat  of  the  ice  sheet  a 
migration  of  the  life  to  and  fro  would,  theoretically,  be  recorded. 
Unfortunately,  however,  no  such  series  of  deposits  has  been  dis- 
covered, either  because  ideal  conditions  did  not  exist  in  any  one 
place,  or  because  the  deposits  are  inaccessible. 

Interglacial  Deposits.  —  Fortunately  two  deposits  are  known  which 
were  laid  down  during  interglacial  periods.  In  one  of  these,  near 
Toronto,  Canada,  the  fossil  beds  were  deposited  upon  the  eroded  sur- 
face of  bowlder  clay  and  were,  in  turn,  eroded  to  some  extent  before 
the  re-advance  of  the  ice  sheet  which  covered  them  with  a  layer  of 
drift.  The  lower  portion  (Don)  of  this  deposit  yields  plants  which 
show  that  the  climate  was  warmer  in  this  interglacial  stage  than  at 
present  on  Lake  Ontario,  being  similar  to  that  of  Virginia  to-day. 
This  is  indicated  by  the  presence  of  the  Judas  tree  (Cercis),  the  Osage 
orange  (Madura)  and  the  papaw  (Asimina).  Besides  these  more 
typically  southern  trees,  there  are  maples,  spruces,  oaks,  elms,  and 


FIG.  575.  —  Gailenreuth  Cavern,  Germany. 


QUATERNARY  665 

hickories.  In  the  upper  portion  (Scarborough  beds)  the  fossil  flora 
indicates  a  colder  climate,  showing  that  the  ice  sheet  was  again  ad- 
vancing. 

A  second  occurrence  of  interglacial  deposit  (Aftonian)  is  in  Iowa  and 
yields  the  bones  of  a  number  of  animals  :  elephants  and  mastodons, 
giant  beavers  (Castoroides),  camels,  horses,  some  dwarf  and  some 
perhaps  larger  than  the  domestic  horse,  and  all  of  extinct  species, 
giant  sloths  (Megalonyx),  etc.  This  deposit  is  of  especial  importance 
since  it  gives  some  idea  of  the  life  of  the  first  interglacial  period  and 
furnishes  a  clue  to  the  age  of  the  fossil  deposits  south  of  the  limit 
of  the  ice  sheet,  since,  if  the  fauna  in  any  of  these  are  the  same  as 
that  of  the  above,  it  is  probable  that  they  lived  at  the  same  time. 

North  and  South  Migrations  during  Glacial  and  Interglacial 
Times.  —  We  have  seen  that  warm  temperate  plants,  such  as  the 
papaw  and  Judas  tree,  lived  in  southern  Canada  during  at  least  one 
interglacial  period.  The  discovery  of  a  fossil  tamarack  (larch) 
in  Georgia,  480  miles  below  its  present  limit,  shows  that  the  Pleisto- 
cene climate  of  the  southern  states  was  colder  at  certain  times  than 
at  present,  and  for  a  period  sufficiently  long  to  permit  trees  to  grow. 
Walruses  of  a  northern  type  lived  on  the  coast  of  Georgia,  and  caribou 
and  moose  ranged  into  Pennsylvania  and  Ohio. 

An  interesting  suggestion  explaining  the  present  migratory  habit  of  birds  is  that 
it  is  due  to  the  reduction  in  the  temperature  of  the  Arctic  regions  during  late  Tertiary 
and  Pleistocene  times.  During  the  earlier  Tertiary  the  comparatively  uniform  climate 
of  the  world  would  not  necessitate  any  extended  periodic  movements,  but  the  cold  of 
the  Glacial  Period  must  have  enforced  prolonged  migration,  and  periodic  migration 
developed  later.  "  During  the  waning  ice  period  the  areas  offering  a  congenial  home 
to  a  great  multitude  of  birds  became  greatly  extended,  from  which,  however,  they 
were  driven  by  semi-arctic  winters  to  seek  favorable  winter  haunts  farther  southward." 

Deposits  beyond  the  Ice  Sheets  or  Protected  from  Them.  —  (i)  Cav- 
erns have  been  the  greatest  source  of  our  knowledge  of  the  mammalian 
life  of  the  Pleistocene,  the  bones  found  in  them  having  been  brought 
there  by  floods  or  by  beasts  of  prey  which  used  the  caverns  as  their 
lairs.  The  term  "  cavern  "  is  used  in  the  broad  sense  to  include  true 
caves,  sink  holes,  and  fissures.  A  section  of  the  cave  of  Gailenreuth 
(Fig.  575),  showing  the  bones  embedded  in  clay  and  the  whole  covered 
by  a  stalagmitic  crust,  is  typical  of  many  European  bone  caverns. 
Sometimes,  however,  there  is  more  than  one  stalagmitic  crust.  The 
number  of  individuals  and  species  represented  by  the  bones  found 
in  some  of  these  caves  is  remarkable.  In  the  Gailenreuth  cave  the 


666  HISTORICAL  GEOLOGY 

remains  of  800  cave  bears  were  found ;  from  one  in  Sicily  20  tons  of 
hippopotamus  bones  were  taken.  A  cave  in  Pennsylvania  (Port 
Kennedy),  60  to  70  feet  deep,  has  yielded  64  species  of  mammals,  of 
which  40  are  extinct,  among  them  being  giant  sloths,  tapirs,  mas- 
todons, and  saber-toothed  tigers.  In  a  California  cave,  horses, 
camels,  ground  sloths,  mastodons,  and  other  extinct  forms  have  been 
identified. 

Caves  were  doubtless  inhabited  by  the  land  animals  of  the  Ter- 
tiary and  other  periods,  but  as  caves  have  a  relatively  short  life,  they 
and  their  contents  are  rarely  preserved  in  the  older  formations. 

(2)  Marshy  ground  in  the  vicinity  of  springs  has  often  preserved 
many  fossils,  since  at  such  places  carnivores  frequently  kill  their  prey 
when  the  latter  are  coming  down  to  drink ;   and  in  times  of  drought 
the  animals  of  the  region  congregate  about  the  water  holes,  where 
they  often  die  in  large  numbers.     Many  also  are    doubtless    mired 
in  wet  seasons.     One  of  the  most  famous  of  such  deposits  (Big  Bone 
Lick,  Kentucky)  has  yielded  100  specimens  of  mastodon,  20  speci- 
mens of  elephant,  as  well  as  bisons,  musk  oxen,  and  other  animals. 

(3)  An  asphalt  deposit  (Fig.  576)  not  far  from  Los  Angeles,  Califor- 
nia is   remarkable,   not  only  because  unusual,  but   also  because  of 
the  number  of  specimens  preserved  in  it.     In  the  early  stages    of 
the   accumulation   of  the   asphalt,   the  gummy   surface   apparently 
acted  as  a  trap  for  unwary  animals ;  where  there  were  pools  of  water, 
aquatic  birds  of  many  kinds  were  entrapped  in  the  soft  tar  about 
their  margins ;    while  land   birds   and   smaller  mammals  were  en- 
snared in  attempting  to  reach  the  water.     Sloths,  mammoths,  horses, 
camels,  saber-toothed  tigers,  together  with  many  birds,  among  which 
is  a  fossil  peacock,  are  only  a  few  of  the  numerous  species  already 
identified. 

(4)  Wind-blown  sand  and  volcanic  dust  have  covered  and    pre- 
served skeletons  which  have  since  been  uncovered  and  studied. 

Difficulty  is  experienced  in  determining  to  what  portion  of  the 
Pleistocene  the  deposits  not  found  between  sheets  of  drift  belong,  but 
four  "  zones,"  or  subdivisions,  of  which  certain  animals  are  character- 
istic, have  been  recognized. 

Deposits  on  the  Last  Drift.  —  The  peat  bogs  which  rest  upon  the 
drift  of  the  last  ice  sheet  (Wisconsin)  have  yielded  a  number  of  masto- 
don and  mammoth  skeletons.  One  mastodon  found  at  Newburg, 
New  York,  had  its  legs  bent  under  the  body  and  the  head  thrown  up, 
evidently  in  the  very  position  in  which  it  was  mired.  The  teeth  were 


QUATERNARY 


667 


FIG.  576.  —  Pleistocene  tar  pool  near  Los  Angeles,  California,  with  entrapped  ani- 
mals. The  elephant  and  wolves  were  caught,  and  the  saber-toothed  tiger  is  about 
to  suffer  the  same  fate.  (After  Prof.  W.  D.  Scott,  History  of  Land  Mammals.) 

still  filled  with  the  half-chewed  remnants  of  its  food  which  consisted  of 
twigs  of  spruce,  fir,  and  other  trees. 

REFERENCES  FOR  THE  LIFE  OF  THE  PLEISTOCENE 

GEIKIE,  J.,  —  Antiquity  of  Man  in  Europe. 

MATTHEW,  W.  D., —  The  Asphalt  Group  of  Fossil  Skeletons:    Am.  Museum  Jour., 

Vol.  13,  pp.  291-297. 

OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  467-480,  487,  498. 
SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  29-49. 

Vegetation.  — The  vegetation  of  the  Pleistocene  is  a  continuation 
of  that  of  the  Tertiary,-and  aside  from  the  extinction  of  a  few  species 
little  change  is  noticed.  Some  minor  effects  of  glaciation  on  vege- 
tation are,  however,  interesting.  It  is  found,  for  example,  that  the 
number  of  genera  of  trees  in  Europe  (33  genera)  is  much  smaller  than 
in  eastern  North  America  (66  genera).  A  study  of  a  glacial  map  of 
Europe  and  North  America  suggests  an  explanation.  It  is  seen  that 
at  the  time  of  maximum  glaciation  in  Europe  the  vegetation  was  con- 
fined between  the  front  of  the  great  ice  sheet  on  the  north  and  the 


668  HISTORICAL  GEOLOGY 

expanded  mountain  glaciers  from  the  east-west  ranges  (Pyrenees 
and  Alps)  at  the  south.  As  a  result,  the  less  hardy  plants  were  killed, 
leaving  a  flora  rather  poor  in  species.  In  eastern  North  America,  on 
the  other  hand,  the  mountain  ranges  have  a  north-south  direction ; 
and  the  broad  plains  offered  few  obstacles  to  the  migration,  back 
and  forth,  of  plants  and  animals.  In  western  North  America  where 
north  and  south  migration  was  more  difficult,  3 1  genera  of  trees  are 
found  as  contrasted  with  66  genera  in  the  east. 

On  the  summits  of  some  mountains  arctic  plants  and  insects  are 
found  whose  presence  is  difficult  to  explain  except  on  the  assumption 
that,  as  the  ice  retreated  northward,  they  followed  the  front,  closely 
moving  up  the  sides  of  the  mountains  as  the  ice  retreated ;  and  that 
they  were  stranded  there  when  the  ice  disappeared  from  the  region. 

Attention  has  been  called  (p.  644)  to  the  fact  that,  apparently  as  a 
result  of  the  oscillations  of  climate  which  marked  the  Pleistocene, 
plants  were  forced  to  special  adaptations  and  habitats,  with  the  result 
that  there  is  now  little  mingling  of  tropical  and  subtropical  types  such 
as  was  the  case  in  the  Tertiary. 

REFERENCE  ON  VEGETATION 
WRIGHT,  G.  F.,  —  The  Ice  Age  in  North  America,  4th  ed.,  pp.  372-391. 

Mammoths  and  Mastodons.  —  Perhaps  the  most  characteristic 
Pleistocene  mammals  were  the  mammoth  and  the  mastodon  (Fig. 
577).  The  mammoths  (Columbian  and  Imperial)  are  true  elephants, 
and  the  mastodon  is  closely  related.  They  have  the  same  general 
appearance;  the  most  conspicuous  differences  being  (i)  in  the  teeth, 
which  in  the  mastodon  have  large,  transverse  ridges,  but  in  the 
mammoth  (as  in  the  living  elephant)  are  made  up  of  many  plates 
of  enamel,  alternating  with  cement  and  dentine  (Fig.  548,  p.  613) ; 
(2)  in  the  forehead,  which  is  low  in  the  mastodon  and  high  and  bulging 
in  the  mammoth;  and  (3)  in  the  shorter  and  more  massive  legs  of 
the  mastodon. 

The  largest  specimens  of  the  mammoth  (Elephas  primigenius)  ex- 
ceed in  size  that  of  any  elephant  now  living,  but  their  average  height 
was  probably  not  much  greater.  The  mastodon  was  somewhat  smaller 
than  the  mammoth.  Both  were  covered  with  long  hair,  with  prob- 
ably an  undercoating  of  fine  wool.  It  is  known  that  the  mammoth 
had  this  additional  protection  against  the  cold,  but  it  is  not  definitely 
known  that  the  mastodon  was  thus  protected. 


QUATERNARY  669 

The  mastodon  was  abundant  in  America,  possibly  as  abundant  as 
the  buffalo  (Clarke) ;  413  specimens  of  mammoths  and  mastodons 
have  been  reported  from  North  America,  of  which  330  are  mastodons. 
The  mastodon  ranged  over  the  whole  of  North  America.  Both  lived 
in  America  after  the  disappearance  of  the  ice  sheets,  as  is  proved  by 
the  burial  of  their  remains  in  peat  bogs  on  top  of  the  latest  drift. 
The  finding  of  charcoal  (perhaps  the  result  of  lightning)  beneath  a 
mastodon  skeleton  in  New  York,  and  charcoal  and  pottery  at  the  same 
level  in  the  same  bog,  suggests  the  possibility  that  man  and  the  mas- 
todon were  contemporaneous  in  America.  Drawings  upon  ivory 


FIG.  577.  —  Model  of  mastodon.     (Restoration  by  C.  R.  Knight  under  direction  of 
Prof.  H.  F.  Osborn.     Copyright,  American  Museum  of  Natural  History.) 

and  sketches  on  cave  walls  of  the  mammoth  prove,  without  question, 
that  in  Europe  man  had  seen  mammoths.  Carcasses  of  the  mammoth 
have  been  found  frozen  in  the  ice  in  Siberia,  where  they  were  so  per- 
fectly preserved  in  this  cold  storage  that  the  body  of  one,  at  least, 
furnished  food  for  dogs,  and  perhaps  even  for  man,  several  thousands 
of  years  after  its  death. 

The  distribution  of  the  elephant  tribe  in  the  New  World  at  that 
time  proves  that  land  connections  must  have  existed  between  Asia 
and  North  America,  and  between  North  and  South  America. 

REFERENCES   FOR  MAMMOTHS  AND   MASTODONS 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  270-282. 

LUCAS,  F.  A.,  —  Animals  of  the  Past,  pp.  177-219. 

LULL,  R.  S.,  —  Evolution  of  the  Elephant:  Am.  Jour.  Sci.,  Vol.  25,  1908,  pp.  181-212. 


670 


HISTORICAL  GEOLOGY 


Edentates.  —  This  class  is  found  abundantly  in  the  Tertiary  for- 
mations of  South  America,  and  towards  its  close  developed  into  gigan- 
tic and  highly  specialized  forms  which  were  among  the  largest  animals 
of  that  continent.  The  Pliocene  land  connections  between  North 
and  South  America  permitted  some  of  these  large  creatures  to  im- 
migrate to  North  America,  where  they  lived  into  the  Pleistocene. 


FIG.  578.  —  Restoration  of  the  gigantic  sloth,  Megatherium.     (After  Prof.  W.  D. 
Scott,  History  of  Land  Mammals.) 

They  never  reached  the  Old  World,  and  in  South  America  their  living 
relatives  are  small  and  inconspicuous.  Some  of  the  Megatherium 
(Greek,  megas,  large,  and  therion,  a  beast)  tribe  (Megatherium,  Mylo- 
don,  Megalonyx)  which  reached  this  continent  attained  the  bulk 
of  a  rhinoceros.  What  strikes  one  most  in  examining  the  skeleton 
of  a  Megatherium  (Fig.  578)  is  its  pyramidal  shape,  the  hind  legs 
being  massive  as  compared  with  the  fore,  and  the  backbone  rapidly 
enlarging  toward  the  hind  quarters.  The  Megatherium  lived  upon 
leaves  and  twigs,  and  when  standing  on  its  hind  legs  could,  if  neces- 
sary, use  its  tail  as  the  third  leg  of  a  tripod,  leaving  the  fore  limbs  free 
to  pull  down  branches  or  even  trees  of  considerable  size. 


QUATERNARY 


67: 


The  edentates  roamed  over  South  America,  and  some  members  of 
the  tribe  (Megalonyx  and  Mylodon)  over  a  large  portion  of  the 
United  States.  The  finding  in  Patagonia  of  a  large  piece  of  skin  (of 
Grypotherium)  covered  with  hair,  whose  edges  showed  the  marks  of 
tools  and  seems  to  have  been  stripped  off  the  carcass  by  man,  indi- 
cates that  some  members  of  the  tribe  were  alive  a  comparatively 
short  time  ago.  But  the  evidence  that  man  was  contemporaneous 
with  the  giant  sloths  in  North  America  is  not  conclusive. 

Another  edentate  of  very  different  appearance,  which  is  distantly 
related  to  the  armadillo,  is  Glyptodon  (Greek,  glyptos,  carved,  and 


FIG.  579.  —  The  great  armored  sloth,  Glyptodon.     (After  Prof.  W.  D.  Scott, 
History  of  Land  Mammals.) 

odont-,  tooth)  (Fig.  579).  The  body  was  covered  with  a  bony 
shell,  similar  in  appearance  to  that  of  a  tortoise,  but  made  up  of  a 
large  number  of  small,  polygonal  bones  united  to  form  an  immovable 
armature,  so  that  this  sloth  has  been  called  the  tortoise  armadillo. 
Not  only  was  the  body  protected,  but  the  tail  also  was  surrounded 
by  bony  plates,  and  the  top  of  the  head  was  similarly  armored.  The 
animals  grew  to  be  15  to  16  feet  long.  In  their  migrations  they 
reached  Texas  and  Florida. 


REFERENCES  FOR  EDENTATES 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  283-293. 
LANKESTER,  E.  R.,  —  Extinct  Animals,  pp.  167-184. 

CLELAND   GEOL. — 43 


672  HISTORICAL  GEOLOGY 

MATTHEW,  W.  D., —  The  Ancestry  of  the  Edentates:   Am.  Museum  Jour.,  Vol.   12, 

1912,  pp.  300-303. 
MATTHEW,  W.  D.,  —  The  Ground  Sloth  Group:  Am.  Museum  Jour.,  Vol.  n,  1911,  pp. 

II3-H9. 
SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  598-625. 

Pleistocene  Carnivores.  — One  of  the  most  characteristic  animals  of 
the  early  Pleistocene  was  the  saber-toothed  tiger  (Figs.  576,  580) 
(Machairodus),  so  named  because  of  the  enormously  developed,  sharp- 
edged,  upper  canine  teeth  which  in  some  species  extended  10  inches 
beyond  the  jaw.  An  examination  shows  that  if  the  jaw  were 

constructed  like  that  of  other 
carnivores,  a  time  would  come  in 
the  evolution  of  the  great  canine 
teeth  when  biting  would  be  im- 
possible. A  more  careful  study 
of  the  jaws,  however,  reveals  the 
fact  that  the  lower  jaw  could  be 

,.,  .        r  dropped  straight  down,  thus  per- 

FIG.    580.  —  Restoration    of  a    saber-          .....  V. 

toothed  tiger.    (Modified  after  Scott.)     mittmg  the  animal  to  use  the  full 

length  of  its  teeth  in  stabbing  its 

prey.     The  tribe  has  had  a  long  history,  small  ancestral  forms  with 
moderate  canine  teeth  being  known  from  the  Oligocene. 

Although  perhaps  the  most  powerful  of  the  carnivorous  animals  of 
the  Pliocene  and  Pleistocene,  they  nevertheless  became  extinct  early 
in  the  Pleistocene  in  Europe,  and  disappeared  from  the  New  World 
before  the  close  of  the  epoch,  their  place  being  taken  by  existing  carni- 
vores, such  as  the  lion,  tiger,  and  leopard. 

REFERENCE  FOR  CARNIVORES 
SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  530-536. 

Horses,  Camels,  etc.  —  Horses  roamed  over  the  plains  of  North 
America  in  great  herds  and  were  of  great  variety,  but  became  fewer 
and  fewer  until  all  had  disappeared  before  the  close  of  the  Glacial 
Period.  Some  (Equus  giganteus}  had  teeth  exceeding  in  size  those 
of  the  largest  modern  horses,  while  others  (Equus  tau)  were  more 
diminutive  than  any  other  true  horses  living  or  extinct. 

True  camels,  as  well  as  llamas,  were  abundant  in  portions  of  the 
United  States  in  the  early  Pleistocene,  living  at  least  as  far  north  in 
the  United  States  as  Nebraska  (Hay  Springs)  and  within  the  Arctic 


QUATERNARY  673 

circle  in  the  Yukon  territory.  They  probably  became  extinct  on  this 
continent  before  the  close  of  the  epoch. 

Bisons  of  many  species  roamed  over  North  America  during  the 
Pleistocene,  some  of  which  were  of  great  size,  if  the  horns  can  be  taken 
as  a  measure,  one  pair  of  horns  measuring  more  than  six  feet  from  tip 
to  tip.  Wolves,  musk  oxen,  bears,  and  rodents  (among  which  is  a 
giant  beaver,  Castoroides),  were  also  present. 

Although  Europe  was  not  invaded  by  the  sloth  tribe,  it  was,  never- 
theless, the  meeting  place  of  many  animals,  those  of  the  tropics  and 
those  of  the  Arctic  regions  coming  at  different  times  and  even  mingling 
at  others. 

REFERENCES  FOR  HORSES  AND   CAMELS 

Camel:  International  Encyclopedia. 
OSBORN,  H.  F.,  —  Age  of  Mammals,  p.  484;  and  others. 

SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  291-308; 
386-402. 

Birds.  —  The  Pleistocene  birds  of  Europe  and  America  were  not 
of  exceptional  size,  nor  did  they  differ  to  any  important  degree  from 
those  now  living,  but  in  New  Zealand  and  Madagascar,  gigantic 
flightless  birds  were  abundant  during  that  epoch.  The  name  moa 
includes,  in  a  general  way,  20  to  25  species  of  these  New  Zealand 
birds,  the  largest  of  which  stood  10  feet  high,  or  from  two  to  three 
feet  higher  than  an  ostrich,  while  the  smallest  were  about  the  size  of  a 
turkey.  In  all  of  these,  wings  are  entirely  wanting.  The  develop- 
ment of  flightless  birds  on  these  islands  seems  to  be  the  indirect  result 
of  the  absence  of  carnivorous  enemies.  With  an  abundance  of  food 
throughout  the  year  and  no  powerful  enemies,  the  New  Zealand  Pleis- 
tocene birds  had  not  the  usual  incentives  to  flight.  Under  such  con- 
ditions, some  of  them  increased  in  bodily  size  until  flight  was  impos- 
sible (25  or  30  pounds  seems  to  have  been  the  limit  of  the  weight  of 
flying  animals).  Once  the  power  of  flight  was  lost,  the  larger  and 
more  powerful  the  bird  the  better  was  the  chance  of  its  preservation 
as  long  as  food  was  abundant,  and  great  size  resulted.  A  change  in 
climatic  conditions,  however,  was  fatal  to  these  bulky  birds,  since 
having  lost  the  power  of  flight  they  were  unable  to  migrate,  and  were, 
therefore,  forced  to  depend  upon  the  food  of  the  islands.  Their  ex- 
tinction appears  to  have  been  due  partly  to  the  cold  of  the  Glacial 
Period  and  partly  to  man  who,  it  is  thought,  completed  their  exter- 
mination about  500  years  ago. 


674 


HISTORICAL  GEOLOGY 


REFERENCES   FOR  BIRDS 

HUTCHINSON,  H.  N.,  —  Extinct  Monsters  and  Creatures  of  Other  Days,  pp.  220-230. 

LANKESTER,  E.  R.,  —  Extinct  Animals,  pp.  240-244. 

LUCAS,  F.  A.,  —  Animals  of  the  Past,  pp.  138-151. 

New  International  Encyclopedia,  and  Encyclopedia  Britannica. 


PREHISTORIC  MAN 

The  record  of  prehistoric  man  and  his  ancestors  is  a  matter  of 
geological  as  well  as  of  anthropological  investigation.  Our  knowledge 
of  the  presence,  although  not  of  the  evolution,  of  prehistoric  man  is 
far  more  complete  than  of  other  animals,  because  the  source  of 
information  is  not  confined  to  his  bones,  but  is  obtained  also  from 
the  implements  of  stone  which  he  made  and  the  discovery  of  which  is 
especially  likely,  as  they  were  frequently  lost  in  fishing  or  in  the  chase. 
Moreover,  since  they  are  practically  indestructible,  they  are  preserved 
after  the  skeletons  of  their  makers  have  been  destroyed. 

It  is  a  mooted  question  whether  or  not  man  existed  in  America 
at  an  early  period,  but  in  Europe  the  remains  of  prehistoric  man  have 
been  found  in  situations  which  prove  beyond  question  their  antiquity. 

A  brief  classification  based  on  the  evolution  of  human  implements 
in  Europe  is  as  follows : 


Recent 

Iron  Age 

The  Present. 

Bronze  Age 

Implements  of  bronze  as  well  as  of  stone. 
Some  tribes  passed  directly  from  the  Stone  to 
the  Iron  Age. 

Stone  Age 

Prehistoric 
in  the  Old  World 

Neolithic  (Greek,  neos,  new,  and  lithos, 
stone).  Implements  of  stone,  often  polished 
and  with  ground  edges. 

Pleistocene 

Paleolithic  (Greek,  paleos,  old,  and  lithos, 
stone).  Stone  implements,  rough  with  chipped 
edges  but  never  ground. 

Eolithic  (Greek,  eos,  dawn,  and  lithos,  stone). 
Dawn  of  the  Stone  Age.  Implements  so  crude 
that  it  is  often  difficult  to  distinguish  them 
from  those  made  by  accident. 

Tertiary 

QUATERNARY 


675 


Eolithic.  —  In  Pliocene  and  Miocene  deposits  and,  it  is  asserted, 
even  in  those  of  the  Oligocene,  extremely  rude  flints,  called  eoliths 
(Fig.  581),  have  been  found.  Although  rough  and  crude,  they  often 
show  one  part  shaped  as  if  to  be  held  in  the  hand,  while  the  other  part 
appears  to  have  been  designed  for  cutting.  It  has  long  been  a  question 
whether  these  flints  were  the  result  of  accident  or  were  made  by  a 
"  tool-making  animal,"  either  very  early  man  or  a  prehuman  type 
given  to  shaping  implements.  If  the  flints  did  not  occur  in  deposits 
earlier  than  the  Pleistocene,  the  question  might  be  answered  more 
certainly,  but  since 
they  are  found  in 
beds  laid  down  more 
than  a  million  years 
ago,  the  difficulty  is 
increased. 

Thediscovery,near 
Heidelberg,  Ger- 
many, of  a  lower  jaw 
of  a  very  low  type  in 
early  Pleistocene  de- 
posits said  to  contain 
eoliths,  is  important, 
since  it  gives  a  clue 
to  the  makers  of 
these  flints.  This 


FIG.  581.  —  Eoliths,  the  crudest  of  flint  implements. 
Believed  to  have  been  made  by  ape-men.  (After 
MacCurdy.) 

lower  jaw  is  massive, 

with  an  essentially  human  set  of  teeth,  its  most  noticeable  feature 
being  the  absence  of  a  chin  projection.  In  other  words,  it  is  the  jaw 
of  an  anthropoid  (manlike)  ape  with  the  dentition  of  a  man.  As 
compared  with  the  oldest  Paleolithic  skulls  (Neanderthal)  (p.  667), 
this  one  is  of  a  much  lower  type.  It  is  possible,  therefore,  that  the 
eoliths  of  the  later  Tertiary  were  made  by  some  tool-making  ape. 

A  creature  (Pithecanthropus  erectus)  whose  fragmentary  remains 
have  been  found  in  Pleistocene  deposits  of  Java,  associated  with  the 
bones  of  extinct  animals,  may  also  have  been  a  member  of  a  race  which 
made  eoliths.  These  remains  consist  of  a  skull  cap,  two  molar  teeth, 
and  a  diseased  thigh  bone,  and  are  remarkable  because  of  the  com- 
bination of  ape  and  human  characters.  The  skull  differs  from  that 
of  an  ape,  its  brain  capacity  being  about  twice  that  of  an  ape  of  equal 
bodily  size.  The  brain  capacity  of  an  ape's  skull  is,  on  an  average, 


676 


HISTORICAL  GEOLOGY 


500  cubic  centimeters ;  of  the  skull  of  this  so-called  ape-man  (Pithe- 
canthropus erectus)  850  cubic  centimeters;  of  an  average  man  1400 
to  1500  cubic  centimeters.  The  skulls  of  aborigines  of  Tasmania 
have  an  average  of  only  1199  cubic  centimeters.  This  skull,  then, 
as  regards  capacity,  occupies  an  intermediate  position  between  the 
large  apes  and  man.  Moreover,  the  forehead  is  low  and  the  frontal 
ridge  prominent,  and  the  characteristic  features  are,  in  general,  inter- 
mediate between  those  of  the  lowest  man  and  the  highest  apes.  The 

teeth  are  human,  with  cer- 
tain apelike  characters,  and 
the  thigh  bone  is  considered 
to  be  intermediate. 

Paleolithic  Man.  —  Al- 
though at  first  merely 
chipped  into  shape  and 
never  ground  at  the  edges 
or  polished,  the  Paleolithic 
stone  implements  (Fig.  582 
A)  indicate  that  their  makers 
had  a  much  greater  intelli- 
gence and  skill  than  that 
possessed  by  the  tool-making 
animals  of  Eolithic  times. 
The  works  of  Paleolithic  man 
are  found  principally  in  caves 
and  in  river  gravels,  often 
associated  with  the  bones  of 
extinct  animals  and  occa- 
sionally with  the  bones  of 
man  himself.  It  seems  to 
be  well  established  that  Paleolithic  man,  together  with  other  south- 
ern animals,  reached  western  Europe  during  one  of  the  interglacial 
periods,  probably  during  the  second. 

The  relative  age  of  Paleolithic  human  relics  can  often  be  determined 
by  a  study  of  the  fauna  with  which  they  are  associated.  The  oldest 
relics  are  found  with  elephants  (Elephas  antiquus)  more  ancient  than 
the  mammoth,  very  old  rhinoceroses  (Rhinoceros  merckii),  and  hip- 
popotamuses (Hippopotamus  amphibius).  In  the  next  oldest  stage 
the  mammoth,  woolly  rhinoceros,  cave  bear,  cave  hyena,  and  other 
extinct  animals  are  common.  The  last  stage  occurred  at  the  close  of 


A  B 

FIG.  582.  —  On  the  left,  a  Paleolithic  imple- 
ment; on  the  right,  a  Neolithic  implement. 
(After  MacCurdy.) 


QUATERNARY 


677 


the  Glacial  Period,  at  which  time  reindeer  were  crossing  Europe  in 
great  numbers;  and  their  remains  often  occur  with  those  of  man, 
giving  it  the  name  of  "  Reindeer  stage." 

We  are  assisted  in  our  conception  of  Paleolithic  man  by  a  study  of 
the  recently  extinct  aborigines  of  Tasmania,  who  were,  though  recent,  a 


FIG.  583.  —  Skulls  of  modern  and  Paleolithic  man.     The  contrast  in  forehead, 
brow,  teeth,  chin,  and  shape  of  skull  is  very  marked. 

true  Paleolithic,  or  perhaps  a  degenerate  race.  Their  clothing  con- 
sisted of  skins  thrown  over  the  shoulders,  and  they  protected  them- 
selves from  the  rain  by  daubing  themselves  with  grease  and  .ocher. 
They  had  no  fixed  place  of  abode ;  and  even  in  winter,  a  screen  of  bark 
served  as  a  shelter.  Their  implements  were  few  and  simple,  and  were 
made  of  wood  and  stone,  the  latter  being  fashioned  by  striking  off 
chips  from  one  flake  with  another.  Cooking  by  boiling  was  unknown  ; 
and  their  sea  food  consisted  of  shellfish,  as  they  knew  nothing  of  fishing 
with  a  hook.  Their  survival  until 
the  present  was  due  to  their  iso- 
lated position. 

The  skulls  and  skeletons  of  the 
older  Paleolithic  men  of   Europe       FlG-  584-  — Thigh  bone  of  modern 
111  r   man   (shaded),   and  of  Paleolithic  man 

show  that  they  were    savages  of  (outline)i 

the  lowest  type  (Figs.  583  B,  584), 

with  low  foreheads  and  rather  large  though  not  highly  organized 
brains.  They  were  small  in  stature  (five  feet,  three  inches  in 
average  height),  with  knees  that  were  bent  slightly  forward,  giving 
them  a  carriage  that  was  not  fully  erect. 

Their  skill  as  hunters  is  shown  by  the  great  quantities  of  bones  of 
animals  about  their  ancient  camps ;    the  environs  of  one  such  camp 


678 


HISTORICAL  GEOLOGY 


FIG.  585.  —  Carvings  on  bone  made  by  Paleolithic  man. 


having  yielded  the  fragments  of  at  least  100,000  horses.  Other 
animals,  such  as  the  mammoth,  rhinoceros,  bison,  and  reindeer  were 
also  eaten.  The  presence  of  charcoal  in  their  caves  shows  that 
Paleolithic  men  knew  how  to  produce  fire  by  friction,  and  that  they 
probably  roasted  the  flesh  upon  which  they  largely  subsisted.  They 
apparently  knew  nothing  of  agriculture  and  had  no  domestic  animals, 

not  even  a  dog.  The 
stone  arrows,  lance 
heads,  and  hatchets, 
as  has  been  said,  were 
never  ground  at  the 
edges  nor  polished. 
In  some  caves  imple- 
ments made  of  bone, 
such  as  arrows,  har- 
poons, fishhooks,  awls  for  piercing  skins,  and  needles  are  not  un- 
common. The  love  of  adornment  is  proved  by  the  occurrence  of 
numerous  perforated  teeth  and  shells  which  were  doubtless  strung 
into  necklaces.  The  artistic  skill  displayed  in  carvings  on  bone 
and  ivory  (Fig.  585),  sketches  on  mammoth  tusks  (Fig.  586),  as 
well  as  pictures  on  the  walls  (Figs.  587,  588)  of  their  caves,  is  unex- 
pected, being  superior  to  that  possessed  by  any  other  primitive  men 
ancient  or  modern.  Indeed,  in  our  own  time  few  people  not  artists 
can  equal  some  of  the  art  of  Paleolithic  man.  Although  there  is  no 
perspective  composition  in  the  pictures,  the  drawing  is  excellent  and 
the  proportions  and  postures  are  unusually  good.  Sketches  were 
made  in  red  and  black  (Figs.  587,  588),  as  well  as 
outline  drawings  in  black.  The  artists  chose 
almost  exclusively  the  large  animals  of  the  time, 
the  bison,  mammoth,  reindeer,  horse,  boar,  and 
rhinoceros.  Man  for  some  reason  was  seldom 
portrayed.  During  the  Paleolithic  the  making 
of  flint  implements  was  gradually  perfected,  and 
before  its  close  bone  and  horn  implements  of  carving  of  a  mammoth. 
highly  useful  and  artistic  forms  were  made. 

Paleolithic  man  had  a  crude  form  of  religion,  and  the  dead  were 
buried  ceremoniously.  There  appears  to  have  been  a  division  of  labor, 
some  men  devoting  themselves  to  hunting,  some  to  flint  making,  and 
some  to  art,  although  it  is  hardly  probable  that  the  specialists  in  any  of 
these  groups  were  not  often  employed  in  other  work. 


FIG.    586.  —  Paleolithic 


QUATERNARY 


679 


I*-*" 


FIG.  587.  —  Paleolithic  painting  of  a  boar.     (Courtesy, 
American  Museum  of  Natural  History.) 


Neolithic  Man.  —  Man  of  Neolithic  times  was  not  contemporane- 
ous with  the  great  extinct  mammals,  with  the  exception  of  the  Irish 
elk.  Their  remains 
have  been  found  in 
caves,  cemeteries, 
and  river  deposits, 
in  peat  bogs,  and 
lake  bottoms  (pile 
dwellings),  and  in 
shell  mounds.  Neo- 
lithic implements  and 
weapons  are  often 
ground  at  the  edge 
(Fig.  582  B)  and 
more  or  less  polished 
and  finely  finished, 
and  are  frequently 
of  graceful  design.  With  some  doubtful  exceptions,  Paleolithic  man 
in  western  Europe  seems  to  have  been  suddenly  replaced  by  Neolithic 
man,  who  brought  with  him  not  only  greater  skill  in  the  manu- 
facture of  implements,  but  domesticated  animals,  such  as  the  dog, 
horse,  sheep,  goat,  and  hog.  Moreover,  he  was  acquainted  with 

agriculture,  as  grains 
and  the  seeds  of 
fruits,  as  well  as 
dried  fruits,  show. 
Spinning,  weaving, 
and  pottery  making 
were  also  practiced. 
An  important  part 
of  our  knowledge  of 
Neolithic  man  comes 
from  the  lake  dwell- 
ings in  Switzerland 
and  Sweden.  These 
FIG.  588.  —  Paleolithic  painting,  in  red  and  black,  of  a  bull,  dwellings  were  on 
(Courtesy,  American  Museum  of  Natural  History.)  .,  .  . 

piles      driven      into 

shallow  lakes,  and  were  connected  with  the  shore  by  drawbridges 
which  could  be  withdrawn  in  case  of  attack. 

The  Age  of  Stone  gradually  merges  into  the  Bronze  Age,  as  that, 


680  HISTORICAL  GEOLOGY 

in  turn,  merges  into  the  Age  of  Iron,  but  since  these  last  two  stages 
belong  to  protohistorical  and  historical  times,  they  are  outside  our 
province.  The  Age  of  Stone  did  not  come  to  an  end  throughout  the 
world  at  the  same  time.  The  natives  of  the  New  World,  Australia, 
and  the  islands  of  the  Pacific  were  in  the  Neolithic  Age,  and  those 
of  Tasmania  in  the  Paleolithic  Age,  when  discovered  by  Europeans ; 
and  some  isolated  tribes  are  to-day  still  using  stone  implements. 


REFERENCES  FOR  PREHISTORIC  MAN 

Anthropology  and  Archaology:  Encyclopedia  Britannica. 

DUCKWORTH,  W.  L.  D.,  —  Prehistoric  Man,  1912. 

HOERNES,  MORITZ,  —  Kultur  der  Urzeit,  Vol.  I,  1912. 

HuNTER-DuvAR,  J., —  The  Stone,  Bronze,  and  Iron  Ages,  pp.  80-138. 

KEITH,  ARTHUR,  —  Ancient  Types  of  Man. 

MACCURDY,  G.  G.,  —  The  Eolithic  Problem,  etc. :   Am.  Anthropologist,  Vol.  7,  1905, 

pp.  425-479. 
MAcCuRDY,  G.  G.,  —  Recent  Discoveries  Bearing  on  the  Antiquity  of  Man  in  Europe: 

Smithsonian  Rept.  for  1909,  pp.  531-583. 
MAcCuRDY,  G.  G.,  —  Ancient  Man,  his  Environment  and  his  Art:  Pop.  Sci.  Monthly, 

Vol.  83,  1913,  pp.  5-23. 
MORRIS,  CHAS.,  —  Man  and  his  Ancestors. 

MUNRO,  ROBERT,  —  Paleolithic  Man  and  Terramara  Settlements  in  Europe. 
OSBORN,  H.  F.,  —  Age  of  Mammals,  pp.  381-385;  403-410;  428. 
OSBORN,  H.  F.,  —  Men  of  the  Old  Stone  Age:  Am.  Museum  Jour!,  Vol.  12,  1912,  pp. 

279-288. 

SOLLAS,  W.  J.,  —  Ancient  Hunters,  1911. 
WISSLER,  C., — The  Art  of  the  Cave  Man:  Am.  Museum  Jour.,  Vol.  12, 1912,  pp.  289-295. 

Man  in  North  America.  —  No  conclusive  proof  of  the  presence  of 
man  in  North  America  during  the  Pleistocene  has  yet  been  offered. 
Indeed,  it  is  doubtful  if  Paleolithic  man  ever  lived  on  this  continent. 

Earlier  investigators  were  led  to  assign  a  greater  antiquity  to  many 
human  relics  than  subsequent  study  has  shown  to  be  possible.  Such 
errors  were  the  result  of  over-enthusiasm  and  a  failure  to  take  into 
consideration  all  of  the  elements  of  the  problem,  some  of  which  are 
the  following. 

(i)  The  presence  in  river  gravels  of  rude  flints  has  led  to  the 
conclusion  that  they  were  made  by  Paleolithic  man.  The  danger  in 
such  a  conclusion  lies  in  the  fact  that,  in  the  shaping  of  a  stone 
tool  the  maker  sometimes  loses  the  half-finished  stone  and  often  rejects 
others  early  in  his  work  because  of  some  imperfection  or  unfavorable 
quality  in  the  stone.  As  a  consequence,  many  unfinished  stone  im- 


QUATERNARY  68 1 

plements  are  left,  especially  along  river  courses  where  the  pebbles 
from  which  the  implements  were  made  occur. 

(2)  If  a  stone  implement  is  found  buried  to  a  great  depth,  the 
thickness  of  the  overlying  deposit  has  often  been  taken  as  a  measure 
of  its  age.     Such  data  are  very  uncertain,  since  during  floods  a  river 
may  scour  out  deep  holes  in  its  bed,  and  within  a  few  weeks,  or  months, 
completely  fill  the  excavation.     The  Missouri,  for  example,  scours 
out  its  bed  to  a  depth  of  40  feet  or  more  during  floods,  and  soon  fills 
it  again  (p.  88).     It  will  readily  be  seen,  therefore,  that  the  finding 
of  a  flint  implement  in  river  gravels  at  a  depth  of  40  feet  might  not 
indicate  any  greater  antiquity  for  it  than  for  one  on  the  surface. 

(3)  The  age  of  flints  in  talus  slopes  is  uncertain,  since  what  was  in 
the  top  portion  of  the  cliff  naturally  becomes  part  of  the  base  of  the 
talus  as  the  cliff  crumbles  back. 

(4)  The  antiquity  of  stone  implements  found  beneath  layers  of 
stalagmite  has  often  been  overstated,  because  a  too  low  rate  of  deposi- 
tion was  used  in  the  estimate.     This,  however,  has  not  been  a  source  of 
difficulty  in  America,  since  cave  deposits  are  rare  on  this  continent. 

(5)  The  admixture  of  human  remains  with  those  of  extinct  animals 
is  not  necessarily  a  proof  of  their  contemporaneousness,  since  it  may 
have  been  due  to  accidental  causes,  such  as  human  burial  in  deposits 
containing  extinct  animals,  or  to  the  washing  out  from  older  deposits 
of  the  bones  of  extinct  animals  and  their  redeposition  with  those  of 
recent  species. 

It  will  readily  be  seen  from  the  above  that  the  error  in  determining 
the  age  of  human  relics  will  usually  be  in  the  direction  of  too  great 
antiquity.  From  the  similarity  in  physical  appearance  of  the  aborig- 
ines of  North  and  South  America,  it  seems  probable  that  the  original 
inhabitants  of  the  New  World  were  immigrants  who  came  from  Asia 
and  spread  over  the  Americas  after  they  had  become  differentiated 
in  Asia,  but  long  enough  ago  to  permit  of  the  development  in  their  new 
home  of  the  many  languages  and  dialects  now  spoken  by  the  Indians. 


REFERENCES  FOR  MAN  IN  NORTH  AMERICA 

Os BORN,  H.  F.,  —  Age  of  Mammals,  pp.  494-500. 

Os BORN,  H.  F.,  —  Men  of  the  Stone  Age. 

SCOTT,  W.  B.,  —  A  History  of  Land  Mammals  in  the  Western  Hemisphere,  pp.  588-590. 

WISSLER,  C., —  The  Art  of  the  Cave  Man:  Am.  Museum  Jour.,  Vol.  12, 1912,  pp.  289-295. 

Birthplace  of  Man.  —  The  location  of  the  original  home  of  man  was 
a  matter  of  speculation  even  by  the  ancients  and  is  still  in  doubt,  but 


682  HISTORICAL  GEOLOGY 

some  suggestions  have  recently  been  made  which  are  worthy  of  con- 
sideration. Science  points,  as  does  the  biblical  account,  to  Asia  as 
the  birthplace  of  man.  It  is  evident  that  the  Americas  were  not 
populated  by  man  until  comparatively  recent  geological  times  (p.  680), 
and  that  Paleolithic  and  Neolithic  man  probably  migrated  to  Europe 
from  some  other  continent.  It  is  a  suggestive  fact  that  all  of  our  do- 
mesticated animals,  with  the  exception  of  the  llama,  the  vicuna,  and 
the  turkey,  had  their  origin  in  Asia,  and  that  they  are  the  most  highly 
specialized  of  their  kinds.  Moreover,  possibly  all  of  the  cereals,  with 
the  exception  of  maize,  are  of  Asiatic  origin.  "  Man  was  born  and 
attained  elemental  civilization  in  Asia  because  there  was  the  place  of 
all  others  upon  the  earth  where  evolution,  in  general,  of  organic  life 
reached  its  highest  development  in  late  Cenozoic  times."  (Williston.) 
The  loss  of  man's  hairy  covering  is  evidence  of  his  origin  in  a  tem- 
perate, or  cold  temperate  climate,  where  he  found  clothing  necessary 
to  protect  himself  from  the  inclemencies  of  the  weather. 


REFERENCE  ON  THE  BIRTHPLACE  OF  MAN 

WILLISTON,  S.  W.,  —  The  Birthplace  of  Man:  Pop.  Sci.  Monthly,  Vol.  77, 1910,  pp.  594- 
597- 

Effect  of  the  Advent  of  Man.  —  The  appearance  of  man  was  one 
of  the  greatest  events  in  the  whole  history  of  the  world,  not  only  be- 
cause, for  the  first  time,  brute  strength  and  agility  were  at  a  disad- 
vantage in  a  struggle  with  higher  intelligence,  but  also  because  of  the 
changes  which  he  directly,  or  indirectly,  caused,  not  only  in  the  life 
of  the  world,  but  also  in  the  very  topography  of  the  earth  itself, 
(i)  Man  has  directly  caused  and  is  still  causing  the  rapid  disappear- 
ance of  many  animals :  such  as  the  bison,  the  moa,  seal,  whale,  fur- 
bearing  animals,  and  the  big  game  of  Africa  and  Asia.  (2)  In- 
directly, by  the  introduction  of  animals  and  plants  into  new  regions, 
he  has  accomplished  as  great,  or  even  greater,  changes  in  life.  The 
introduction  of  the  mongoose  into  Cuba,  which  soon  destroyed  not 
only  the  snakes,  but  the  birds  that  nested  on  the  ground,  has  almost 
revolutionized  the  fauna  of  that  island.  The  rabbits  brought  to 
Australia  have  overrun  that  continent,  with  a  marked  effect  on  the 
indigenous  life.  The  various  insects  introduced  into  North  America  by 
man  are  changing  the  flora  of  this  country.  Many  other  examples 
might  be  added.  (3)  His  work  has  not,  however,  been  entirely  de- 
structive to  life.  Animals  and  plants  on  the  verge  of  extinction  have 


QUATERNARY  683 

been  preserved.  The  ginkgo  tree  (p.  567),  for  example,  would  have 
been  to  us  an  extinct  species  if  man  had  not  preserved  it  by  cultivation. 
(4)  Not  only  has  his  contact  with  the  life  of  the  world  been  important, 
but  his  indirect  effect  upon  inanimate  nature  has  been  stupendous. 
The  cutting  and  burning  of  forests  in  certain  regions  has  resulted  in  the 
rapid  erosion  of  large  areas,  and  the  pulverization  of  the  soil  in  plow- 
ing has  permitted  rainwash  to  carry  away  the  best  of  the  soil.  By 
deforestation  alone  a  single  lumber  merchant  may  in  50  years  deprive 
the  human  race  of  soil  that  required  thousands  of  years  to  form. 
Another  effect  which  will  eventually  greatly  lessen  the  fertility  of  the 
soil  is  the  enormous  and  irrecoverable  loss  of  phosphates  in  the 
sewerage  of  cities.  As  the  result  of  these  and  many  other  effects  of 
man's  supremacy,  the  earth  has  suffered  a  vastly  greater  change  in 
the  past  few  hundred  years  than  in  many  thousands  of  years  in  the 
most  destructive  periods  of  the  past. 

FUTURE  HABITABILITY  OF  THE  EARTH 

Two  statements  are  often  made  concerning  the  future  of  the 
earth :  one  that  the  climate  will  become  progressively  cooler  until 
it  will  be  unsuited  for  the  existence  of  plants  and  animals;  the 
other,  that  the  earth  will  eventually  be  consumed  by  fire.  The 
former  statement  is  based  on  the  assumption  that  the  heat  of  the 
sun  is  diminishing,  and  that,  since  the  earth  depends  upon  it  for 
its  heat,  a  cooling  of  the  sun  will  cause  refrigeration.  There  is  no 
question  but  that  this  would  be  the  case  were  the  sun's  heat  to 
decrease,  but  no  such  change  can  be  detected.  The  present  heat  of 
the  sun  is  apparently  maintained  by  the  infalling  of  meteorites,  as 
well  as  by  that  given  off  by  radioactive  minerals,  and,  consequently, 
sufficient  heat  to  produce  a  favorable  climate  may  exist  for  many 
millions  of  years.  A  convincing  proof  lies  in  the  fact  that,  since  life 
began,  the  climate  has  not  changed  sufficiently  to  cause  a  widespread 
destruction  of  life.  Periods  of  aridity,  glaciation,  and  other  climatic 
changes  have  frequently  occurred,  but  none  that  was  universally  fatal. 

The  statement  that  the  earth  will  eventually  be  consumed  by  fire 
assumes  that  the  sun  or  earth  may  collide  with  some  other  star.  No 
such  catastrophe,  however,  has  occurred  in  the  past,  none  seems  to 
be  impending.  It  seems  safe,  consequently,  to  predict  that  for  many 
years  —  hundred  of  thousands,  perhaps  millions  —  the  conditions 
favorable  to  man's  existence  will  be  present. 


684  HISTORICAL  GEOLOGY 

When  it  is  remembered  that  man  has  come  up  from  the  cave  and 
the  stone  hammer  in  the  past  50,000  or  75,000  years,  and  that  in  the 
past  100  years  the  greatest  achievements  of  science  have  been  accom- 
plished, so  that  man  to-day  lives  under  conditions  radically  different 
from  those  of  his  ancestors  of  a  few  generations  past,  it  would  seem 
that  the  evolution  which  will  take  place  will  change  profoundly  the 
human  race,  if  not  interfered  with.  The  progress  of  evolution  does 
not,  however,  have  a  free  course  since,  as  never  before  in  the  history 
of  animal  life,  the  unfit  do  not  disappear  in  the  struggle  for  existence, 
but  the  life  of  the  physically  and  mentally  unfit  is  lengthened  through 
the  aid  of  medical  science  and  chanty.  The  future  will,  doubtless, 
bring  solution  for  such  vital  problems,  and  the  evolution  of  the 
human  race  can  confidently  be  expected  to  continue,  with  the  develop- 
ment of  a  type  of  man  much  superior  to  that  now  on  earth. 


APPENDIX 
COMMON  MINERALS 

EVERY  student  of  geology  should  be  able  to  recognize  the  common 
minerals  by  sight  and  know  their  approximate  chemical  composition. 
In  order  to  determine  minerals  without  the  aid  of  chemical  tests,  one 
must  depend  upon  their  physical  properties.  Of  these  the  color, 
streak,  hardness,  specific  gravity,  and  crystalline  form  are  important. 

The  color  sometimes  varies  greatly  in  the  same  mineral,  but  never- 
theless often  affords  a  strong  clue  to  its  identity.  The  color  of  a  min- 
eral is  often  due  to  the  inclusion  of  foreign  matter,  such  as  iron  oxide 
and  organic  matter,  but  some  minerals,  such  as  the  carbonate  of  cop- 
per, malachite,  vary  slightly. 

Each  mineral  has  a  characteristic  hardness  and  this  quality  often  af- 
fords an  easy  means  of  positive  identification.  The  scale  of  hardness 
in  common  use  is:  I,  Talc;  2,  Gypsum;  3,  Calcite;  4,  Fluorite; 
5,  Apatite ;  6,  Orthoclase ;  7,  Quartz ;  8,  Topaz ;  9,  Corundum  ; 
10,  Diamond.  Minerals  with  a  hardness  of  I  and  2  can  be  scratched 
with  the  finger  nail.  If  a  mineral  will  barely  scratch  a  copper  coin, 
it  may  be  considered  as  about  3  in  hardness ;  if  it  fails  to  scratch 
glass,  its  hardness  is  less  than  5  ;  if  it  scratches  glass  but  fails  to  scratch 
quartz,  its  hardness  is  between  5  and  7.  A  knife  point  is  almost  in- 
dispensable in  determining  hardness,  since  with  a  little  practice,  the 
hardness  of  all  minerals  between  i  and  6  can  be  readily  determined. 

The  streak  or  mark  that  a  mineral  makes  on  a  hard  white  substance, 
such  as  a  piece  of  unglazed  porcelain,  is  often  important  in  distinguish- 
ing between  minerals.  The  color  of  the  streak  is  the  same  as  that  of 
the  fine  powder. 

When  a  mineral  breaks  or  cleaves  in  definite  directions  so  as  to  form 
plane  surfaces,  it  is  said  to  have  a  cleavage.  Since  cleavage  is  caused 
by  the  separation  along  and  between  layers  of  molecules,  it  occurs 
only  in  crystals.  The  thin  leaves  of  mica  are  formed  by  the  splitting 
of  the  mineral  along  cleavage  planes. 

The  relative  weight  of  a  mineral,  or  its  specific  gravity,  is  often  an 
important  aid  in  determining  a  mineral  by  its  physical  properties. 

685 


686  APPENDIX 

IRON  MINERALS 

Magnetite  (magnetic  iron  ore),  Fes 04.  —  Color,  black.  A  black 
streak  is  made  when  the  mineral  is  scratched  on  a  hard  white  surface. 
The  hardness  is  slightly  greater  than  steel  (H=  6).  It  is  always  at- 
tracted by  a  magnet  and  is  sometimes  capable  itself  of  lifting  particles 
of  iron  and  steel.  It  is  a  valuable  ore  of  iron.  The  Adirondack  iron 
ore  is  largely  magnetite. 

Hematite  (red  iron  ore),  Fe2O3.  —  The  color  is  black  to  brick  red. 
Streak,  red.  Slightly  harder  than  steel  (H=  6).  Occurs  in  compact 
masses  composed  of  micalike  flakes,  in  an  earthy  form,  and  in  thin 
crystals  set  on  edge.  It  is  the  most  widely  used  iron  ore  in  North 
America,  the  most  famous  localities  of  which  are  in  the  Lake  Superior 
region  and  in  Alabama. 

Limonite  (brown  hematite,  iron  hydroxide),  ^  Fe2O3-3  H2O.  —  The 
color  is  usually  dark  brown,  but  is  sometimes  yellow.  The  streak  is 
yellow.  The  hardness  of  compact  kinds  is  slightly  less  than  steel 
(H=  5).  The  ocher  which  occurs  with  limonite  is  composed  of  clay 
and  limonite  in  a  finely  divided  condition.  Limonite  is  really  iron 
rust  and  is  formed  from  the  hydration  of  many  iron  minerals,  and  con- 
sequently occurs  in  many  situations  and  is  widespread.  It  is  an  ore 
of  excellent  quality,  but  is  little  used  in  this  country  because  of  the 
more  abundant  and  more  easily  mined  hematite.  It  is  common  in 
New  England  and  the  Appalachians. 

Siderite  (spathic  ore),  FeCO3.  —  The  color  is  gray  on  freshly 
broken  surfaces;  surfaces  exposed  to  the  weather,  even  for  a  few 
weeks,  are  brown.  The  streak  is  white,  or  nearly  so.  It  can  be 
easily  scratched  with  a  knife  (H  =  4).  It  occurs  commonly  in 
masses  which  show  shiny,  bent,  cleavage  surfaces.  Siderite  effer- 
vesces with  warm  hydrochloric  acid,  giving  off  carbon  dioxide. 
It  is  an  ore  of  iron  which,  however,  is  little  used  in  the  United 
States. 

Pyrite  or  Iron  Pyrites  (fool's  gold),  FeS2.  —  The  color  is  brass-yellow 
when  fresh,  but  oxidizes  on  the  outside  to  brown  limonite.  It  is 
harder  than  steel  (H  =  6.5).  It  occurs  in  veins  and  is  disseminated 
throughout  many  igneous  and  sedimentary  rocks.  It  often  occurs 
in  cubical  crystals  or  in  crystalline  masses.  The  yellow  stains  on 
rocks  are  often  due  to  the  weathering  of  grains  of  pyrite.  Pyrite 
is  a  common  mineral.  It  is  not  used  as  an  ore  of  iron,  but  is  used 
in  the  manufacture  of  sulphuric  acid. 


APPENDIX  687 

Pyrrhotite  (magnetic  pyrites),  FenSi2.  — The  color  is  bronze-yellow 
when  fresh,  but  weathers  readily  to  brown  on  the  outside.  It  is  darker 
than  pyrite.  Pyrrhotite  is  softer  than  pyrite  and  can  easily  be  scratched 
with  a  knife  (H  =  4).  Small  fragments  are  attracted  by  a  magnet. 
Pyrrhotite  is  of  little  use  in  itself  but,  since  it  often  bears  nickel,  it 
is  mined  for  that  metal.  The  most  valuable  deposits  occur  in  Can- 
ada, but  large  quantities  are  found  in  Vermont,  Pennsylvania,  and 
elsewhere. 

ZINC  MINERALS 

Sphalerite  (blend,  black  jack,  jack,  zinc  sulphide),  ZnS. — The 
color  varies  from  yellow  to  brown-black.  When  a  fragment  is  crushed, 
the  pieces  look  like  resin.  This  resinous  luster  can  usually  be  seen 
whenever  a  specimen  is  fractured.  The  streak  is  light  yellow.  Sphal- 
erite is  softer  than  steel  (H  =  3.5)  and  occurs  in  crystals  or  in  masses 
with  well-developed  cleavage  faces.  It  is  an  important  ore  of  zinc 
and  is  often  associated  with  lead  and  silver  ores.  It  is  extensively 
mined  in  Missouri  and  is  of  common  occurrence  in  smaller  quantities 
elsewhere. 

CALCIUM   MINERALS 

Calcite  (calc  spar),  CaCOs.  —  The  color,  when  pure,  is  white  or  color- 
less^ but  when  impurities  are  present  the  color  depends  upon  the 
foreign  substance ;  yellow,  green,  gray,  salmon,  lavender,  and  other 
colors  are  common.  Calcite  is  much  softer  than  glass  (H  =  3).  It  is 
readily  distinguished  from  other  minerals  by  its  strong  rhomboidal 
cleavage,  its  hardness,  and  its  effervescence  with  acids.  It  is  one  of  the 
most  widespread  and  abundant  minerals.  There  are  a  number  of 
varieties,  including  dogtooth  spar,  so-called  because  of  the  shape  of  the 
crystals;  marble,  a  crystalline  rock  composed  of  large  and  small 
grains  of  calcite;  Mexican  onyx,  an  agatelike  rock  formed  by  suc- 
cessive layers  of  lime  deposited  from  solution  in  a  cavity. 

Dolomite  (pearl  spar),  CaMg  (CO3)2-  —  The  color  is  usually  white  or 
with  a  yellow  tint.  It  is  softer  than  steel  (H  =  3.5).  Dolomite  is 
distinguished  from  calcite,  which  it  resembles,  by  its  curved  cleavage 
surfaces,  its  pearly  luster,  and  its  lack  of  effervescence  with  cold  hydro- 
chloric acid.  It  occurs  in  distinct  crystals  and  forms  thick  strata  of 
limestone.  It  is  a  common  vein  mineral. 

Gypsum,  CaSO4-2  H2O.  — This  mineral  is  colorless  or  white  unless 
tinted  by  impurities.  It  is  softer  than  calcite  and  can  be  scratched 

CLELAND  GEOL. — 44 


688  APPENDIX 

with  the  finger  nail  (H  =  2).  Gypsum  occurs  in  veins  or  beds;  in 
crystals,  or  compact,  rocklike  masses.  The  most  important  variety 
is  selenite,  a  crystalline  gypsum  with  a  perfect  cleavage,  thin  leaves 
of  which  may  be  split  off  and  resemble  those  of  mica.  They  differ 
from  the  latter  in  their  inelasticity  and  vertical  cleavage.  Alabaster 
is  a  compact,  fine-grained,  usually  translucent  gypsum,  used  in  making 
ornaments  and  statuary.  Satin  spar  is  a  fibrous  gypsum  which  has 
somewhat  the  appearance  of  satin.  It  is  occasionally  used  in  the 
manufacture  of  cheap  jewelry.  Rock  gypsum  is  compact  and  rock- 
like.  It  is  used  for  plaster  of  Paris.  Gypsum  is  common  in  many 
portions  of  North  America,  but  is  especially  abundant  in  New 
York,  Iowa,  Michigan,  and  Ohio. 

Fluorite  (fluorspar,  blue  John),  CaF2. — The  color  is  commonly 
blue  or  green,  but  is  occasionally  white  or  yellow.  It  is  slightly  harder 
than  calcite  and  can  be  scratched  with  a  knife  (H  =  4).  Its  principal 
use  is  as  a  flux  in  reducing  iron,  but  it  is  used  to  some  extent  for  orna- 
mental purposes.  It  occurs  in  clear,  cubical  crystals,  and  in  masses. 
Fluorite  is  mined  in  Illinois  and  Kentucky. 

Apatite  (asparagus  stone,  phosphate  rock,  calcium  phosphate).  — 
The  color  is  usually  green  or  reddish  brown.  It  is  harder  than  fluorite 
and  cannot  easily  be  scratched  with  a  knife  (H  =  5).  After  being 
treated  with  sulphuric  acid,  it  becomes  a  valuable  fertilizer.  It 
is  found  in  many  parts  of  North  America,  but  the  most  valuable  de- 
posits occur  in  Canada. 


COPPER  MINERALS 

Chalcopyrite  (copper  pyrites),  CuFeS2.  —  The  color  is  a  deeper  yellow 
than  pyrite.  Chalcopyrite  can  be  easily  scratched  with  a  knife  (H  = 
3.5)  and  this  character  alone  easily  distinguishes  it  from  pyrite,  but 
not  from  pyrrhotite.  The  bluish  tarnish  of  chalcopyrite  is  also  dis- 
tinctive. Since  it  is  not  attracted  by  a  magnet,  it  is  easily  distinguish- 
able from  pyrrhotite.  It  is  a  valuable  and  widespread  ore  of  copper 
and  is  mined  in  many  of  the  Western  States. 

Malachite  (green  copper  carbonate),  (CuOH)2CO3. — The  color 
is  bright  green.  The  color,  hardness,  which  is  less  than  that  of  steel 
(H  =  3.5),  and  its  effervescence  with  acids  readily  distinguish  it  from 
other  minerals.  Its  principal  use  in  the  United  States  is  as  an  ore  of 
copper,  although  in  Europe  the  compact  varieties  have  long  been  much 
sought  after  for  vases,  table  tops,  and  mosaics. 


APPENDIX  689 

LEAD   MINERALS 

Galenite  (galena),  PbS.  —  The  color  is  lead  gray.  Its  softness 
(H  =  2.5),  its  high  specific  gravity  which  is  greater  than  that  of  iron, 
and  its  strong  cubical  cleavage  make  it  one  of  the  most  easily  recog- 
nizable minerals.  It  occurs  in  masses  and  as  cubical  crystals.  Galena 
is  valuable  as  an  ore  of  lead,  as  well  as  for  the  silver  which  it  usually 
carries. 

SILICA   MINERALS 

Quartz  and  its  Varieties,  SiO2.  —  When  pure,  quartz  is  colorless  or 
white,  but  in  no  other  mineral  do  the  colors  vary  so  widely ;  red,  pink, 
yellow,  brown,  green,  blue,  lavender,  and  black,  in  fact  almost  every 
conceivable  color  is  found  in  quartz.  Quartz  is  harder  than  steel  and 
scratches  glass  (H  =  7).  It  is  the  commonest  of  minerals.  "  It 
makes  up  most  of  the  sand  of  the  seashore ;  it  occurs  as  a  rock  in  the 
forms  of  sandstone  and  quartzite,  and  is  a  prominent  part  of  many 
other  important  rocks,  as  granite  and  gneiss."  It  is  readily  dis- 
tinguished from  other  minerals  by  its  hardness  and  its  lack  of  cleavage. 
The  crystals  are  six-sided  (hexagonal).  The  principal  varieties  are: 
rock  crystal,  as  the  clear  quartz  crystals  are  called,  which  is  used 
for  making  "  pebble  lenses,"  "  Japanese  balls,"  and  other  objects ; 
amethyst,  purple  crystalline  quartz  which  is  cut  for  gem  stones ;  rose 
quartz,  which  is  light  pink  or  rose  color;  milky,  smoky,  and  yellow 
quartz,  named  because  of  their  color.  Chalcedony  is  a  translucent 
variety  with  a  waxy  luster  which  varies  greatly  in  color.  Agate  is  a 
banded  chalcedony  in  which  the  bands  are  variously  colored.  Flint 
(p.  524)  and  chert  are  gray  to  black  translucent  or  opaque  quartz 
masses  which  occur  in  chalk  and  limestone.  Jasper  is  similar  to  flint 
in  appearance,  but  is  usually  red,  black,  white,  or  yellow. 

SILICATE   MINERALS 

Orthoclase  Feldspar  (potash  feldspar),  KAlSiaOg. — The  color   is 

usually  white,  gray,  or  flesh.  The  hardness  is  about  that  of  steel  (H  = 
6).  The  mineral  cleaves  readily,  the  cleavage  planes  being  at  right 
angles  to  each  other.  Orthoclase  feldspar  is  an  important  constitu- 
ent of  granite  and  sometimes  occurs  in  large  crystals.  Pure  feldspar 
is  used  to  make  the  glaze  on  porcelain. 

Labradorite  Feldspar  (lime  feldspar).  — The  color  is  dark  gray,  often 
with  blue,  green,  and  red  iridescence.  It  is  slightly  harder  than  steel 


690  APPENDIX 

(H  =  6).  The  cleavage  planes  are  often  striated  and  are  not  at  right 
angles  to  each  other  as  in  orthoclase.  It  is  an  important  constituent 
of  some  igneous  rocks.  Labradorite  is  used  to  a  limited  extent  for 
ornamental  purposes. 

Muscovite  Mica  (isinglass,  white  mica),  H2KAl3(SiO4)3.  —  It  is 
usually  transparent  or  gray.  It  can  be  scratched  with  the  finger  nail 
(H  =  2).  The  most  distinctive  characters  of  muscovite  are  its  abil- 
ity to  be  cleaved  into  thin  leaves,  its  hardness,  the  elasticity  of  its  leaves, 
and  its  color.  It  is  used  in  stove  doors,  for  insulation  in  electrical 
apparatus,  and,  when  ground,  as  a  lubricant. 

Biotite  Mica  (black  mica),  a  complex  silicate.  —  With  the  exception 
of  the  color  and  chemical  composition,  biotite  has  the  same  characters 
as  muscovite. 

Chlorite,  a  complex  silicate.  The  color  is  usually  dark  green.  It  is 
so  soft  that  it  can  be  easily  scratched  with  the  finger  nail  (H  =  1-2). 
It  occurs  in  dark  green  masses  in  which  the  flakes  are  usually  so  small 
as  to  be  distinguished  with  difficulty.  Chlorite  occurs  commonly  in 
metamorphic  rocks. 

Talc,  a  hydro-magnesian  silicate.  —  The  color  is  white,  greenish, 
or  gray.  It  is  readily  distinguished  by  its  soapy  feel  (H  =  i),  in  which 
it  differs  from  gypsum.  Talc  commonly  occurs  in  plates  or  leaves  like 
mica.  It  occasionally  occurs  in  beds  15  or  more  feet  in  thickness.  It 
is  ground  to  make  "  talcum  powder  "  and  has  many  other  uses,  such 
as  a  filler  for  paper,  a  lubricant,  and  an  adulterant.  Large  deposits 
of  talc  occur  in  New  York,  Massachusetts,  North  Carolina,  and  other 
states. 

Serpentine,  a  hydro-magnesian  silicate.  —  The  color  is  usually 
green  or  yellow,  and  the  hardness  is  less  than  that  of  steel  (H  =  usually 
about  3).  There  are  two  principal  varieties,  massive  serpentine,  a 
compact  mineral  with  a  greasy  or  waxy  luster,  and  asbestos  or  chrysotile, 
a  fibrous  variety.  The  massive  serpentine  is  polished  for  table  tops 
and  other  ornamental  purposes ;  the  asbestos  is  used  in  the  manufac- 
ture of  fire-proof  articles,  such  as  theater  curtains,  coverings  of  steam 
pipes  and  boilers,  and  for  firemen's  suits.  The  province  of  Quebec 
is  the  great  center  for  asbestos. 

Hornblende,  a  silicate  of  several  elements.  — The  color  is  commonly 
black,  and  the  hardness  about  that  of  steel  (H  =  5.6).  The  most  dis- 
tinctive character  of  hornblende  is  its  occurrence,  usually,  in  slender, 
flat  crystals,  the  larger  angles  of  the  crystals  being  about  124  degrees. 
A  fibrous  variety  known  as  hornblende  asbestos  has  much  the  same  ap- 


APPENDIX  691 

pearance,  and  is  used  for  the  same  purpose,  as  serpentine  asbestos. 
Hornblende  is  a  constituent  of  some  igneous  rocks. 

Augite,  a  silicate  of  several  elements.  —  The  color  is  black  or 
dark  green  and  the  hardness  about  that  of  steel  (H  =  5-6).  It  usually 
occurs  in  shorty  thick  crystals.  It  is  a  rock-making  mineral  of  wide 
distribution  and  is  an  important  constituent  of  "  trap." 

Olivine  (chrysolite,  peridot),  an  iron  magnesium  silicate.  —  The 
color  is  usually  yellowish  green,  and  the  hardness  that  of  quartz 
(H  =  6.5-7).  It  is  an  important  constituent  of  some  igneous  rocks. 
Large,  clear  crystals  are  cut  for  gem  stones. 

Garnet,  variable  silicates  of  various  bases.  —  The  color  is  com- 
monly red  or  black,  but  brown  and  green  garnets  also  occur.  The 
hardness  is  that  of  quartz  (H  =  7).  Garnets  usually  occur  in  crystals 
with  12  similar  faces  (dodecahedrons  or  trapezohedrons)  and  are  found 
embedded  in  metamorphic  rocks  of  various  kinds.  Garnets  are  crushed 
and  manufactured  into  sandpaper,  and  fine,  clear  specimens  of  good 
color  are  cut  for  gem  stones. 


INDEX 


INDEX 


Aa  lava,  300. 

Aar  glacier,  movement  of,  150. 

pond  on,  148. 

rock  flour  from,  159. 
Ablation,  147. 
Abrasion,  glacial,  157-158. 
Abyssal  injection  hypothesis  of  volcanism, 

336. 

Acadian  epoch,  402. 
Accumulation,  mountains  of,  352. 
Acervularia  davidsoni,  456. 
Acid  rocks,  329. 
Acrothele  subsidua,  414. 
Actinocrinus  multiradiatus,  481. 
Actinopteri,  465,  485. 
Adams,  F.  D.,  59,  258,  388. 
Adeloblatta  columbiana,  484. 
Aftonian  stage,  649,  665. 
Aganides  rotatorius,  483. 
Agassiz,  L.,  171. 
Agates,  78. 
Agnostus,  412. 
Agnostus  interstrictus,  412. 
Agriochaerus,  620. 
Alachua  Lake,  Florida,  69. 
Alderney     breakwater,     England,     storm 

wave  at,  201. 
Algae,  agents  of  deposition  from  solution, 

66. 

Algonkian,  see  Proterozoic. 
Alkaline  lakes,  136. 
Allorisma  terminale,  483. 
Allosaurus,  541. 
Alluvial  cones,  124-127. 

fans,  124-127. 

plains,  126,  127. 

terraces,  128. 
Altitude,  effect  on  disintegration  of  rocks, 

32- 

Amazon  River,  bores  in,  202. 
Amblypoda,  592-594- 


Ammonites,  Carboniferous,  481. 

Mesozoic,  528,  530. 
Amphibians,  Carboniferous,  485-489. 

Mesozoic,  536. 

origin  of,  488. 

rise  of,  489. 

Tertiary,  624-625. 
Amphiuma,  488. 
Amygdalocystites  florealis,  430. 
Amygdaloidal  rocks,  331. 
Anchisaurus,  540. 
Anchura  americana,  528. 
Angiosperms,  effect  of  introduction  on  life 
568. 

Mesozoic,  568. 

Animals,  mechanical  action  of,  33. 
Antarctic  glaciation,  171. 
Antecedent  streams,  101,  102. 
Anticlines,  254,  363. 
Anticlinorium,  256. 
Apatite,  688. 

Apiocrinus  parkinsoni,  525. 
Appalachia,  in  Cambrian,  406. 

Devonian,  455. 

Ordovician,  420. 

Permian,  478. 

Silurian,  440. 

Appalachian  coal  field,  474. 
Appalachian  deformation,  477-478,  505. 

peneplain,  116. 
Appalachian  trough,  Devonian,  453. 

Mississippian,  469. 

Ordovician,  420. 

Permian,  478. 

Silurian,  439. 

Ararat,  Little,  fulgurites  on,  34. 
Archaean,  see  Archaeozoic. 
Archaeocyathus  rensselaericus,  416. 
Archaeopteryx,  560-561. 
Archaeozoic  era,  389-392. 

batholiths  of,  390. 


695 


696 


INDEX 


Archaeozoic  era  —  (Continued) 

characteristics  of  rocks  of,  390. 

conditions  during,  391-392. 

contrasted  with  Proterozoic,  393. 

distribution  of  rocks  of,  389. 

duration  of,  392. 

foreign,  389. 

limestone  of,  391. 

metamorphism  of,  391. 

thickness  of  rocks  of,  391. 
Archelon,  557. 
Archemedes  wortheni,  481. 
Arcoptera  aviculaeformis,  629. 
Arctinurus     (Lichas)     bigsbyi    (boltoni), 

448. 

Arikaree,  Miocene,  Nebraska,  76. 
Arnold,  R.,  77,  581. 
Arrhenius,  274. 
Artesian  wells,  59-60. 
Arthrolycosa  antiqua,  484. 
Arthropods,  Carboniferous,  481-484. 

Silurian,  448-450. 
Artiodactyls,  615-619. 

divergence    from    perissidactyls,     599- 

600. 

Arve  River,  confluence  with  Rhone,  162. 
Asama  volcano,  Japan,  298. 
Asimina,  664. 

Asphalt,  deposits  in,  666,  667. 
Aspidorhynchus,  534. 
Astraeospongia  meniscus,  445. 
Astronomic  geology,  definition  of,  21. 
Athyris  lamellosa,  482. 
Atikokania,  397. 

Atlantic  City,  marine  erosion  and  deposi- 
tion at,  224. 
Atlantic  coast  of  North  America,  stability 

of,  230. 
Atmosphere,  work  of  the   weather,    27- 

43- 

work  of  the  wind,  44-53. 
Atolls,  243-245. 
Atrypa  reticularis,  458. 
Aucella  pioche,  527. 
Augite,  691. 
Auriferous  gravels,  582. 
Aviculopecten  occidentalis,  483. 


Bacteria  and  disintegration  of  rocks,  37. 

Baculites,  529,  530. 

Bad  lands,  588. 

Balanced  bowlders,  155,  156. 


Bandai-san,  307. 

Barnacles,  460. 

Barrell,  J.,  216,  264,  454,  455,  469,  479. 

Barrier  beaches,  221-223. 

Barrier  reefs,  243. 

Bars  and  spits,  220. 

Basal  conglomerate,  240. 

Basal  unconformity,  405. 

Basalts,  331. 

Base  level  of  erosion,  86. 

Basic  rocks,  329. 

Basins,  hydrographical  or  drainage,  103. 

Batholiths,  327,  328. 

Archaeozoic,  390. 
Bathyurus  longispinus,  435. 
Batocrinus  (Dizygocrinus)  rotundus,  481. 
Bayhead  beaches,  219. 
Bay  of  Naples,  sand  of,  236. 
Beach  deposits,  235-237. 
Beaches,  218,  220. 

raised,  214. 
Bedding  faults,  265. 
Bedding  planes,  24,  234.. 

effect  of  weathering  on,  28,  29. 
Beekmantown  stage,  422. 
Beheaded  stream,  107. 
Belemnites,  531,  554. 
Bellerophon  percarinatus,  482. 

sublaevis,  482. 
Bell  Sound,  glacier  on,  153. 
Beltina  danai,  397,  408. 
Belts  of  weathering  and  cementation,  6l. 
Bennettitales,  565,  566,  567. 
Bergschrund,  145. 
Bermuda  Islands,  waves  of,  229. 
Bern,  glaciers  of,  150. 
Billingsella  coloradrensis,  414. 
Biotite  mica,  690. 
Birds,  Mesozoic,  560-563. 

Pleistocene,  673. 

Tertiary,  623-624. 
Birmingham,  Alabama,  443. 
Bison,  673. 
Bittern,  136. 
Bituminous  coal,  501. 
Black  Hills,  355-356. 

artesian  water  from,  59. 

Proterozoic  of,  395-396. 
Black  River  stage,  422. 
Blastoids,  Carboniferous,  480,  481. 

Devonian,  457. 

Ordovician,  431. 

Silurian,  446. 


INDEX 


697 


Block  mountains,  354. 

Blowholes,  209,  210. 

Blow-outs,  44. 

Blue  grass  region  of   Kentucky,   soil 

41. 

Blue  Grotto,  229. 
Blue  mud,  241. 
Bonnersheim,  155. 
Bores,  tidal,  202. 
Bos,  663. 
Bosses,  328. 

Bossons  Glacier,  movement  of,  151. 
Boston  harbor,  drumlins  of,  177. 
Bosworth,  T.  O.,  425. 
Bothriolepis,  462,  464. 
Bottom-set  beds,  131. 
Bovidae,  619. 

Boulder,  Montana,  gold  of,  371. 
Bowlder  clay,  160,  172,  173. 

trains,  172. 

Bowlders,  balanced,  155,  156. 
Bowman,  I.,  43. 
Brachiopods,  Cambrian,  414-415. 

Carboniferous,  481,  482. 

Devonian,  457-458. 

Mesozoic,  526. 

Ordovician,  431-432. 

Proterozoic,  397. 

Silurian,  446-447. 

Tertiary,  626. 

Brachiospongia  digitata,  427. 
Brachyurans,  532. 
Braided  stream,  87,  178. 
Branchiosaurus,  487,  488. 
Brandon,  fossil  plants  of,  633. 
Breakers,  200. 
Breccia,  249. 

volcanic,  332. 

Brenva  Glacier,  retreat  of,  153. 
Bridger  formation,  624. 
Brittle  stars,  Carboniferous,  481. 

Ordovician,  431. 
Brontosaurus,  542-543. 
Brontotherium,  604. 
Broom,  R.,  537,  560. 
Bryograptus,  428. 
Bryozoans,  Carboniferous,  480,  481. 

Devonian,  457,  458. 

Ordovician,  432,  433. 

Silurian,  447. 

Buchiola  retrostriata,  459. 
Bumastus  (Ilhenus)  ioxus,  448. 

trentonensis,  435. 


of, 


Buttes,  106,  328. 
Byssonychia  radiata,  433. 


Calabria,  earthquake  of,  285. 

Calamites,  496-497. 

Calcite,  687. 

Calcium  minerals,  687-688. 

Calderas,  307,  308,  309. 

California  earthquake,  275-276. 

Callopora  elegantula,  447. 

pulchella,  432. 
Calymene  callicephala,  435. 
Camarotcechia  endlichi,  458. 
Cambrian  period,  climate  of,  417. 

close  of,  407. 

duration  of,  417. 

evolution  during,  416. 

life  of,  408-417. 

of  other  continents,  408. 

physical  geography  of,  405. 

plants  of,  409-410. 

subdivisions  of,  402. 

submergence  during,  406. 

trilobites  of,  402,  410-413. 

volcanic  activity  of,  407. 
Cambrian  rocks,  character  of,  406. 

location  of,  402. 

present  condition  of  sediments,  407. 

thickness  of,  406. 
Camels,  Pleistocene,  672. 

Tertiary,  615-617. 
Camptonectes  bellistriatus,  527. 
Camptosaurus,  544. 
Canadian  epoch,  422. 
Cannel  coal,  495,  501. 
Canoe  valleys,  363. 
Canyons,  95-96. 

Cape  Charles,  marine  erosion  at,  213. 
Cape  Cod,  marine  erosion  at,  233. 
Cape  de  la  Heve,  erosion  at,  205. 
Carbonation,  37. 
Carboniferous  periods,  469-507. 

climate  of,  503. 

life  of,  480-498. 

Lower,  see  Mississippian. 

plants  of,  491-498. 

Upper,  see  Pennsylvanian. 
Carcharodon  megalodon,  625,  626. 
Cardioceras  cordatum,  529. 
Carnivores,  Pleistocene,  672. 

Tertiary,  622. 
Caryocrinus  ornatus,  446. 


698 


INDEX 


Cassidulus  subconicus,  526. 
Castoroides,  665,  673. 
Casts,  379. 

Catskill  stage,  452,  454. 
Cave  deposits,  71. 
Caverns,  70-71. 

Pleistocene  deposits  of,  665-666. 
Caves,  sea,  209. 
Cementation,  belt  of,  61. 

of  sediments,  248. 
Cenozoic  era,  572-683. 

life    compared     with    Mesozoic,    572- 

573- 

Cephalaspis,  461,  462,  464. 
Cephalopods,  Carboniferous,  481,  483. 

Devonian,  459-460. 

Mesozoic,  528,  531. 

Ordovician,  434-435. 

Silurian,  448. 

Tertiary,  627-628. 
Ceratites,  528. 
Ceratites  nodosus,  529. 
Ceratocephala  dufrenoyi,  448. 
Ceratodus,  465,  535. 
Ceratosaurus,  541. 
Ceraurus  pleurexanthemus,  435. 
Cercis,  664. 
Cestracion,  463,  533. 
Chalcopyrite,  688. 
Chalk,  249. 

marine  erosion  of,  205. 

Mesozoic,  523-524. 
Chamberlin    and     Salisbury,     336,     365, 

662. 

Chamonix,  glaciers  of,  144. 
Champlain  subsidence,  655-656. 
Champsosaurus,  624. 
Changes  of  level,  228-231. 
Charleston  earthquake,  286,  291. 
Chazy  stage,  422,  427. 
Chemical  deposits  in  lakes,  134. 
Chemung  stage,  452,  454. 
Chert,  689. 

Child  Glacier,  movement  of,  150,  151. 
Chimborazo,  volcano,  310. 
Chlorite,  690. 
Chonetes  coronatus,  458. 
Chronology,  geological,  380. 

shown  by  fossils,  380. 
Chrysolite,  691. 
Cidaris  coronata,  526. 
Cincinnati  anticline,  423. 
Cincinnatian  epoch,  422. 


Cinder  cones,  312. 
Cirques,  143-146. 
development  of,  146. 
origin  of,  145. 
Cladoselache,  462,  463. 
Clarke,  F.  W.,  198,  425. 
Clarke,  J.  M.,  457,  669. 
Clastic  deposits,  239. 
Clay  stones,  76,  77. 
Cleavage,  344~34S- 

relation  to  pressure,  349. 
Cleveland,  glacial  drift  near,  174. 
Climacograptus  bicornis,  428. 
Climate,  and  erosion  of  canyons,  95. 
and  stream  erosion,  128. 
of  Cambrian,  417. 

Carboniferous,  503. 

Cretaceous,  570. 

Devonian,  468. 

Eocene,  634-635. 

Jurassic,  570. 

Miocene,  635. 

Oligocene,  635. 

Ordovician,  437. 

Paleozoic,  507. 

Pliocene,  635-636. 

Proterozoic,  398. 

Silurian,  451. 

Tertiary,  634-636. 

Triassic,  569-570. 
Clinton  iron  ore,  375,  442. 
Clinton  shale,  439. 
Cleverly  stage,  515. 
Coal    beds,    climate    during    deposition, 

.  .S°3> 
conditions  necessary  for  formation  of, 

499- 

Cretaceous,  571. 

Eocene,  575. 

modes  of  occurrence,  499. 

origin  of,  499. 

Pennsylvanian,  501-503. 

Triassic,  571. 
Coal,  cannel,  495. 

varieties  of,  501. 

Coal    fields,     extent    and    structure     of, 
502. 

location    of,    in    North    America,  473- 

474- 

Coal  measures,  502. 
Coal      plants,      conditions      of     growth, 

497- 
Coastal  Plain,  91,  224. 


INDEX 


699 


Coast  line,  mature,  232. 

old  age  of,  232. 

proofs  of  depression,  228-230. 

proofs  of  elevation,  228-230. 

rough,  226. 

smooth,  224. 

submerged,  226. 

youthful,  231. 
Coasts  in  non-glaciated  regions,  indented, 

228. 

Cobbleskill  stage,  439. 
Coccosteus,  464. 
Cockerell,  T.  D.  A.,  634. 
Coelenterata,  Cambrian,  415-416. 

Carboniferous,  480. 

Devonian,  456-457. 

Ordovician,  427-430. 

Silurian,  444-445. 
Colima,  volcano,  312,  314. 
Colorado  Plateau,  faults  in,  262,  263. 

ground  water  of,  57. 
Colorado  stage,  517. 
Columnar  structure,  333-334. 
Columnana  halli,  429. 
Comanchean,  see  Cretaceous,  Lower. 
Competent  strata,  257. 
Compsognathus,  540. 
Concentration  of  ores,  373. 
Concretions,  75-77. 
Condylarthra,  598,  622. 
Conformity,  270. 
Conglomerates,  basal,  240. 

lens  shape  of  deposits,  239. 
Conifers,  Carboniferous,  497,  498. 

Mesozoic,  567. 
Conocardium  ohioense,  459. 
Consequent  stream,  101-102. 
Constant  springs,  64. 
Constellaria  florida,  432. 
Contact  metamorphism,  341-343. 
Continental  glaciers,  168-171. 
Continental  shelf,  195,  197. 
Continents,  permanence  of,  368. 
Contraction,  due  to  cooling,  364. 
Copper  minerals,  688. 
Copper  of  Keweenawan  system,  396. 
Coquina  limestone,  248. 
Coral  islands,  243. 

formation  and  growth  of,  243-244. 

glacial-control  theory  of,  246-247. 

submarine  bank  theory  of,  246. 

subsidence  theory  of  Darwin  of,   245, 
247. 


Coral  reefs,  Devonian,  456. 
Corals,  Cambrian,  415-416. 

Devonian,  456-457^^ 

Mesozoic,  524. 

Ordovician,  430. 

Silurian,  444-445. 

Tertiary,  626. 
Cordaites,  496. 

Cordilleran  ice  sheet,  646-647. 
Corrasion,  83. 

Corrasion  and  weathering,  85. 
Correlation  of  strata,  382. 
Corrosion,  83. 
Corynotrypa  inflata,  432. 
Cosmic  geology,  definition  of,  21. 
Cotopaxi,  volcano,  298,  322. 
Cotylosaurs,  489. 
Coves  and  headlands,  208. 
Crater  Lake,  309,  310,  314. 
Craterlets,  291. 
Craters,  294,  3 14. 
Creep,  hillside,  41. 
Creep  of  soils,  31. 
Creodonta,  Mesozoic,  594-595. 

Tertiary,  622. 

Cretaceous,  Lower,  of  Atlantic  and  Gulf 
coasts,  514-515. 

of  other  continents,  515. 

of  Pacific  coast,  515. 

of  western  interior,  515. 

physical  geography  of,  514-516. 
Cretaceous  periods,  508. 

climate  of,  570. 

peneplain  of,  518-519. 
Cretaceous,  Upper,  of  Atlantic  and  Gulf 
coasts,  516. 

of  other  continents,   518. 

of  Pacific  coast,  517. 

of  western  interior,  517-518. 

of  physical  geography  of,  516-520. 

subsidence  during,  51.6. 
Crevasses  in  glaciers,  147,  151. 
Crinoids,  Carboniferous,  480,  481. 

Devonian,  457. 

Mesozoic,  524-526. 

Ordovician,  430-431. 

Silurian,  445-446. 

Tertiary,  626. 
Crioceras,  528,  530. 
Crocodiles,  Mesozoic,  552. 

Tertiary,  624. 

CrolPs  hypothesis,  660-661. 
Cross-bedding,  234,  235,  238. 


yoo 


INDEX 


Cross-bedding,  —  (Continued) 

of  dunes,  47,  48. 
Crossopterygians,  465,  488. 
Croxian  stage,  402. 
Crush  breccia,  268. 
Crustacea,  Cambrian,  410-413. 

Devonian,  460. 

Mesozoic,  532. 

Ordovician,  435-436. 

Proterozoic,  397. 
Crustal  movements,  112,  257. 
Crust  of  earth,  273-274. 
Crystallization,  349. 
Ctenodonta  nasuta,  433. 
Cuestas,  225. 
Cyathophyllum,  445. 
Cycadales,  565. 
Cycads,  565-566. 
Cycle  of  shore  erosion,  231-233. 

of  stream  erosion,  109-114. 
Cynognathus,  537. 
Cyrtodonta  billingsi,  433. 
Cyrtolites  ornatus,  434. 
Cystodictya  hamiltonensis,  458. 
Cystoids,  Cambrian,  415. 

Devonian,  457. 

Ordovician,  430. 

Silurian,  446. 


Daemonhelix,  631-632. 

Dakota  formation,  517. 

Dalmanella  testudinaria,  432. 

Dalmanites  limulurus,  448. 

Daly,  R.  A.,  246,  247,  309,  336,  392. 

Dana,  J.  D.,  309,  456,  488. 

Dapedius,  534. 

Darwin,  C.  R.,  245. 

Davis,  W.  M.,  86,  246. 

Dawson,  W.,  632. 

Dawsonoceras  americanum,  448. 

Dead  Sea,  263. 

Dean,  B.,  533. 

Deep-sea  deposits,  241-243. 

Deer,  618-619. 

Deforestation,    effect     on     streams, 

98. 

Deformation,  365. 
Degradation,  87. 
Deltas,  depth  of,  132. 

growth  of,  130. 

in  glacial  lakes,  180. 

structure  of,  131. 


96- 


De  Martonne,  E.,  53. 
Dendritic  river  systems,  103. 
Density  of  the  earth,  273-274. 
Denudation  of  continents,  rate  of,  118. 
Deposition  along  shores,  233. 

by  ground  water,  60. 

by  streams,  causes  of,  119. 

in  lakes,  250. 

Depression,  effect  on  streams,  in. 
Derbya  crassa,  482. 
Dermochelys,  557. 
Desert  limestone,  62. 
Deserts,  wind  erosion  in,  46. 
Devonian  period,  452-468. 

climate  of,  468. 

duration  of,  468. 

in  other  continents,  455-456. 

life  of,  456-468. 

migration    and   evolution   during,  467- 
468. 

of  New  York  state,  452,  454. 

oil  and  gas  of,  455. 

physical  geography  of,  453-455. 

plants  of,  467. 

thickness  of  formations  of,  453. 

volcanic  activity  of,  455. 
Dew  and  hoarfrost  as  weathering  agents, 

33- 

Diatomaceous  earth,  134,  580. 
Diatoms,  134,  581,  633. 
Dibelodon,  614. 
Dicellocephalus,  402. 
Dicellocephalus  minnesotensis,  412. 
Diceras,  527. 
Diceras  arietinum,  527. 
Diceratherium,  606,  607. 
Dichograptus,  428,  429. 
Dichograptus  octobrachiatus,  428. 
Dicroceros,  618. 
Dictyonema  flabelliforme,  428. 
Dicynodon,  537. 
Didymograptus,  428,  429. 
Didymograptus  nitidus,  428. 
Dielasma  bovidens,  482. 
Differential  weathering,  40. 
Dikes,  324-325,  327- 
Dimetrodon,  490. 
Dimorphodon,  558-559. 
Dinichthys,  464. 
Dinosaurs,  539-552,  572. 

migration  and  extinction  of,  549-551. 
Dinotherium,  615. 
Diorite,  330. 


INDEX 


701 


Dip  and  strike,  252. 
Dipleura  dekayi,  460. 
Diplograptus  pristis,  428. 
Diplopodia  texanum,  526. 
Diptera,  533. 

Dirt  cones  on  glaciers,  150. 
Displacement  of  faults,  262,  281,  282. 
Distributaries,  on  alluvial  cones,  126. 

on  deltas,  130. 
Divides,  103. 
Divisional  planes,  25. 
Dole  and  Stabler,  83. 
Dolichopterus  macrocheirus,  449. 
Dolines,  73. 
Dolomite,  249,  687. 
Domed  mountains,  355-356. 
Don  stage,  664. 

Dovetailing  of  sediments,  239-240. 
Downthrow  of  fault,  262. 
Dreikanter,  45. 
Driftless  area,  174. 
Drift   sheets,    characteristics   of  ancient, 

650-651. 

Dromatherium,  563-564. 
Drowned  rivers,  114. 
Drumlins,  177. 
Dryopithecus,  622. 
Dima  River,  rapid  erosion  by,  96. 
Dunbar,  storm  waves  at,  201. 
Dunes,  46-52. 

beneficial  effects  of,  51. 

distribution  of,  46. 

height  of,  52. 

material  of,  51. 

migration  of,  49-51. 

origin  of,  47,  48,  49. 

shape  of,  47,  48,  49. 

structure  of,  47. 
Dunwich,    England,    marine    erosion    at, 

213. 
Dust,  origin  of,  54. 

wind-blown,  52—54. 
Dust  wells  on  glaciers,  150. 
Dynamical  geology,  definition  of,  21. 


Earth,  future  habitability  of,  683-684. 

interior  of,  272-274. 

origin  of,  385-387. 

specific  gravity  of,  273-275,  387. 
Earthquake,  craterlets,  291. 

gases,  291. 

rift  of  California,  275-278,  286,  289. 


Earthquake  —  (Continued) 

topography,  289. 

vibrations,  amplitude  of,  284. 

waves,  202,  283. 
Earthquakes,  areas  affected  by,  286. 

causes  of,  281. 

construction    of    buildings    in    regions 
affected  by,  291-292. 

destruction  by,  275,  290. 

distribution  of,  278-280. 

duration  of,  286. 

focus  of,  282. 

frequency  of,  286. 

geological  effects  of,  287-291. 

great  sea  waves  caused  by,  292. 

sounds  accompanying,  290. 

vorticose  movements  of,  285. 
Eatonia  medialis,  458. 
Echinocaris  punctata,  460. 
Echinodermata,  Cambrian,  415. 

Carboniferous,  480,  481. 

Devonian,  457. 

Ordovician,  430-431. 

Silurian,  445-446. 
Echinoids,  Carboniferous,  480. 

Mesozoic,  525-526. 

Tertiary,  626,  627. 
Economic  geology,  definition  of,  22. 
Ecphora  quadricostata,  628. 
Ectenocrinus  grandis,  431. 
Eddystone  lighthouse,  England,  203. 
Edentates,  Mesozoic,  621-622. 

Pleistocene,  670-671. 
Elasmosaurus,  554. 
Elephants,  612-615. 
Elephas  antiquus,  676. 

primigemus,  668. 
Elevation,  effect  on  streams,  in. 
Endoceras,  435. 
Endothiodon,  537. 
Englacial  debris,  156. 
Enostracophori,  464. 
Entelodon,  619. 
Eobasileus,  593. 
Eocene  epoch,  574. 

climate  of,  634-635. 

close  of,  576. 

coal  in,  575. 

migration  during,  639. 

of  Atlantic  and  Gulf  coasts,  574. 

of  other  continents,  576-577. 

of  Pacific  coast,  574-575. 

of  western  interior,  575-576. 


702 


INDEX 


Eocene  epoch  —  (Continued) 

physical  geography  of,  574-577. 
Eocetus,  596. 

Eocystites  longidactylus,  415. 
Eohippus,  608-609. 
Eolian  sandstone,  52. 
Eolithic  man,  675-676. 
Eoliths,  622,  675. 
Eosiren,  597. 
Eotitanops,  604. 
Epicontinental  seas,  405. 
Equisetales,  495. 
Equus,  610. 
Equus  giganteus,  672. 

tau,  672. 

Erosion,    by    continental    glaciers,     182- 
184. 

by  glaciers,  157-159. 

by  shore  ice,  203-204. 

by  streams,  83-89. 

by  waves,  202. 

of  volcanic  cones,  314-316. 

rate  of,  by  streams,  88. 
Erratics,  156. 
Eryon  propinquus,  532. 
Eryops,  486,  487. 
Escharopora  subrecta,  432. 
Eskers,  180. 
Eucalyptocrinus  crassus,  446. 

elrodi,  446. 

Eumetria  marcyi,  482. 
Eumicrotis  curta,  527. 
Euparkeia,  560. 
Eurypterids,  436,  449,  460. 
Eutheria,  590,  591. 

Evolutional  geology,  definition  of,  22. 
Exfoliation,  32,  328. 
Exogyra,  526. 
Expansion    and     contraction    theory    of 

glacial  movement,  190. 
Extrusion  of  lava,  334-339. 


Fall  Line,  92,  224. 
Falls,  89-95. 

from  hanging  valleys,  163. 

not  the  result  of  erosion,  93. 

of  the  Ohio,  corals  at,  456. 
Falls  Creek,  travertine  dams  of,  65. 
Fan  folds,  255. 
Fault,  breccia,  268. 

line,  269. 

rift,  277,  278,  281,  282,  289,  290. 


Fault  —  (Continued) 

scarps,  265,  266. 

surface,  262. 
Faulted  mountains,  354. 
Faulting    and    the  direction   of  streams, 

102. 

Faults,  261-269. 

definition  of,  25. 

detection  of,  268. 

influence  on  topography,  265-268. 

origin  of,  269. 
Faults  and  fissures,  caused  by  earthquakes, 

287,  288. 

Faults  and  folds,  359. 
Favpsites,  444,  456. 
Felsites,  331. 
Felsitic  rocks,  329. 
Fens   of  Lincolnshire,   marine  deposition 

on,  224. 
Ferns,  Carboniferous,  492. 

Mesozoic,  566. 
Fingal's  Cave,  StafFa,  209. 
Fiords,  166,  228. 
Fire  clay,  499. 
Fisher,  E.  H.,  129. 
Fishes,  Carboniferous,  485. 

comparison  of  Devonian  and   modern, 
466. 

Devonian,  461-467. 

Mesozoic,  533-536. 

Ordovician,  436. 

Silurian,  450. 

Tertiary,  625-626. 
Fissure  eruptions,  310-311. 
Fissures,  caused  by  earthquakes,  287. 
Fistulipora  micropora,  458. 
Flint  nodules,  77,  524. 
Flood  plains,  119-121,  128. 
Florissant  beds,  580,  629,  634. 
Flow  of  rocks,  258. 
Fluorite,  688. 
Flying  reptiles,  558-560. 
Folded  mountains,  356-358. 

origin  of,  358-362. 

topographic  features  of,  362-363. 
Folded  regions,  coasts  of,  227. 
Folds,  254-257. 

and  faults,  359. 

origin  of,  257. 
Foliation,  345. 
Foot  wall,  262. 
Foraminifera,  Carboniferous,  482. 

Mesozoic,  524. 


INDEX 


703 


Foraminifera  —  (Continued) 

Tertiary,  626. 
Forbes,  J.  D.,  155,  189. 
Fordilla,  433. 
Fore-set  beds,  131. 
Fort  Union  stage,  519,  571,  576,  635. 
"Fossil"  ore,  442. 
Fossils,  formation  of,  377. 

indicators  of  chronology,  381. 
of  physical  conditions,  382. 

preservation  of,  378. 
Fragmental  volcanic  rocks,  296-298,  332- 

334- 

Frankfort  stage,  422. 
Frost,  effect  on  disintegration  of  rocks,  27, 

28. 

Fulgurites,  34. 
Fumaroles,  295. 
Fuson  formation,  515. 
Fusulina,  480,  482. 
Fusulina  secalica,  482. 


Gabbro,  330. 
Gailenreuth  Cavern,  664. 
Galenite,  689. 
Galleries  of  caves,  71. 
Galoon-goon,  322. 
Ganges  delta,  131. 
Gangue,  370. 
Ganodonta,  621. 
Ganoids,  Devonian,  465. 

Mesozoic,  535. 
Garnet,  691. 
Garwood,  E.  J.,  410. 
Gas,  see  Oil  and  Gas. 
Gaseous    interior    of    earth,    theory   of, 

274. 

Gastornis,  623. 
Gastropods,  Cambrian,  413. 

Carboniferous,  481,  482. 

Devonian,  457,  459. 

Mesozoic,  528. 

Ordovician,  433-434. 

Silurian,  447-448. 

Tertiary,  626-^27. 
Geikie,  A.,  34,  44,  83,  170. 
Geikie,  J.,  77. 
Genesee  stage,  452. 
Geodes,  78. 
Geological  chronology,  381. 

time,  divisions  of,  383,  384. 
length  of,  22,  377. 

CLELAND  GEOL. 45 


Geology,  definition  of,  21. 

divisions  of,  21-22. 

present  status  of,  22. 
Geosyncline,  359. 

of  Sierra  Nevada  Mountains,  513. 
Geyserite,  67. 
Geysers,  67-68,  323. 
Gibraltar,  a  tied  island,  223. 
Gidley,J.  W.,  611. 
Gilbert,  G.  K.,  276. 
Ginkgo,  567. 
Glacial  abrasion,  157. 

bowlders,  155,  171. 

debris,  amount  and  varied  character  of, 
155- 

deposition,  effect  on  topography,  172- 

I7S- 

erosion,  operation  of,  157-159. 
rate  of,  159. 

topography  modified  by,  163-167. 
formations,  Permian,  476-477,  505. 

Proterozoic,  398. 
lakes,  Lake  Agassiz,  656. 
Lake  Algonquin,  653. 
Bascom,  656. 
Bonneville,  657. 
Chicago,  653. 
Duluth,  653. 
Iroquois,  655. 
Lahonton,  657. 
Lundy,  653. 
Maumee,  653. 
Nipissing,  655. 
Warren,  653. 
movement,  150-154. 
differential,  151. 
rate  of,  150,  152. 
theories  of,  189-191. 
planation,  183. 
streams,  150. 
water  of,  159. 
work  of,  161-163. 
striation,  183. 
transfer  of  load,  156. 
Glaciated  areas,  645-647. 
pebbles,  158. 
valleys,  characteristics  of,  163. 

disappearance  of,  165. 
Glaciation,  ancient,  171-188. 
Permian,  476-477,  505. 
Proterozoic,  398. 
causes  of,  660-662. 
theories  of,  astronomical,  660-661. 


704 


INDEX 


Glaciation  —  (Continued) 

theories  of,  atmospheric,  661-662. 

theories  of,  elevation,  660. 
Glacier  milk,  159. 
Glaciers,  Alpine,  141. 

continental,  168-171. 

deposition  by,  159-163,  171-182. 

distribution  of,  141. 

fluctuations  of,  152. 

formation  of,  143. 

limits  of,  152. 

Piedmont,  167-168. 

transportation  by,  154-156. 

size  of,  141,  144. 

slope  of,  152. 

surface  of,  147-150. 
Glassy  rocks,  329,  331. 
Glauconite,  516. 
Globigerina  ooze,  241. 
Glyptocrinus  decadactylus,  431. 
Glyptodon,  671. 
Glyptosaurus,  625. 
Gneiss,  346. 
Gomphoceras,  460. 
Goniatite,  Devonian,  459. 

Mesozoic,  530. 

Goniograptus  postremus,  428. 
Goniophyllum  pyramidale,  444. 
Gossan,  373. 
Graben,  263.        « 
Grade,  88. 

Graham's  Island,  erosion  of,  213. 
Grammysia  hannibalensis,  483. 
Grand  Calumet  River,  course  changed  by 

drifting  sand,  50,  51. 
Grand  Canyon  of  the  Colorado,  95, 109, 

352. 
Granite,  24,  330. 

marine  erosion  of,  206. 
Granitoid  rocks,  329. 
Granulation,  349. 
Graphite,  501. 
Graptolites,  Cambrian,  416. 

Ordovician,  427-429. 

Silurian,  445. 
Grasses,  630-631. 
Gravity  fault,  261-263. 
Great  Lakes,  origin  of  basins,  651-652. 

preglacial  drainage  of,  651. 

stages  in  history,  652-655. 
Great  Rift  valley,  Africa,  267. 
Greenland  glaciation,  168-169. 
Green  River  formation,  575,  625. 


Gregory,  H.  E.,  57. 
Gregory,  J.  W.,  166. 
Grindelwald,  glaciers  ot    144. 
Ground  moraine,  161,  177. 
Ground  water,  affected   by  earthquakes, 
290. 

chemical  work  of,  60-62. 

deposition  by,  60. 

depth  of,  58. 

level  of,  56. 

mechanical  work  of,  see  Landslides. 

mineral  matter  in  solution,  58. 

movement  of,  58. 

quantity  of,  56. 

replacement  by,  60. 

solution  by,  60. 

striking  effects  of,  69-75. 
Growth    of   granules,    theory    of   glacial 

movement,  191. 
Gryphaea,  526. 
Gryphaea  arcuata,  527. 
Grypotherium,  671. 
Guelph  stage,  439. 
Gunnison  River,  contrasted  with  Uncom- 

pahgre,  113-114. 
Gymnosperms,  496-497,  567. 
Gypidula  galeata,  458. 
Gypsum,  687. 

Mississippian,  470. 

Permian,  476. 


Hade,  261. 

Halobia  (Daonella)  lommeli,  527. 

Halysites,  444,  456. 

Halysites  catenulatus,  444. 

Hamilton  stage,  452. 

Hanging  valley,  93,  163,  206. 

Hanging  wall,  262. 

Hapsiphyllum  calcareforme,  480. 

Harrison  formation,  606. 

Hastings,     England,     storm     waves    at, 

201. 

Haug,  E.,  348,  360. 
Hawaiian  volcanoes,  307-310. 
Headlands  and  coves,  208. 
Headward  erosion,  99. 
Heat,  interior,  273,  365. 
Heave,  262. 

Hebertella  borealis,  432. 
Height  of  land,  104. 
Helderberg  limestones,  452. 
Helgoland,  erosion  of,  213,  214. 


INDEX 


70S 


1  leliophyllmn  halli,  456. 

, 
'emipnstis  serra,'625. 

,>rera,  533. 
Mercoglossa  tuomeyi,  628. 

562. 

inus)  subcrassus,  431. 
i  n  xa<  wro!!<.,  -,_^.. 
Hidden  Glacier,  179. 
High  plains,  588. 
Hillside  creep,  73. 
Hinge  fault,  282. 
Hipparion,  610. 
Hipparionyx  proximus,  458. 
Hippopotamus  amphibius,  676. 
Historical  geology,  definition  of,  22. 
Hoang  Ho,  delta  of,  132,  133. 

destructiveness  of,  123. 
Hobbs,  W.  H.,  33,  285,  287. 
Hogback,  105. 
Holoptychius,  465. 

Hooks,  220-221. 

Horizontal  faults,  264-265. 
Hormatoma  gracilis,  434. 
Hornblende,  690. 

schist,  346. 
Horses,  evolution  of,  608-612. 

extinction  of  in  North  America,  611- 
612. 

Pleistocene,  672. 
Horseshoe  lakes,  122. 
Horsetails,  565. 
Horst,  263,  354. 
Hot  springs,  66. 

Hudson  River,  Palisades  of,  326,  511. 
Humboldt  Glacier,  Greenland,  170. 
Huronian  system,  393. 
Hustedia  mormoni,  482. 
Hybocrinus  tumidus,  431. 
Hydration,  36. 
Hydrographical  basins,  103. 
Hymenocaris  perfecta,  412. 
Hyolithes  carinatus,  413. 
Hypohippus,  610. 
Hypothyris  cuboides,  458. 
Hyracodon,  606. 


Ice,  formation  from  snow,  142. 

in  lakes,  204. 

in  sea,  203. 

texture  of,  142-143. 
Icebergs,  formation  of,  188. 


Icebergs  —  (Continued) 

size  of,  189,  190. 

work  of,  189. 

Icefalls  on  glaciers,  148,  149. 
Icelandic  volcanoes,  311. 
Ice  sheets,  168-171. 

centers  of,  646-647. 

deposition  by,  171-182. 

development  of,  647-648. 

effect  on  drainage,  184-188. 
on  rivers,  187. 

erosion  by,  182-184. 

in  foreign  countries,  645. 

in  North  America,  176,  646-647. 

moraines  of,  175. 

thickness  of,  647-648. 

work  of,  662—663. 
Ice  tables  on  glaciers,  149,  150. 
Ichthyornis,  1562-563. 
Ichthyosaurus,  553-554. 
Igneous    intrusions,    and    ore     deposits, 
372. 

rocks,  age  of,  334. 
definition  of,  24. 
subdivisions  of,  329. 
weathering  of,  32. 
Iguanodon,  544. 
Illinoian  epoch,  649. 
Impact  marks,  75. 
Imperial  valley,  Cal.,  132. 
Incompetent  strata,  257. 
Industrial  geology,  definition  of,  22. 
Injected  igneous  rock,  324-327. 
Inoceramus,  528. 
Inoceramus  vanuxemi,  527. 
Inostranservia,  539. 
Insectivores,  620. 
Insects,  Carboniferous,  481,  484. 

Mesozoic,  532-533. 

Tertiary,  627-630. 
Interglacial  deposits,  664-665. 

migrations,  665. 

stages,  648-649. 
Interior  heat,  273,  365. 
Interior  of  earth,  272-274,  335,  365. 
Intermittent  springs,  64. 
Internal  density,  theories  of,  273. 
Internal  fluidity  theory,  273. 
Intrenched  meanders,  112. 
Intrusions  of  lava,  324-328. 
lowan  stage,  649. 
Iron  minerals,  686-687. 
Iron  ore,  Clinton,  442. 


706 


INDEX 


Iron  ore  —  (Continued} 

in  Pennsylvanian,  475. 

in  Proterozoic,  396. 

sedimentary  deposits,  374. 
Isoclines,  254. 

Isolation,  effect  on  faunas,  636-642. 
Isostasy,  365-367. 
Isotelus  gigas,  435. 


Jeffrey,  E.  C.,  500,  501.      . 
Jellyfish,  416. 
Jerseyan  stage,  649. 
Johnstown  flood,  82,  83. 
Joints,  258-260. 

influence  on  erosion  by  streams,  88. 
on  topography,  260. 
on  wave  erosion,  207,  208. 
on  weathering,  29,  39. 

origin  of,  259. 

vertical,  cause  of  fall,  92. 
Jorullo,  volcano,  295. 
Jupiter     Serapis,     Pozzuoli,     temple     of, 

229. 
Jurassic  period,  508. 

climate  of,  570. 

mountain  forming  of,  513. 

of  Atlantic  and  Gulf  coasts,  512. 

of  other  continents,  513-514. 

of  western  interior,  512. 

physical  geography  of,  512-514. 


Kaaterskill  Creek,  108. 
Kahl,  51. 
Kames,  181-182. 
Kansan  glacial  stage,  649. 
Karst  topography,  72-73. 
Katmai,  volcano,  305. 
Keewatin  ice  sheet,  646. 
Kemp,  J.  F.,  332. 
Kettle  holes,  179. 
Keuper  formation,  512. 
Keweenawan  system,  388. 
Kilauea,  308. 
King,  F.  H.,  44,  131. 
Kittatinny  peneplain,  516. 
Klamath  Mountains,  581. 
Knowlton,  F.  H.,  634. 
Kootenai  formation,  515. 
Krakatao,  304-305. 
Kutorgina  cingulata,  414. 


Labidosaurus,  490. 
Labradorean  ice  sheet,  646-647. 
Labradorite  feldspar,  689. 
Laccolith  mountains,  354-355. 
Laccoliths,  327. 

Laccoliths  of  Henry  Mountains,  519. 
Lafayette  formation,  586. 
Lake  Agassiz,  656. 

Algonquin,  653. 

Bascom,  656. 

Bonneville,  137,  657. 

Chicago,  653. 

Duluth,  653. 

Iroquois,  655. 

Lahontan,  657. 

Lundy,  653. 

Marjelen,  186. 

Maumee,  653. 

Nipissing,  655. 

Superior  formations,  375. 
iron  deposits  of,  375. 
Proterozoic  of,  393. 

Tanganyika,  267. 

Tulare,  125,  126. 

Warren,  653. 
Lakes,  dammed  by  glaciers,  186. 

deposition  in,  133-135,  250. 

extinct,  136-138. 

formed  by  alluvial  fan,  126. 
dam  of  drift,  i86f 
earthquakes,  288,  289. 
landslide  dam,  74,  75. 
•lava  dam,  321. 
morainic  dam,  160. 
natural  levees,  124. 
sand-dune  dam,  50. 
talus  dam,  39. 

in  cirques,  145. 
«»in  craters,  321. 

in  glacial  drift,  iS6f 

in  glaciated  valleys,  165. 

in  moraines,  175. 

in  overdeepened  river  valleys,  186.* 

in  rock  basins,  i86*» 
Lakota  formation,  515. 
Lambdotherium,  604. 
Laminae,  24,  234. 
Lance  formation,  518,  572,  624. 
Landslides,  62,  207. 

causes  of,  73-75,  289. 

form  falls,  93. 
Landslide  topography,  75. 
Lapilli,  332. 


INDEX 


707 


Laplacian  hypothesis,  385. 
Laramie  formation,  517,  518,  624. 
Lassen  Peak,  California,  318. 
Lateral  erosion  by  streams,  89. 
Lateral  moraines,  154,  155,  156. 
Lateral  pressure,  359,  364. 
Laurentian  peneplain,  117. 
Laurentian  period,  390,  393. 
Laurentian  shield,  117,  118,  389. 
Lava,  298. 

columnar  structure  of,  333-334. 

composition  of,  30x^-302. 

crystallization  of,  302. 

extrusion  of,  365. 

fluidity  of,  299. 

rise  of,  338. 

temperature  of,  299. 
Lava  cones,  310,  312. 
Lava  dam  and  waterfall,  93. 
Lava  sheets,  311. 
Lava  streams,  309. 

flow  of,  298. 

surface  of,  299. 

velocity  of,  300. 
Lead  minerals,  689. 
Lee  side,  158,  183. 
Lege,  dunes  of,  49. 
Leith,  C.  K.,  281,  349. 
Leperditia  inflata,  435. 
Lepidodendron,  493-494,  497. 
Lepidodiscus  (Agelacrinus)  cincinnatiensis, 

430. 

Leptaena  rhomboidalis,  432. 
Leptomeryx,  618. 
Levees,  natural,  123. 
Leverett,  F.,  653. 
Lichas  (Gaspelichas)  forillonia  (cephalon), 

460. 

Lichenalia  concentrica,  447. 
Life,  before  fossils,  398. 

imperfection  of  record  of,  380. 
Lifting,  84. 

Lightning,  mechanical  work  of,  34. 
Limestone,  249. 

deposition  of,  238. 

oolitic,  77. 
Limnoscelis,  486. 
Limonite,  134,  375,  686. 
Limulus,  449. 
Lingula  brevirostra,  526. 
Lingula  rectilateralis,  432. 
Lingulepis  acuminata,  414. 
Liquid-thread  theory  of  volcanism,  336. 


Tjithnstrfft 
Litoptern* 


Lithodomus,  229,  230. 

ion  canadense,  480. 
itopterna,  641. 
Littoral  currents,  see  Shore  currents. 
Littoral  deposits,  219. 
Load,  effect  of,  86. 
Load  of  streams,  how  measured,  118. 
Lockport  stage,  439. 
Lodes,  370. 

Loess,  43,  52-54,  588,  657-658. 
Loops,  221. 

Lophophyllum  profundum,  480. 
Lophospira  bicincta,  434. 
Lowville,  stage,  422. 
Loxonema  noe,  459. 
Lucas,  F.  A.,  533. 
Lungfish,  464-465. 
Lycopods,  493-495- 
Lyell,  Sir  Charles,  303. 
Lyginodendron,  493. 
Lysorphus,  488. 
Lytoceras  fimbriatum,  529. 


Machairodus,  672. 
Maclura,  664. 
Macluria  logani,  434. 
Macrura,  532. 
Magmatic  segregation,  374. 
Magmatic  waters  and  ore  deposits,  372. 
Magnetite,  686. 
Malachite,  688. 
Malaspina  Glacier,  167,  168. 
Mammals,   factors  in  evolution  of,  600- 
603. 

marine,  595-59?- 

Mesozoic,  563-565. 

rise  of,  590. 

Tertiary,  590-623. 
Mammoths,  668. 
Man,  antiquity  of,  674-680. 

birthplace  of,  681-682. 

effect  of  advent,  682-683. 

Eolithic,  675-676. 

in  North  America,  680-681. 

Neolithic,  679-680. 

Paleolithic,  676-678. 
Manticoceras  oxy,  459. 
Mantle  rock,  41,  42. 
Marattiaceae,  492. 
Marble,  344. 

Marblehead,   Mass.,    marine    erosion    at, 
206. 


INDEX 


Marcellus  stage,  452. 

Marine  deposits,  classification  of,  235. 

erosion,  202-214. 

life,  distribution  of,  197. 

plains,  ancient,  215-217. 
New  England,  216. 

terraces,  211,  212. 
Marl,  134. 
Marsupials,  591. 
Mass  action,  372. 
Mastodon,  614,  669. 
Mastodonsaurus,  536. 
Matthew,  W.  D.,  485,  551,  620. 
Mature  topography,  no. 
Mauch  Chunk  shale,  488. 
Mauna  Loa,  308. 
Meanders,  121. 

Mechanical  deposits  in  lakes,  133. 
Medial  moraines,  154,  155,  156. 
Medina  stage,  439,  441. 
Mediterranean  Sea,  temperature  of,  196, 

197. 

Medlicottia  copei,  483. 
Meekoceras  gracilitatis,  529. 
Megalonyx,  665,  670,  671. 
Meganeura  monyi,  484. 
Megatherium,  670. 
Melina  (Perma)  maxillata,  629. 
Melocrinus  milwaukeensis,  457,  671. 
Melting   and    pressure   theory   of  glacial 

movement,  191. 

Mer  de  Glace,  movement  of,  152. 
Mesas,  105,  106,  354. 
Mesohippus,  609. 
Mesozoic  era,  508-571. 

coal  of,  571. 

duration  of,  520. 

invertebrates  of,  523-533. 

life  of,  521-569. 

compared  with  Cenozoic,  572-573. 
compared  with  Paleozoic,  521. 

mountain  making  at  close  of,  519. 

plants  of,  565-569. 
Metacheiromys,  621. 
Metamorphic  rocks,  classification  of,  344- 

.  .347' 
definition  of,  25. 

development  of,  349. 
importance  of,  351. 
weathering  of,  350. 
Metamorphism,  328,  341,  358. 
by  contact,  341-343. 
by  pressure,  343. 


Metamorphism  —  (Continued) 

causes  of,  347-348. 

of  Archaeozoic,  391. 
Metamynodon,  606. 
Mica  schist,  346. 
Micromitra  bella,  414. 
Microzeuglodon,  597. 
Migration,  effect  of,   on  mammals,  636- 

642. 

Milne,  J.,  274. 
Mineral  matter  in  ground  water,  58. 

in  springs,  64-66. 
Mineralogy,  definition  of,  21. 
Minerals,  cleavage  of,  685. 

hardness  of,  685. 

specific  gravity  of,  685. 

streak  of,  685. 
Mineral  springs,  66. 
Miocene  epoch,  573. 

climate  of,  635. 

disturbances  on  Pacific  Coast,  582. 

economic  products  of,  579-580. 

migration  during,  640. 

mountain  building  of,  582. 

of  Atlantic  and  Gulf  coasts,  579. 

of  other  continents,  584. 

of  Pacific  coast,  580-582. 

of  western  interior,  580. 

physical  geography  of,  579-585. 

volcanic  activity  of,  581,  583-584. 
Mississippian  period,  401,  469-471. 

close  of,  470. 

gypsum  of,  470. 

of  other  continents,  471. 

physical  geography  of,  469-471. 
Mississippi    River,     meanders     of,     121, 

122. 
Mississippi   Valley,    earthquake   of,    279, 

285,  286,  287. 
Moa,  673. 
Modioloides,  433. 
Modiomorpha  concentrica,  459. 
Moeritherium,  612. 
Mohawkian  stage,  422. 
Mohawk  valley,  faults  in,  26? 
Molds,  379. 
Mollusca,  Cambrian,  413. 

Carboniferous,  481-483. 

Devonian,  457. 

Ordovician,  433-435. 

Silurian,  447-448. 
Molluscoidea,  Cambrian,  414-415. 

Carboniferous,  480-482. 


INDEX 


709 


Molluscoidea  —  (Continued) 

Devonian,  457. 

Ordovician,  431-432. 

Silurian,  446-447. 
Monadnocks,  in,  118. 
Monoclines,  256. 
Monograptus  clintonensis,  428. 
Monopteria  longispina,  483. 
Montana  formation,  517,  571. 
Monterey  formation,  581. 
Moraines,  ground,  161,  177. 

lateral,  154,  155,  156. 

medial,  154,  155,  156. 

recessional,  160,  175. 

submarginal  or  bank,  161. 

terminal,  159-160,  175. 
Morrison  formation,  515. 
Mosasaurs,  555-556. 
Moulins  on  glaciers,  149. 
Mountain  folding,  358-363. 

and  crustal  shortening,  359. 

causes  of,  364-367. 

experiments  in,  360-361. 

rate  of,  362. 
Mountains,  age  of,  369. 

classification  of,  352. 

cycle  of  erosion  of,  364. 

distribution  of,  368. 
Mount  Etna,  322. 

Greylock,  73,  256. 

Hood,  318. 

Pelee,  306,  307. 

Shasta,  318,  321. 
Mud  cracks,  238. 
Mud  volcanoes,  323. 
Muensteroceras  oweni,  483. 
Muir,  John,  633. 

Muschelkalk  beds,  Germany,  511. 
Muscovite  mita,  690. 
Myalina  recurvirostris,  483. 
Mylodon,  670. 


Nahant,  a  tied  island,  223. 

Nansen,  F.,  169. 

Naraoia  compacta,  412. 

Narrows,  86. 

Naticopsis  altonensis,  482. 

Natural  bridges,  formed,  by  fall,  91. 

by   perforation   of  neck   of    intrenched 
meander,  112. 

by  swallow  holes,  71. 

from  caves,  71. 


Natural  levees,  123. 
Nautilus,  530. 
Neanderthal  man,  675. 
Nebular  hypothesis,  385. 

contrasted  with  planetesimal,  387. 
Neolithic  man,  679-680. 
Neuroptera,  532. 
Neve,  142. 

New  England  peneplain,  114. 
Niagara  Falls,  89,  441. 

recession  of,  658-659. 

slight  erosion  at  brink,  85. 
Niagaran  formation,  441. 
Niagaran  limestone,  440. 
Nile  valley,  flood  plain  of,  121. 
Niobrara  formation,  524. 
Nipponites  mirabilis,  530. 
Nivation,  146. 
Nodular  flint,  524. 
Nome,  Alaska,  gold  of,  374. 
Normal  fault,  261-263. 
Norton,  W.  H.,  175. 
Nostoceras  stantoni,  530. 
Nucleocrinus  verneuili,  457. 
Nummulites,  577,  626. 
Nunataks,  170. 


Oases  of  Kerid,  64. 

Obelisk,  City  of  New  York,  28. 

Obolella  atlantica,  414. 

Obsidian,  332. 

Ocean,  age  of,  198. 

depth  of,  194,  195. 

extent  of,  194. 

general  character  of,  194-202. 

temperature  of,  196-197. 
Ocean  basins,  permanence  of,  368. 
Ocean  currents,  202. 
Ocean  floor,  topography  of,  194. 
Ocean      water,     composition      of,      195- 
196. 

movement  of,  198-202. 
Odontaspis  cuspidata,  625. 
Odontaspis  elegans,  625. 
Oil  and  gas,  in  Devonian,  455. 

in  Ordovician,  425. 

in  Pennsylvanian,  475. 

origin  of,  425-426. 
Old  age  of  valleys,  in. 
Oldhamia  antiqua,  409. 
Old  Red  Sandstone,  455. 
Olenellus,  402. 


7io 


INDEX 


Olenellus  thompsoni,  412. 
Oligocene,  573. 

climate  of,  635. 

migration  during,  639-640. 

of    Atlantic    and    Gulf    coasts,     577- 
578. 

of  other  continents,  578. 

of  Pacific  coast,  578. 

of  western  interior,  578. 

physical  geography  of,  577-578. 
Olivine,  691. 
Oncoceras  pandion,  434. 
Oneida  conglomerate,  441. 
Oneida  stage,  439. 
Onondaga  limestone,  452,  454. 
Ony chaster  flexilis,  481. 
Onychocrinus  exsculptus,  481. 
Oolitic  limestone,  77-78,  249. 
Ooze,  241. 

Ophileta  compacta,  434. 
Ordovician,  401,  418-438. 

climate  of,  437. 

close  of,  422. 

duration  of,  437. 

in  New  York  state,  422. 

life  of,  427-437. 

of  other  continents,  423. 

oil  and  gas  of,  424-426. 

physical  geography  of,  418-424. 

volcanic  activity  of,  423. 
Ore  deposits,  37°-375- 

and  magmatic  waters,  371. 

in  fissures,  370-371. 

origin  of,  370. 

weathering  and  concentration  of,  373. 
Oreodon,  619. 

Organic  deposits  in  lakes,  134. 
Organisms  and  rock  disintegration,  37. 
Origin  of  the  earth,  385-387. 
Oriskany  sandstones,  452,  454. 
Orkney  Islands,  work  of  wind  in,  44. 
Orthaulax  gabbi,  628. 
Orthis  tricenaria,  432. 
Orthoceras,  Devonian,  460. 

Mesozoic,  530. 

Ordovician,  435. 

Orthoceras  multicameratum,  434. 
Orthoceratites,  Carboniferous,  481. 

Mesozoic,  528. 
Orthoclase  feldspar,  689. 
Orthoptera,  532. 

Osborn,  H.  F.,  572,  578,  605,  611,  623, 
631,663. 


Ostracoderms,  Devonian,  461-462. 

Silurian,  450. 
Ostracods,  435,  436. 
Ostrea  sellaeformis,  628. 
Ottoia  prolifica,  415. 
Oudenodon,  537. 
Outcrops,  width  of,  253. 
Outer  Hebrides,  205. 
Outliers,  106. 
Outwash  plains,  178. 
Overdeepening,  163. 
Overlap,  271. 
Oversteepening,  163. 
Overturned  folds,  255. 
Owens  valley,  earthquake  of,  282. 
Oxbow  lakes,  121,  122. 
Oxidation,  36. 
Ozarkian,  401,  408. 


Pachyaena,  595. 
Pahoehoe  lava,  299,  300. 
Palaeaster  eucharis,  457. 
Palaeaster  simplex,  431. 
Palaeomastodon,  613. 
Palaeoneilo  constricta,  459. 
Paleodictyoptera,  483. 
Paleogeography,  definition  of,  22. 
Paleolithic  man,  676-678. 
Paleontology,  definition  of,  22. 
Paleozoic  era,  401-507. 

climate  of,  507. 

life  compared  with  Mesozoic,  521. 

change  in  life  at  close  of,  521. 

evolution  and  extinction  during,  506. 

physical  geography  of,  505-506. 
Panama  Canal,  34,  74,  280. 
Paradoxides,  402,  412. 
Paradoxides  harlani,  412. 
Parahippus,  610. 
Pareiasaurus,  538. 
Paris  basin,  383. 
Partiot,  218. 
Pea  ore,  442. 
Peat,  134. 
Pecopteris,  492. 
Pecten  madisonius,  628. 
Pelagiella  (Platyceras)  primzevum,  413. 
Pelecypods,  Carboniferous,  481,  483. 

Devonian,  457,  459. 

Mesozoic,  526-527. 

Ordovician,  433. 

Silurian,  447-448. 


INDEX 


711 


Pelecypods  —  (Continued) 
Tertiary,  626,  627,  629. 
Pelee,  Mount,  306. 
Pelycosaurs,  490. 
Penaeus  meyeri,  532. 
Penck,  A.,  658. 
Peneplain,  114,  115,  116. 
Peneplanation,  ill,  114-118. 
Pennsylvanian  period,  401,  472-475. 
coal  of,  473. 
duration  of,  475. 
in  other  continents,  475. 
iron  and  oil  of,  475. 
oil  of,  475. 

physical  geography  of,  472-473. 
thickness  of,  473. 
Pentacrinus,  525. 
Pentacrinus  fossilis,  525. 
Pentamerus  oblongus,  446. 
Pentremites  pyriformis,  481. 

robustus,  481. 
Peorian  stage,  649. 
Perce  Rock,  Quebec,  210,  211. 
Peridot,  691. 
Peridotite,  330. 
Perisphinctes  achilles,  529. 
Perissidactyls,  599-60x3. 
Permian,  401,  476-480. 
aridity  of,  505. 
decrease  in  marine  life,  504. 
deserts,  477. 

glaciation,  476-477,  505. 
gypsum,  476. 
in  other  continents,  479. 
physical  geography  of,  476-480. 
problems  of,  504-505. 
volcanic  activity  of,  477. 
Petrifaction,  378. 
"  Petrified  moss,"  66. 
Petroleum,  see  Oil. 
Petrology,  definition  of,  21. 
Phacops  rana,  460. 
Phenacodus,  598,  600. 
Phillipsia  major,  484. 
Phororhachus,  623. 
Phosphates,  Miocene,  580. 
Phragmoceras  parvum,  448. 
Phyllograptus  angustifolius,  428. 

typus,  428. 

Physical   geography,   ancient,   determina- 
tion of,  403. 

Physiographic  geology,  definition  of,  21. 
Piedmont  glaciers,  167-168. 


Piedmont  plains,  127. 
Piedmont  Plateau,  91,  92. 
Pine,  Arizona,  natural  bridge  of,  65. 
Pipestem  concretions,  77. 
Piracy,  stream,  102,  107-109. 
Pirsson,  L.  V.,  332. 
Pitch,  256. 
Pitchstone,  332. 
Pithecanthropus,  623. 
Pithecanthropus  erectus,  675. 
Placer  gold  deposits,  374. 
Placodermata,  464. 
Plagiaulacidae,  591. 
Planes  of  stratification,  24. 
Planetesimal  hypothesis,  386-387. 

contrasted  with  nebular,  387. 
Planetesimals,  386. 
Plants  and  chemical  disintegration,  37. 

mechanical  action  of,  33. 
Plants,  Cambrian,  409-410. 

Carboniferous,  491-498. 

Devonian,  467. 

Mesozoic,  565-568. 

Ordovician,  436. 

Pleistocene,  667-668. 

Tertiary,  630-634. 
Plastostoma  broadheadi,  482. 
Plateaus,  see  Mountains. 
Platte  River,  a  braided  stream,  87. 
Platyceras  dumosum,  459. 
Platycrinus  discoideus,  481. 
Platyostoma  (Diaphorostoma)  niagarense, 

447- 

Platystrophia  lynx,  432. 
Playas,  134. 

Plectambonites  sericeus,  432. 
Pleistocene  period,  573,  643-674. 

changes  of  elevation   at   beginning  of, 
643-644. 

duration  of,  658-660. 

glaciation  of,  644. 

life  of,  663-683. 

plants  of,  667-668. 

sources  of  knowledge  of  life   of,   663- 

666. 

Plesiosaurs,  554-555. 
Pleurocystis  filitextus,  430. 
Pleurodictyum  stylopora,  456. 
Pleurophorus  tropidophorus,  483. 
Pleurotomaria  nodulostriata,  482. 
Pliocene  epoch,  573. 

climate  of,  635-636. 

elevation  during,  587. 


712 


INDEX 


Pliocene  epoch  —  (Continued) 

migration  during,  640-641. 

of  Atlantic  and  Gulf  coasts,  585-586. 

of  other  continents,  588-590. 

of  Pacific  coast,  587. 

physical  geography  of,  585-590. 

of  western  interior,  586. 
Pliosaurus,  554. 
Plucking  by  glaciers,  157. 

by  ice  sheets,  183. 
Plugs,  3 16. 

Plutonic  rocks,  324-332. 
Plymouth,    England,    storm    waves    at, 

201. 

Pbebrotherium,  616. 

Polypora  lilaea,  458. 

Porphyry,  331. 

Portage  stage,  452,  454. 

Portheus,  534,  535. 

Port  Jackson  shark,  463. 

Postglacial  stage,  649. 

Potato  Creek,  effect  of  deforestation  on, 

97- 

Potholes,  93-95. 
Potomac  formation,  514. 
Pottsville  stage,  472. 
Powers,  S.,  342. 

Pre-Cambrian  formations,  388-400. 
Precession     of    equinoxes     and     climate, 

661. 

Precipitation  of  ores,  372. 
Predentata,  armored,  539,  545-549. 
Predentata,  unarmored,  539,  544-545. 
Prehistoric  man,  674-680. 
Pressure,  lateral,  359. 
Primates,  622-623. 
Procamelus,  617. 
Productella  spinulicosta,  458. 
Productus,  481. 
Productus  burlingtonensis,  482. 

costatus,  482. 
Proterozoic  era,  393-399. 

climate  of,  398. 

contrasted  with  Archaeozoic,  393. 

duration  of,  397. 

fossils  of,  396-397. 

glaciation  of,  398. 

iron  and  copper  of,  396. 

life  of,  396-397- 

of  Black    Hills,    South    Dakota,    395- 
396. 

of  Grand  Canyon,  Arizona,  394-395. 

of  Lake  Superior  region,  393. 


Proterozoic  era  —  (Continued) 

in  other  continents,  396. 

unconformities  of,  394. 

volcanic  activity  of,  394. 
Protoceras,  619. 
Protocetus,  596. 
Protohippus,  610. 
Protorohippus,  609. 
Protowarthia  cancellata,  434. 
Protozoans,  Cambrian,  416. 

Carboniferous,  480. 

Ordovician,  427. 
Protylopus,  616.     ..   .%<-. 
Prozeuglodon,  596. 
Pteranodon,  559-560. 
Pteraspis,  450. 
Pteridosperms,  492-493. 
Pterinea  demissa,  433. 

emacerata,  447. 

flabellum,  459. 
Pteropods,  Cambrian,  413. 

Silurian,  447. 
Pterosaurs,  558-560. 
Ptilodus,  591. 
Puerco  formation,  591.     . 
Pulaski  stage,  422. 
Pumice,  297. 
Pumiceous  lava,  302. 
Pyrite,  686. 

weathering  of,  37.    .  -je^ 
Pyropsis  bairdi,  528. 
Pyrrhotite,  687. 


Quartz,  689. 
Quartzite,  248,  344,  ,350. 
Quaternary  period,  573,  643-684. 
Quebec,  landslide  at,  73. 


Race,  tidal,  201. 

Radioactivity  and  interior  of  the  earth, 

274. 

Radiolaria,  427. 
Radiolarian  ooze,  242. 
Radiolites,  527. 
Radiolites  cornu-pastoris,  527. 
Rain,  mechanical  effect  of,  33,  34. 

effect  on  heated  rocks,  33. 
Raindrop  impressions,  236. 
Raised  beaches,  214. 
Raphinesquina  alternata,  432. 
Receptaculites,  445. 


INDEX 


713 


Receptaculites  ohioensis,  427,  445. 

Recessional  moraines,  160,  175. 

Recumbent  folds,  255. 

Red  beds,  476. 

Red  clay,  242. 

Red    River,    N.   Dak.,    a   young   stream, 

109. 

Red  snow  (Sphaerilla  nivalis),  169. 
Reelfoot    Lake,    formed    by   earthquake, 

288. 
Regelation,  theory  of  glacial  movement, 

190. 

Regional  metamorphism,  343. 
Rejuvenation  of  streams,  112. 
Rensselaeria  ovoides,  458. 
Replacement  by  ground  water,  60. 
Replacement  deposits,  372,  375,  378. 
Reptiles,  489-491. 

marine,  552-557. 

Meso/oic  536-560. 

rise  of,  491. 

Tertiary,  624-625. 
Requienia,  527. 
Requienia  patagiata,  527. 
Residual  mountains,  352-354. 
Reverse  faults,  263-264. 
Rhaetic  formation,  512,  565. 
Rhamphorynchus,  558. 
Rhine  River,  dissolved  minerals  in,  83. 
Rhine  valley,  a  graben,  263. 
Rhinobatus,  535. 
Rhinoceroses,  605-607. 
Rhinoceros  merckii,  676. 
Rhipidomella  burlingtonensis,  482. 

oblata,  458. 

Rhone  River,  confluence  with  Arve,  162. 
Rhynchonella  aequiplicata,  526. 

gnathophora,  526. 
Rhynchotrema  capax,  432. 
Rhynchotreta  cuneata  americana,  446. 
Rhytimya  radiata,  433. 
Richards  and  Mansfield,  264. 
Richthofen,  Baron  von,  54. 
Ries,  H.,  425. 
Rift  valley,  100. 
Rill  marks,  236,  237. 
Ripple  marks,  236,  237. 
Rivers,  see  Streams. 
Roches  moutonnees,  158,  184. 
Rochester  shale,  439. 
Rock  basins,  lakes  in,  186. 
Rock  flour,  159. 
Rock  glaciers,  30,  31. 


Rocking  stones,  155,  156. 
Rock  salt,  origin  of,  136,  443. 
Rock  terraces,  106. 
Rocks,  classification  of,  23. 

acid,  329. 

decolorization  of,  37. 

igneous,  definition  of,  24. 

metamorphic,  definition  of,  25. 

sedimentary,  definition  of,  23. 
Rodents,  620-621. 
Rondout  stage,  439. 
Rossberg,  Switzerland,  landslide  at,  74. 
Ruedemann,  R.,  429. 
Russell,  I.  C.,  354. 

Rye,  England,  marine  erosion  and  deposi- 
tion at,  224. 


Saber-toothed  tiger,  672. 

Sabrina,  volcano  in  the  Azores,  295. 

Sagenites  herbichi,  529. 

Salina  stage,  439,  441,  443,  451. 

Salisbury,    Chamberlin    and,     336,    365, 

662. 

Salt  beds,  136. 
Salt  in  New  York  state,  443. 

in  the  ocean,  198. 

origin  of,  443. 

Salina  series,  443. 
Salt  lakes,  135. 
Salton  sink,  132. 
Sand  dunes,  see  Dunes. 
Sand  reefs,  221-223. 
Sandstone,  24,  249. 

lens  shape  of  deposits,  239. 
San  Francisco,  earthquake  of,  275. 
Sangamon  stage,  649. 
Sauropoda,  539,  541-543- 
Scaphites,  528. 
Scaphites  nodosus,  529. 
Scarborough  beds,  665. 
Scaumenacia,  464. 
Scenella  varians,  413. 
Schists,  345. 
Schizocrania  filosa,  432. 
Schroederoceras  eatoni,  434. 
Schuchert,  Chas.,  398,  401,  418. 
Schwartz,  E.  H.  L.,  335. 
Scolithus,  415. 
Scoriaceous  lava,  301,  302. 
Scorpions,  450. 
Scott,  D.  H.,  56,  467,  521. 
Scour  and  fill,  88. 


714 


INDEX 


Scutella  aberti,  627. 

Sea  arches,  210. 

Sea-captured  streams,  214. 

Sea  caves,  209. 

Sea  cliffs,  erosion  by  waves,  208. 

by  weather,  208. 
Sea  cucumbers,  415. 
Seattle,  lowering  of  hill  at,  84. 
Sea  urchins,  Carboniferous,  480. 

Ordovician,  431. 
Sedimentary  rocks,  23. 

classification  of,  249-250. 

influence  upon  topography,  250. 
Sediment  of  streams,  how  carried,  81. 

source  of,  81. 
Sediments,  cementation  of,  248. 

consolidation  of,  248. 

effect  of  heat  on,  249. 

effect  of  pressure  on,  248. 
Seismographs,  287. 
Seminula  argentea,  482. 

subquadrata,  482. 
Septaria,  77. 
Sequoia  gigantea,  632. 
Sequoias,  632-633. 
Seracs  on  glaciers,  148. 
Serpentine,  690. 
Shaler,  N.  S.,  303. 
Shales,  250. 
Sharks,  Devonian,  462-463. 

Mesozoic,  533. 

Shetland   Islands,  work  of  wind   in,  44. 
Shingle,  217. 

Shoal-water  deposits,  237-241. 
Shore  currents,  200. 

deposition  by,  219,  223-224,  233. 

transportation  by,  217. 
Shore  ice,  203. 
Shore  line,  cycle  of  erosion  of,  231-233. 

rough,  226. 

smooth,  224. 
Shores,  maturity  of,  232. 

rough,  226. 
Shoulders,  165. 
Siderite,  686. 

Sierra  Nevada  Mountains,  513,  582. 
Sigillaria,  494-495,  497. 
Silicate  minerals,  689-690. 
Sills,  326-327. 

Sill  tunnel,  Austria,  rapid  erosion  of,  96. 
Silurian  period,  401,  439-451. 

climate  of,  451. 

close  of,  451. 


Silurian  period  —  (Continued) 

deserts  of,  443. 

duration  of,  451. 

formations,  character  of,  440-442. 
thickness  of,  442. 

life  of,  444-450. 

migration  during,  451. 

of  New  York  state,  439. 

in  other  continents,  444. 

oscillations  of  level  during,  441. 

physical  geography  of,  439-442. 

volcanic  activity  of,  444. 
Silver  Spring,  Fla.,  63,  65. 
Slate,  344-345- 
Slickensides,  268. 
Slip,  262. 

Slope  of  ice  sheets,  647-648. 
Sloths,  621,  670. 
Slumping,  73. 
Smith,  J.  P.,  528. 
Snakes,  624. 

Snow  crystals,  growth  of,  143. 
Snow  fields,  142. 
Snow  line,  141. 
Sogne  fiord,  Norway,  166. 
Soil,  creep  of,  31,  71.    v  ^f 

kinds  of,  42. 

removal  of,  43. 
Solfataras,  323. 
Solution  by  ground  water,  60. 

by  streams,  83. 

in  disintegration  of  rocks,  35,  36. 
Sphalerite,  687. 
Sphenophycus  latifolius,  436. 
Sphenophylls,  495. 
Spherical  concretions,  77. 
Spheroidal  weathering,  39,  40. 
Spirifer  acuminatus,  458. 

cameratus,  482. 

.disjunctus,  458. 

mucronatus,  458. 

radiatus,  446. 
Spiriferina,  526. 
Spiriferina  spinosa,  482. 
Spirifers,  457. 
Spirula,  531. 
Spits  and  bars,  220. 
Sponges,  Cambrian,  416. 

Mesozoic,  524. 

Ordovician,  427. 

Proterozoic,  397. 

Silurian,  445. 
Spouting  horns,  210. 


INDEX 


715 


Springs,  constant  and  intermittent,  64. 

mineral  matter  contained  in,  64-66. 

origin  of,  62-64. 

temperature  of,  66. 

thermal,  66. 
Squalodonta,  597. 
Stabler,  Dole  and,  83. 
Stacks,  211. 
Stalactites,  71,  72. 
Stalagmites,  71,  72. 
Starfish,  Devonian,  457. 

Mesozoic,  526. 

Ordovician,  431. 

Staunton,  Va.,  swallow  holes  of,  69. 
Stegocephalians,  486-489,  536. 
Stegodon,  614. 
Stegosaurus,  546-548. 
Stenotheca  rugosa,  413. 
Stigmaria,  495. 
Stocks,  327-328. 
Stopes,  M.,  498. 
Stoping,  337. 
Storm  waves,  force  of,  200. 

height  of,  201. 
Stoss  side,  158,  183. 
Stratification,  24,  233-234. 
Stratified  drift,  160,  178-182. 
Stratigraphy,  definition  of,  22. 
Stream  deposits,  characteristics  of,  130. 
Stream  erosion,  cycle  of,  109-114. 

features  due  to,  89-114. 
Stream  piracy,  102,  107-109. 
Streams,  sea-captured,  214. 

sediment  of,  81. 
Streptelasma       (Enterolasma)       calicula, 

444- 

profundum,  429. 
Streptis  grayi,  446. 
Striations,  giacial,  157,  183. 
Strike,  252. 
Stromatopora,  Ordovician,  429. 

Silurian,  445. 
Stromboli,  339. 
Stropheodonta  demissa,  458. 
Strophomena  rugosa,  432. 
Strophostylus  cyclostomus,  447. 

expansus,  459. 
Structural  adjustment,  102. 
Structural  geology,  definition  of,  21. 
Structural  valleys,  100. 
Strutt,  274. 
Stylonurus,  461. 
Subaftonian  stage,  649. 


Subcrust  theory  of  interior  of  the  earth, 

274. 

Subglacial  material,  156. 
Subjacent  igneous  rock,  327-328. 
Submarginal  moraines,  161. 
Submarine  delta,  see  Continental  shelf. 
Submarine  earthquakes,  282. 
Submerged  streams,  227. 
Submerged  valleys,  195. 
Submergence,  226. 
Subsequent  streams,  100-102. 
Subsequent  valleys,  116. 
Subsoil,  42. 
Suez  Canal,  50. 
Sumatra,  earthquake  of,  281. 
Sumbawa,  volcano,  298. 
Sun  cracks,  236,  238. 
Superglacial  debris,  150,  154-156. 
Superposition,  order  of,  381. 
Surface  moraines,  154-155. 
Swallow  holes,  69. 
Swamps,  conditions  for,  472. 
Swine,  619. 
Syenite,  330. 
Synclines,  254,  363. 
Synclinorium,  256. 
Syndyoceras,  619. 
Synechodus,  534. 
Syringopora,  445,  456. 
Syringopora  retiformis,  444. 


Taconic  deformation,  422. 

Talc,  690. 

Talus,  angle  of  slope,  30. 

damming  lake,  39. 

in  arid  regions,  33. 

origin  of,  29. 
Tapirs,  607-608. 
Tarr,  R.  S.,  179,  648. 
Taylor,  F.  B.,  652. 
Teleoceros,  607. 
Teleosts,  535. 

Temnocheilus  forbesianus,  483. 
Temperature    and    the    disintegration    of 
rocks,  31. 

of  ocean,  196-197. 

of  springs,  66. 

Temperature,  changes  in  daily,  31. 
Tentaculites  gyracanthus,  447. 
Terabratula  humboldtensis,  526. 
Terminal  moraines,  159-160,  175. 
Terraces,  alluvial,  127. 


7i6 


INDEX 


Terraces  —  (Continued) 

discontinuity  of,  129. 

marine,  211-212. 

of  glacial  origin,  179. 

rock,  128. 
Tertiary  period,  572-642. 

changes  at  close  of,  643-644. 

climate  of,  634-636. 

duration  of,  642. 

effect  of  isolation  and  migration  during, 
636-642. 

life  of,  590-634. 

physical  geography  of,  574-590. 

vegetation  of,  630-634. 
Tetrabelodon,  614. 
Tetracoralla,  524. 
Tetragraptus,  428,  429. 
Tetragraptus  fructicosus,  428. 
Thalattosuchia,  552. 
Thaleops  ovata,  435. 
Thalweg,  63. 

Thames,  dissolved  minerals  in,  83. 
Thamnastraea  prolifera,  524. 
Thecosmilia  trichotoma,  524. 
Thelodus,  450. 
Thermal  springs,  66. 
Theromorphs,  536-539. 
Theropoda,  539. 
Thompson,  J.,  191. 
Throw,  262. 
Thrust  faults,  263-264. 
Tidal  bores,  202. 
Tidal  currents,  201,  218. 
Tidal  scour,  202. 
Tides,  201. 
Tied  islands,  223. 
Till,  160,  172,  173. 

Tillamook  Rock,  storm  waves  at,  201. 
Tillotherium,  621. 
Titanotheres,  603-605. 
Tivoli,  lime  deposits  of,  65. 
Topography,  mature,  no. 

youthful,  109. 
Top-set  beds,  131. 
Tornoceras  mithras,  459. 
Torosaurus,  548. 
Trachodon,  544,  545,  546,  572. 
Transportation,  and  velocity  of  streams, 
82,  86. 

by  marine  currents,  219-224. 
Travertine,  249. 
Travertine  dams,  65. 

cause  falls,  93,  94. 


Trematis  ottawaensis,  432. 
Trematonotus  alpheus,  447. 
Trenton  formation,  422. 
Trenton  stage,  422,  423. 
Triassic  period,  508-512. 

tlimate  of,  569-570. 

in  other  continents,  5.11. 

of  Atlantic  and  Gulf  coasts,  508-511. 

of  Pacific  coast,  511. 

of  western  interior,  511. 

physical  geography  of,  508-512. 

volcanic  activity  of,  511. 
Tribes  Hill  stage,  422. 
Triceratops,  547~549»  572- 
Trigonia  clavellata,  527. 
Trigonolestes,  616. 
Trilobites,  Cambrian,  402,  410-413. 

Carboniferous,  481,  484. 

Devonian,  460. 

Ordovician,  435-436. 

Silurian,  448-449. 
Trinity  formation,  515. 
Trinucleus      (Cryptolithus)       tessallatus, 

435- 

Trochoceras  desplainense,  448. 
Trocholites  ammonius,  434. 
Trochonema  umbilicatum,  434. 
Trochus  saratogensis,  413. 
Troostocrinus  reinwardti,  446. 
Tropidoleptus  carinatus,  458. 
Tropites  subbullatus,  529. 
Tsientang  River,  bores  in,  202. 
Tsunamis,  292. 

Tuckasegee    and    Davidson    rivers    con- 
trasted, 98. 
Tuff,  332. 
Tully  stage,  452. 
Turrilites,  530. 
Turrilites  catenatus,  529. 
Turritella  humerosa,  628. 

mortoni,  379,  628. 

variabilis,  628. 
Turtles,  Mesozoic,  557. 

Tertiary,  624. 
Tyndall,  J.,  190. 
Tyrannosaurus,  540,  572. 


Uintacrinus,  524. 
Uinta  Mountains,  356. 
Ulrich,  E.  O.,  408. 

Uncompahgre  River,  contrasted  with  Gun- 
nison,  114. 


INDEX 


717 


Unconformity,  270-271. 

basal,  405. 
Undertow,  200. 
Ungulates,  ancestors  of,  598. 

climbing,  619. 

Eocene,  592. 
Unstratified  drift,  160,  172-174. 

relation  to  stratified,  182. 
Unsymmetrical  valleys,  89. 
Upham,  W.,  658. 
Upthrow  of  fault,  262. 
Utica  stage,  422. 


Valley  of  Virginia,  soil  of,  41. 
Valley  trains,  162,  178. 
Valleys,  direction  of,  101. 

glacial,  163. 

growth  of,  98. 

structural,  100. 

widened  by  weathering,  41. 

young,  85. 

Van  Hise,  C.  R.,  393. 
Vaqueros  formation,  580,  581. 
Varanosaurus,  490,  491. 
Varieties  of  coat,  501. 
Veins,  370,  371-372. 
Velocity   and   transportation   of  streams, 

82,  86. 

Venericardia  planticosta,  628. 
Vermilion    River,   decolorization   of  cliffs, 

37- 

Vertical  faults,  264-265. 
Vertical  range  of  species,  381. 
Vesuvius,  295,  302-303,  309,  312,  322. 
Viscosity  theory  of  glacial  movement,  189- 

190. 

Visp,  earthquake  at,  281. 
Volcanic  activity  in  Archaeozoic,  391. 

Cambrian,  407. 

Devonian,  455. 

Miocene,  581,  583-584. 

Ordovician,  423. 

Permian,  477. 

Proterozoic,  394. 

Silurian,  444. 

Triassic,  510-511. 
Volcanic  ash,  297,  321. 

bomb,  297. 

cinders,  297. 

cones,  erosion  of,  314-316. 
slope  of,  311-314. 

dust,  297. 


Volcanic  —  (Continued) 

eruptions,  periodicity  of,  338. 

gases,  295,  338. 

necks,  316,  317. 
Volcanism,  early,  320. 

importance  of,  321-322. 

theories  of,  334-339. 
Volcanoes,  classification  of,  295. 

distribution  of,  318-320. 

in  Iceland,  311. 

in  United  States,  317. 

materials  erupted  by,  295-298. 

new,  294. 

number  of,  318. 

types  of,  302-311. 
Volutilithes  petrosus,  628. 


Waagenoceras  cumminsi,  483. 
Walchia,  497,  498. 
Walcott,  C.  D.,  398,  408. 
AValled  lakes,  204,  205. 
Warping,  112,  257. 
Wasatch  formation,  519. 
Waterfalls,  formed  by  drift,  187. 
Water  gaps,  116,  117. 
Water  table,  56-58. 
Water  wear  in  streams,  83. 
Waucobian,  402. 
Wave-built  terrace,  212. 
Wave-cut  terrace,  212. 
Wave  motion,  198. 
Waves,  199. 

breaking  of,  200. 

deposition  by,  218-224,  233. 

earthquake,  202. 

erosion  by,  202,  213-214,  231. 
Weathering,  27. 

and  erosion,  85. 

and  ores,  373. 

chemical  effects  of,  35-38. 

comparison  of  chemical  and  mechanical, 
38. 

differential,  40-42. 

mechanical  effects  of,  27-34. 

of  metamorphic  rocks,  350. 

rate  of,  27. 

results  of,  38-43. 

spheroidal,  39-40. 
Wedge  work  of  ice,  28. 
Wells,  57-58. 
Whales,  597. 
Wheeler,  W.  H.,  49,  199,  205. 


7i8 


INDEX 


Wheeler,  W.  M.,  630. 
White,  D.,  569. 
White  River  formation,  604. 
Willis,  B.,  575. 
Williston,  S.  W.,  682. 
Wilson,  A.  W.  G.,  118. 
Wind,  abrasion  by,  34. 

and  sand,  44-52. 

deflation  by,  44. 
Wind  cave,  South  Dakota,  70. 
Wind  gaps,  108. 
Wind  River  formation,  604. 
Wisconsin  ice  sheet,  666. 
Wisconsin  stage,  649. 
Worms,  Cambrian,  415. 

Proterozoic,  397. 


Worthenella  cambria,  415. 
Worthenia  tabulata,  482. 

Yarmouth  stage,  649. 

Yellowstone  National  Park,  66,  67,  323, 

Young  rivers,  109-110. 

Youthful  shore  line,  231. 

Youthful  topography,  109. 

Zambezi  River,  course  of,  260. 
Zeuglodon,  595~597- 
Zinc  minerals,  687. 
Zone  of  flow,  258. 

of  fracture,  58,  258. 
Zygospira  recurvirostris,  432. 


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